sustainability

Review Sulfolane: Magic Extractor or Bad Actor? Pilot-Scale Study on Solvent Corrosion Potential

Andrzej Bak 1,* , Violetta Kozik 1, Paulina Dybal 1, Slawomir Kus 2, Aleksandra Swietlicka 1 and Josef Jampilek 3,*

1 Department of Synthesis Chemistry, Faculty of Mathematics, Physics and Chemistry, University of Silesia, Szkolna 9, 40007 Katowice, Poland; [email protected] (V.K.); [email protected] (P.D.); [email protected] (A.S.) 2 Honeywell Process Solutions, 11201 Greens Crossing Blvd, Suite 700 Houston, TX 77067, USA; [email protected] 3 Department of Pharmaceutical Chemistry, Faculty of Pharmacy, Comenius University, Odbojarov 10, 83232 Bratislava, Slovakia * Correspondence: [email protected] (A.B.); [email protected] (J.J.); Tel.: +48-32-359-1197 (A.B.)



Received: 17 September 2018; Accepted: 11 October 2018; Published: 14 October 2018


Abstract: The sulfur-containing derivatives and their metabolites, regarded as ‘old devils of green’ chemistry, constitute a relevant class of air/water/soil contaminants in over-polluted world. In fact, some industrially-engineered solvents have become environmentally unfavorable. An attractive alternative to commonly used industrial liquids is sulfolane (C4H8SO2), an anthropogenic medium. The main objective of this paper is the comprehensive review focusing mainly on the state-of-the-art aspects of the sulfolane synthesis, application of sulfolane as an extractive solvent due to its ‘unique’ physicochemical properties as well as the potential of sulfolane to cause equipment corrosion and subsequent spills. The potential risk for groundwater contamination, danger for human health and ways of sulfolane biodegradation were briefly reviewed as well. Interestingly, the analysis performed on data stored in the Reaxys database revealed an alternating tendency of waxing and waning interest in sulfolane during the space of the last fifty years. Moreover, the primary goal of the presented case study was to verify applicability of industrial, multi-electrochemical technique for reliable detection of corrosion in low conductive process fluids. Several aspects of corrosion measurement including the impact of process parameters (temperature) and impurities (oxygen and chlorides) on stainless steel corrosion in pure sulfolane were investigated briefly.

Keywords: sulfolane; aprotic solvent; liquid extraction; biodegradation; on-line corrosion monitoring; electrochemical techniques

1. Introduction Pollutants emitted from certain solids or liquids originate from anthropogenic activity or from the biogenic emissions of certain reactive hydrocarbon derivatives formed as chemical (by-)products of natural transformations [1]. The odor-causing mixtures of sulfur-containing derivatives and their metabolites constitute a main class of irritant and toxic contaminants with increasingly detrimental impact on human health in the long-term exposure, being also classified as a potential mutagenic and carcinogenic risk factors [2,3]. A diverse ‘suite’ of environmental contaminants, potentially resistant to degradation, were designed and engineered for industrially-specific purposes becoming problematic, when released into the Nature since recalcitrance correlates to persistence [4]. On the other hand, natural gases also contain ‘sour’ impurities such as hydrogen sulfide (H2S) or carbon dioxide (CO2), the removal of which is technologically and/or economically relevant to ‘soften’ the

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Sustainability 2018, 10, x FOR PEER REVIEW 2 of 22 natural/industrial (off)gases [5,6]. Consequently, the eradication of sulfur-containing compounds 45 technologically(e.g., H2S, COS, and/or mercaptans, economically organic sulfides) relevant from to unprocessed ‘soften’ the natural natural/industrial gases can be (off)gases performed [5,6]. in the 46 Cliquid-liquidonsequently, extractionthe eradication process, of sul wherefur-containing industrial solventscompounds areused (e.g. inH2 aS, ‘closed’ COS, mercaptans loop, recovered, organic and 47 sulregeneratedfides) fromon-site, unprocessedthat theoretically natural gas preventes can extractorbe performed disposal in the [7, 8li].quid In this-liquid context extraction questions process about, 48 wherethe ‘green’ industrial solvents solvents and manufacturing are used in a procedures ‘closed’ loop, used recovered in the liquid-liquid and regenerated extraction on are-site looked, that 49 theoreticallyupon favourably. prevent extractor disposal [7,8]. In this context questions about the ‘green’ solvents 50 and manufacturingIn fact, a range procedure of solventss canused be in employed the liquid in-liquid the extraction extraction process are looked used inupon oil refiningfavourably. including 51 (di/tri/tetra)In fact, a ethylene range of glycolsolvents (D/T/TT/EG), can be employed diglycol in the amine extraction (DGA), processn-methyl used pyrolidone in oil refining (NMP), 52 includingdimethylsulphoxide (di/tri/tetra)(DMSO), ethylene dimethylformamide glycol (D/T/TT/EG), (DMF),diglycol morpholine amine (DGA), and carbonaten-methyl pyrolidone derivatives. 53 (NMP),An attractive dimethylsulph alternativeoxide to commonly (DMSO), used dimethylformamide industrial extractive (DMF) liquids, ismorpholine sulfolane, anand anthropogenic carbonate 54 derivatives.organosulfur An medium attractive regarded alternative as toa versatilecommonly dipolar used aprotic industrial solvent extractive[9,10]. Sulfolaneliquids is sulfolane (C4H8SO,2 an) is 55 anthropogenicthe generic name organosul for hydrogenatedfur medium sulfonesregarded of as butadiene a versatile [ dipolar11]. In aprotic the present solvent review [9,10]. weSulfolane mainly 56 (Cfocus4H8SO on2) theis the sulfolane generic synthesis, name for application hydrogenated of sulfolane sulfones asof anbutadiene extractive [11]. solvent In the due present to its ‘unique’review 57 wephysicochemical mainly focus on properties the sulfolane as well synthesis, as on the application potential of of sulfolane sulfolane to as cause an extractive equipment solvent corrosion due andto 58 itssubsequent ‘unique’ physicochemical spills. The possible properties risk for as groundwater well as on the contamination, potential of sulfolane danger for to humancause equipment health and 59 corrosionways of sulfolane and subsequent biodegradation spills. areThe reviewed possible briefly risk asfor well. groundwater Interestingly, contamination, the analysis performed danger for on 60 humandata stored health in and the Reaxysways of databasesulfolane revealed biodegradation an alternating are reviewed tendency briefly of waxing as well. and Interestingly, waning interest the 61 analysisin sulfolane performed during on the data space stored of the in last the fifty Reaxys years. database Roughly revealed speaking, an the alternating revealed ‘wavy’ tendency trends of 62 waxingin sulfolane and waning interests interest provide in ansulfolane illustration during of the the partial space correlationsof the last fifty with years oil prices.. Roughly Moreover, speaking, the 63 theprimary revealed goal ‘wavy’ of the presentedtrends in casesulfolane study wasinterests to verify provide applicability an illustration of industrial, of the multi-electrochemicalpartial correlations 64 withtechnique oil prices. for reliableMoreover, detection the primary of corrosion goal of the in lowpresented conductive case study process was (sulfolane-based) to verify applicability fluids. 65 ofSeveral industrial, aspects multi of corrosion-electrochemical measurement technique of general for reliable and localized detection corrosion of corrosion modes in werelow conductive investigated 66 processbriefly, (sulfolane since corrosion-based) control fluids. and Several monitoring aspects inof sulfolanecorrosion unitsmeasurement does not of find general prominent and localized presence 67 corrosionin published modes literature. were investigated Incipient attempts briefly, to since quantify corrosion impact control of process and parameters monitoring (temperature) in sulfolane 68 unitsand impurities does not find(oxygen prominent and chlorides) presence on in stainless published steel literature (UNS S304033). Incipient corrosion attempts in pure to quantifysulfolane 69 impactprovided of process reasonably parameters meaningful (temperature) data and and are detailedimpurities elsewhere (oxygen inand this chlorides) paper. For on thestainless purpose steel of 70 (UNSthe proposed S304033) investigationscorrosion in pure a dedicated sulfolane testing provided vessel reasonably was designed meaning andful constructed. data and are It shoulddetailed be 71 elsewhereemphasized in thatthis therepaper. is For a clear the absence purpose of of detailed the proposed and quantitative investigations evaluation a dedicated of individual testing sulfolane vessel 72 wasimpurities designed (like and water, constructed. chlorides, It should etc.), their be emphasized limits and interactionsthat there is ona clear corrosion absence rate of ofdetailed carbon and and 73 quantitativealloyed steels; evaluation therefore of more indi detailedvidual sulfolane control ofimpurities experiments, (like a water, wider range chlorides, of solution etc.), their conditions, limits 74 anda wider interactions range of on temperatures, corrosion ra andte of with carbon this theand corresponding alloyed steels; surface therefore analysis more of detailed the electrodes control must of 75 experiments,be performed a inwider the future.range of solution conditions, a wider range of temperatures, and with this the 76 corresponding surface analysis of the electrodes must be performed in the future. 2. Beware of Sulfolane 77 2. Beware of Sulfolane Formally, sulfolane is a cyclic sulfone, containing a four-membered carbon ring and sulfonyl 78 functionalFormally, group sulfolane (R2SO is2) a with cyclic a sulfursulfone, atom containin double-bondedg a four-membered to two oxygen carbon atoms ring and with sul afonyl quite 79 functionallarge dipole group moment (R2SO (2µ) with= 4.7 a debye), sulfur atom dielectric double constant-bonded (ε υto= two 43.4) oxygen and low atoms vapor with pressure a quite [12large,13 ]. 80 dipoleThe structure moment of ( sulfolaneμ=4.7 debye with), dielectric its zwitterionic constant and ( dritterionicευ=43.4) and resonance low vapor structures pressure is [ illustrated12,13]. The in 81 structureFigure1. of sulfolane with its zwitterionic and dritterionic resonance structures is illustrated in 82 Figure 1.

(a) (b) 83 Figure 1. Cont. Sustainability 2018, 10, 3677 3 of 21 Sustainability 2018, 10, x FOR PEER REVIEW 3 of 22

sulfolane “zwitterion” “dritterion” (c)

84 FigureFigure 1. 1. StructureStructure of of sulfolane sulfolane with with (a ()a hydrophilic) hydrophilic and and hydrophobic hydrophobic parts parts (b ()b 3D) 3D geometry geometry (c ()c ) 85 resonanceresonance structures structures..

86 DueDue to to its its combination combination of of physical physical and and chemical chemical properties properties (inertness (inertness and and stability stability),), sulfolane sulfolane 87 isis a aselective selective solvent solvent in in liquid liquid-liquid-liquid and and liquid liquid-vapour-vapour extraction extraction processed processed,, used used for for the the removal removal 88 ofof close close-boiling-boiling alkanes alkanes from from cycloalkanes cycloalkanes ( (CC5-5C–C10 10derivedderived from from petroleum petroleum fractions) fractions) or or for for the the 89 separationseparation of compounds compounds with with different different degree degree of saturation of saturation and polarity and polarity in the extractive in the rectificationextractive 90 rectificationof arenes (BTX—benzene, of arenes (BTX toluene, - benzene, xylene) toluene, from non-aromaticxylene) from saturatednon-aromatic hydrocarbon saturated mixtures) hydrocarbon [14,15 ]. 91 mixtures)Sulfolane [14 is the,15]. preferred Sulfolane solvent is the for preferred these processes solvent due for to itsthese satisfactory processes selectivity due to for its the satisfactory aromatics 92 selectivityof interest, for low the boilingaromatics temperature of interest and, low its boiling capacity temperature for dissolving and its large capacity quantities for dissolving of aromatics large in 93 quantitiesrelatively of low aromatics quantities in of relatively solvent—the low sulfolanequantities extraction of solvent process - the sulfolane is less expensive extraction to operate process than is 94 lesssimilar expensive processes to operate using otherthan similar solvents. processes using other solvents. 95 ApartApart from from the the traditional traditional applications application ins thein extraction the extraction of aromatics of aromatics and in ‘sour and gas in sweetening’,‘sour gas 96 sweetening(e.g., removal’, (e.g. of, removal acid gases of acid from g aases natural from gasa natural stream) gas sulfolane stream) sulfolane can be used can inbe aused wide in rangea wide of 97 rangeengineering of engineering and biomedical and biomedical applications, applications, e.g., fractionation e.g., fractionation of wood of wood tars, curing tars, curing agent agent for epoxy for 98 epoxyresins, resins, production production of electronics of electronics and polymers and polymers [12]. Moreover, [12]. Moreover, sulfolane andsulfolane its analogues and its demonstrate analogues 99 demonstratethe properties the ofproperties inflammation of inflammation inhibitors (sulfolane-induced inhibitors (sulfolane hypothermia)-induced hypothermia) and tumor and cell tumor growth 100 cellinhibitors growth [ 16 inhibitors–18]; however [16–18 these]; howeve effects arer the probablyse effect species-specifics are probably and speciesthe extrapolation-specific and to human the 101 extrapolationhealth is speculative to human, as health noticed is speculative one of Reviewers., as noticed one of Reviewers. 102 SulfolaneSulfolane with with a a high high worldwide productionproduction volumevolume (approximately (approximately 18 18 000–36 000 – 00036 000 tons tons per year)per 103 yearis a) readily is a readily available available commodity commodity chemical chemical that can that be ca purchasedn be purchase as anhydrousd as anhydrous or aqueous or aqueous chemical 104 chemicalsubstance substance (3–5% water (3-5% content water prevents content prevents the transition the transition to solid state) to solid [19]. state) According [19]. According to ECHA data to 105 ECHA(European data Chemical(European Agency), Chemical sulfolane Agency), is manufactured sulfolane is and/or manufactured imported and/or to the European imported Economic to the 106 EuropeanArea in quantity Economic of Area 1 000–10 in quanti 000 tonsty of annually1 000 – 10 [20 000]. In tons theory, annually during [20 the]. In large-scale theory, during industrial the 107 largeusage-scale of sulfolane industrial disposal/leakage usage of sulfolane of disposal/leakage solvent is not encountered of solvent isdue not to encountered its on-site recovery due to and its 108 onregeneration;-site recovery however and regeneration; a typical loss however rate is approximateda typical loss rate to 5–10 is approximated ppm (parts per to million) 5-10 ppm of the(parts feed 109 prateer million) [21]. Practically, of the feed some rate unpredicted/accidental [21]. Practically, some unpredicted spills as well/accidental as leaks from spills extraction as well as units leaks in 110 fromrefineries extraction or gas units plants in wererefineries reported or gas worldwide, plants were resulting reported in inadvertent worldwide contamination, resulting in ofinadvertent the vadose 111 contaminationzone, groundwater of the andvadose wetland zone, ecosystemsgroundwat [er22 and]. Moreover, wetland leachatesecosystems from [22]. disposal Moreover, areas, leachates leakage 112 fromduring disposal processes areas, and leakage seepage during from producing processes wells and and seepage unlined from storage producing ponds have wells led and to unlinedsulfolane 113 storagecontaminated ponds have soil and led groundwater to sulfolane contaminatedaround gas processing soil and plants groundwater [4]. High around aqueous gas miscibility processing of 114 plantssulfolane, [4]. High combined aqueous with miscibility the low octanol-water of sulfolane, partition combined (K withow), carbon-water the low octanol partition-water coefficient partition 115 (K(Kowoc),) carbonand pK-awatervalue partition have resulted coefficient in substantial (Koc) and off-site pKa value migration have from resulted contaminated in substantial sites, therefore off-site 116 migrationthe environmental from contamina fate of theted compound sites, therefore is being the extensively environmental studied fate [23 of]. the compound is being 117 extensivelyReduction studied of leaks [23]. and spills of sulfolane highlight an important question about its corrosivity and 118 impactReduction on integrity of leaks of industrialand spills systemsof sulfolane (pipes, highlight vessels, an pumps important etc.). Itquestion was noticed about that its corrosiv impellersity of 119 andsulfolane impact pumps on integrity are the components of industrial particularly systems (pipes, sensitive vessels, to corrosion, pumps due etc.). to the It velocity was noticed of sulfolane that 120 impellersagainst the of metal.sulfolane It seems pumps that aresulfolane-induced the components particularly corrosion is sensitive purely industrial to corrosion problem, due with to the no 121 velocityapparent of crossoversulfolane intoagainst the the public metal. realm, It seems because that research sulfolane did-induced not indicate corrosion that corrosion is purely ever industrial became 122 problemsevere enoughwith no to apparent cause leakage/spills crossover into of the sulfolane public outsiderealm, because of the plant research [24]. did In fact, not pureindicate sulfolane that 123 corrosionunder standard ever became operating severe conditions enough to is consideredcause leakage/spills to be a stable of sulfolane compound outside and of non-aggressive the plant [24]. to 124 Insteel, fact, butpure sulfolane sulfolane systems, under standard if contaminated operating by conditions traces of oxygen is considered and at typicalto be a processstable compound conditions 125 and(170–180 non-aggressive◦C), may lead to steel, to sulfolane but sulfolane decomposition systems, and if contaminated formation of by corrosive traces of(by-)products—the oxygen and at 126 typical process conditions (170-180ºC), may lead to sulfolane decomposition and formation of Sustainability 2018, 10, 3677 4 of 21 corrosion of steel can be quite rapid, causing severe damage to industrial installations. Decomposition of sulfolane may be further enhanced by presence of water, used commonly as a sulfolane diluent, as well as by presence of oxygen and/or chlorides, respectively. Some general correlations between oxygen and chloride concentration leading to sulfolane corrosivity are known and described, e.g., oxygen above 0.5% and water above 3% accelerate sulfolane degradation and production of acidic corrodents [25]. However, sulfolane-corrosion knowledge is still lacking detailed and quantitative evaluation of individual impurities, their limits and interactions on corrosion rate of carbon and alloyed steels. Is it important to monitor the fate of sulfolane in the environment and the primary potential routes of human exposure to this solvent? The answer to above question is positive, because sulfolane poses a potential risk for contamination of surface and groundwater, domestic wells around sour gas plants and off-site migration—the transfer of contaminant mass between the liquid and solid phases was noticed [22]. Sulfolane has been detected in groundwater in several regions nearby industrial areas, therefore some assessment of the potential hazard for human beings were undertaken with toxicity benchmarks specification including several genotoxicity studies, acute toxicity studies in multiple species via multiple routes of exposure or a chronic oral toxicity analysis as well [19]. In fact, animal data suggest that reproductive and developmental toxicity is only likely to occur at concentrations much higher than those, that elicit other milder adverse effects. Sulfolane is known to be acutely toxic at doses over 200 mg/kg in lab rats and guinea pigs [12]. Luckily, sulfolane is not likely to be mutagenic, clastogenic or carcinogenic, or pose reproductive or developmental health risks except perhaps at very high exposure concentrations [19]. Generally, two procedures of removing sulfolane-containing contaminants from industrial/environmental areas can be distinguished based on the nature of applied transformations: physical (filtration) and chemical (oxidation). The first technique comprises transfer and/or concentration of pollutants in a liquid phase onto solid phase (activated carbon adsorption), whereas the second one is related to the destruction/decomposition of molecules by thermal/catalytic oxidation, flaring or degradation by microorganism community to harmless (by-)products [26]. The complete oxidation of sulfolane can be described as follows:

+ 2− C4H8O2S + 6.5O2 → 4CO2 + 3H2O + 2H + SO4 (1)

Exploiting the metabolic capabilities of microorganisms naturally occurring in areas of contamination, using techniques such as monitored natural attenuation or biostimulation, may be a way to remediate sulfolane-contaminated aquifers according to ‘clean at the end of pipe’ paradigm [4]. Biodegradation of sulfolane using microbes residing in an activated sludge system, in wastewater treatment or in laboratory microcosm using contaminated aquifer sediments was evaluated under aerobic and anaerobic conditions, respectively. It turned out that sulfolane-contaminated environmental samples are pretty easily biodegradable in nutrient-enriched aerobic microcosms [27,28].

2.1. Synthesis and Manufacturing Sulfolane is a non-reactive, water-soluble dipolar aprotic industrial solvent that was first engineered and patented by Shell Oil Company in the early 1940s [10]. The original process of sulfolane synthesis is based on the hydrogenation of 3-sulfolene (3) obtained as a product of sulfur dioxide (2) and butadiene (1) reaction (Scheme1). Sustainability 2018, 10, x FOR PEER REVIEW 4 of 22

127 corrosive (by-)products - the corrosion of steel can be quite rapid, causing severe damage to 128 industrial installations. Decomposition of sulfolane may be further enhanced by presence of water, 129 used commonly as a sulfolane diluent, as well as by presence of oxygen and/or chlorides, 130 respectively. Some general correlations between oxygen and chloride concentration leading to 131 sulfolane corrosivity are known and described, e.g., oxygen above 0.5% and water above 3% 132 accelerate sulfolane degradation and production of acidic corrodents [25]. However, 133 sulfolane-corrosion knowledge is still lacking detailed and quantitative evaluation of individual 134 impurities, their limits and interactions on corrosion rate of carbon and alloyed steels. 135 Is it important to monitor the fate of sulfolane in the environment and the primary potential 136 routes of human exposure to this solvent? The answer to above question is positive, because 137 sulfolane poses a potential risk for contamination of surface and groundwater, domestic wells 138 around sour gas plants and off-site migration – the transfer of contaminant mass between the liquid 139 and solid phases was noticed [22]. Sulfolane has been detected in groundwater in several regions 140 nearby industrial areas, therefore some assessment of the potential hazard for human beings were 141 undertaken with toxicity benchmarks specification including several genotoxicity studies, acute 142 toxicity studies in multiple species via multiple routes of exposure or a chronic oral toxicity analysis 143 as well [19]. In fact, animal data suggest that reproductive and developmental toxicity is only likely 144 to occur at concentrations much higher than those, that elicit other milder adverse effects. Sulfolane 145 is known to be acutely toxic at doses over 200 mg/kg in lab rats and guinea pigs [12]. Luckily, 146 sulfolane is not likely to be mutagenic, clastogenic or carcinogenic, or pose reproductive or developmental 147 health risks except perhaps at very high exposure concentrations [19]. 148 Generally, two procedures of removing sulfolane-containing contaminants from 149 industrial/environmental areas can be distinguished based on the nature of applied 150 transformations: physical (filtration) and chemical (oxidation). The first technique comprises 151 transfer and/or concentration of pollutants in a liquid phase onto solid phase (activated carbon 152 adsorption), whereas the second one is related to the destruction/decomposition of molecules by 153 thermal/catalytic oxidation, flaring or degradation by microorganism community to harmless 154 (by-)products [26]. The complete oxidation of sulfolane can be described as follows:

C4H8O2S + 6.5O2 → 4CO2 + 3H2O + 2H+ + SO42- (1) 155 Exploiting the metabolic capabilities of microorganisms naturally occurring in areas of 156 contamination, using techniques such as monitored natural attenuation or biostimulation, may be a 157 way to remediate sulfolane-contaminated aquifers according to ‘clean at the end of pipe’ paradigm 158 [4]. Biodegradation of sulfolane using microbes residing in an activated sludge system, in 159 wastewater treatment or in laboratory microcosm using contaminated aquifer sediments was 160 evaluated under aerobic and anaerobic conditions, respectively. It turned out that 161 sulfolane-contaminated environmental samples are pretty easily biodegradable in nutrient-enriched 162 aerobic microcosms [27,28].

163 2.1. Synthesis and Manufacturing 164 Sulfolane is a non-reactive, water-soluble dipolar aprotic industrial solvent that was first 165 engineered and patented by Shell Oil Company in the early 1940s [10]. The original process of Sustainability 2018, 10, 3677 5 of 21 166 sulfolane synthesis is based on the hydrogenation of 3-sulfolene (3) obtained as a product of sulfur 167 dioxide (2) and butadiene (1) reaction (Scheme 1).

(1) (2) (3)

168 Scheme 1. Industrial processScheme of sulfolane 1. Industrial synthesis. process of sulfolane synthesis.

We illustrated some selected examples of sulfolane synthesis as well. In 1982 the synthesis of sulfolane was proposed with 1,4-dihalogenobutane as the starting reagent and sulfoxide as the intermediate [29]., A few years later, the silica-supported titanium/tartaric acid catalysts were used in the sulfide→sulfoxide oxidation [30]. In fact, several titanium derivatives supported on silica or molecular sieves suspended in a solution containing hydrogen peroxide (H2O2) led to high yields of the selective oxidation of sulfides to sulfoxides. Due to the significance of sulfoxides as intermediates in organic synthesis, the selective sulfide→sulfoxide oxidation has been a challenging issue; therefore a large number of supported reagents (hydrogen peroxide, chromic acid, nitric acid, ozone, manganese dioxide, peracids) was tested revealing some process drawbacks such as extended reaction time or formation of toxic and irritant intermediates [31]. In 1999 the use of urea-hydrogen peroxide (UHP) adduct under solvent-free conditions was proposed in response to the need for mild and selective/efficient methods for conversion of sulfide to the corresponding sulfoxides or sulfone [32]. Highly chemoselective and non-aqueous procedure of sulfides→sulfones oxidation with magnesium monoperoxyphthalate (MMPP) on hydrated silica gel support in methylene chloride solvent was demonstrated by Ali et al. [33]. The employment of magnesium reagent resulted in a simple and environmentally friendly procedure with high yields. Similarly, the excellent yield (≥94%) and selectivity (>95%) of sulfides→sulfoxides oxidation was reached using tetra-(tetraalkylammonium)octamolybdate catalysts with 30% aqueous hydrogen peroxide as oxidant in high ratio of substrate to catalyst (10000:1) [34]. Following the principles of ‘green chemisty’ a new environmentally friendly method for the sulfide oxidation at the ambient temperature by H2O2/H2O (oxidant/solvent) mixture was documented with high yields and non-toxicity of the corresponding (by-)products [35]. In 2017 an eco-friendly and catalyst-free method for oxidation of sulfides to sulfones, using Oxone (a stable 2:1:1 ternary composite of KHSO5, K2SO4 and KHSO4) was provided by Kupwade et al. [36]. The previously available protocols in the chemoselective sulfides→sulfoxides oxidation by Oxone suffered from the drawbacks of thermal activation; therefore the energy-efficient combination of Oxone-KBr was proposed as an oxidant. Moreover, the chromatography-free and scalable procedure for the transformation of sulfides to sulfones with an unprecedented catalyst-oxidant combination (diethylamine-Oxone) was provided as well. It should be emphasized that recyclable pollution-free catalysts reused in the mild reaction conditions with halogen-free solvents and ‘green’ oxidants provide a benign alternative to environmentally unfavorable (by-)products formed in the other reactions. The detailed description of all available methods for sulfolane synthesis is beyond the scope of this paper, but can be found elsewhere [35]. Is sulfolane still an eye-catching compound for the scientific community? In fact, we found some highly interesting regularity while mining the commercially available Reaxys database [37]. Interestingly, the analysis performed on data stored in the Reaxys database revealed an alternating tendency of waxing and waning interest in sulfolane during the space of the last fifty years as illustrated in Figure2. Extensive database screening was performed to identify 1644 hits, where word ‘sulfolane’ was queried in the title or abstract of the paper published during last five decades (from 1969 to 2018). Most of them describe sulfolane physicochemical properties (704), its chemical transformations (462), physicochemical analysis methods (450) or quantum chemical calculation procedures (33), respectively. More detailed analysis revealed, that many papers referred to solubilizers (320), ionic liquids (168), Sustainability 2018, 10, 3677 6 of 21

viscosity (140), solubility (106), organic solvents (100), solvent extraction (91), oxidation reaction (72), catalysts (72), separation methods (70), chemical stability (52), aprotic solvents (37), polar solvents (35), chemical engineering (35), solvent mixtures (28), toxicity (19), biodegradation (19), petroleum (18), liquid-liquid equilibrium (18), corrosion (9), etc. Furthermore, 189 patents and patent applications that relate to the synthesis of sulfolane, its purification and use as a solvent or extractor, are mostly owned bySustainability Philips Petroleum 2018, 10, x Company FOR PEER REVIEW (24), Lloyd (15), Berg (15) and Mobil Oil Corporation (10), respectively.6 of 22

219 FigureFigure 2. Trends 2. Trends in thein the average average annual annual crude crude oil pricesoil prices and and interests interests in sulfolanein sulfolane during during the the space space of of 220 lastlast fifty fifty years. years. Plot Plot of theof the number number of papers,of papers, where where sulfolane sulfolane was was searched searched in paperin paper title title or abstractor abstract 221 andand average average annual annual oil pricesoil prices ($/barrel) ($/barrel) based based on on Reaxys Reaxys and an OPECd OPE dataC data [37 [,3837,38]. ].

222 A naturalA natural question question appears, appears what, what actually actually triggers triggers an an increasing increasing interests interests in insulfolane sulfolane that that 223 fadesfades after after some some years? years? Surprisingly, Surprisingly, the the waves waves of interestof interest superpose superpose on on the the fluctuations fluctuations in averagein average 224 annualannual crude crude oil prices oil pr ($/barrel)ices ($/barrel provided) provid by theed Organization by the Organization of the Petroleum of the Exporting Petroleum Countries Exporting 225 (OPEC)Countries [37,38 (OPEC)]. Generally, [37,38 the]. Generally, constant growth the constant in number growth of publicationsin number of related publications to sulfolane related is to 226 observedsulfolane throughout is observed the throughoutyears with a the peak years reached with in a 2016. peak Roughly reached speaking, in 2016. Roughly the revealed speaking, ‘wavy’ the 227 trendsrevealed in sulfolane ‘wavy’ intereststrends in provide sulfolane an illustrationinterests provide of the partialan illustration correlations of the with partial oil prices correlations as shown with 228 in Figureoil prices2. In as the shown period in of theFigure relative 2. In oil the prices period stability of the (1987–1999) relative oil the prices amount stability of sulfolane-related (1987-1999) the 229 papersamount was of more sulfolane or less-related constant, papers while was the more growth or of less the constant, oil prices while induced the noticeablegrowth of increase the oil prices of 230 theinduced scientific noticeable activity, as increase observed of the between scientific 1973 activity and 1985, as orobserved in a new between millennium 1973 asand well. 1985 Obviously, or in a new 231 sulfolanemillennium is used as in well. the aromatic Obviously, extraction sulfolane in the is usedoil industry, in the where aromatic price extraction per barrel in put the pressure oil industry, on 232 thewhere economy price of per the barrel refining put processes. pressure on the economy of the refining processes. 233 Briefly,Briefly, in the in aromatic the aromati extractionc extraction process process sulfolane sulfolane is initially is mixed initially with mixed the petroleum with the feedstock petroleum 234 to dissolvefeedstock aromatics to dissolve in thearomatics extractor in column.the extractor A subsequent column. A stripper subsequent column stripper removes column any remainingremoves any 235 non-aromaticsremaining no fromn-aromatics the aromatic-laden from the aromatic sulfolane.-laden Then, sulfolane. the sulfolane/aromatics Then, the sulfolane/aromatics mixture is fed through mixture 236 a recoveryis fed through column, a where recovery aromatics column are, where vaporized aromatics and captured. are vaporized Sulfolane and is captured. recycled Sulfolane for reuse. is 237 A simplifiedrecycled flow for diagram reuse. of A this simplified process with flow a typical diagram extractor-stripper-regenerator of this process configurationwith a typical is 238 shownextractor in Figure-stripper3. -regenerator configuration is shown in Figure 3. Sustainability 2018, 10, 3677 7 of 21 Sustainability 2018, 10, x FOR PEER REVIEW 7 of 22

239 Figure 3.3. Sulfolane extraction process with areasareas ofof highhigh corrosioncorrosion potentialpotential [[2525].].

240 SulfolaneSulfolane isis vitalvital inin thethe processingprocessing technology offered by Shell forfor thethe (pre)treatment(pre)treatment ofof naturalnatural 241 gases.gases. CombinationCombination of of sulfolane sulfolane (40–60%) (40-60%) with with alkanolamines alkanolamines (30–45% (30−45% of diisopropanolamine of diisopropanolamine (DIPA) 242 or(DIPA) 30–45% or of30−45% methyl of diethanolamine methyl diethanolamine (MDEA)) and (MDEA) water) (5–15%) and water is used (5−15%) as a physicalis used solventas a physical in the ® 243 Sulfinolsolvent ingas the sweetening Sulfinol® gas process sweetening to remove process toxic levelsto remove of sour toxic gases levels (e.g., of H sou2S,r CO gases2) from (e.g. raw, H2 naturalS, CO2) 244 gasfrom condensate raw natural [39 gas]. The condensate Sulfinol® [process39]. The is Sulfinol a liquid–liquid® process extraction, is a liquid blending–liquid extraction, liquid gas feedblending into 245 liquidliquid solvent gas feed feed into to dissolve liquid the solvent sulfur feed compounds to dissolve of interest, the sul wherefur compounds sulfolane flows of interest,counter-current where 246 tosulfolane the flow flows of the feed counter stream-current in the to extractor the flow column. of the Sulfur-containing feed stream in compounds the extractor are separated column. 247 fromSulfur the-containing solventby compounds heating, subsequently are separated condensed from the solvent and recovered. by heating, Sulfolane subsequently is regenerated condensed to 248 removeand recov impuritiesered. Sulfolane and recycled is regenerated for repeated to remove usage. impurities and recycled for repeated usage.

249 2.2.2.2. Physicochemical PropertiesProperties 250 DichotomicDichotomic (lipo/hydro-philic) (lipo/hydro-philic) nature nature of of sulfolanesulfolane,, directly related to its chemical chemical structure structure 251 wherewhere the five five-membered-membered ring ring allows allows the the sul sulfonylfonyl moiety moiety to be to exposed be exposed at the at interface the interface (see Figure (see 252 Figure1a), makes1a), makes it broadly it broadly used used industrially industrially,, due due to toits its advantageous advantageous combination combination of physicochemicalphysicochemical 253 propertiesproperties[ 13[13].]. In In fact, fact, a strong a strong solvation solvation of cations of cations by the by oxygen the oxygen atoms inatoms thesulfone in the group, sulfone possibly group, 254 increasespossibly increases nucleophilicity nucleophilicity of the corresponding, of the corresponding, less solvated less anions. solvated As anions. a dipolar As aprotic a dipolar substance, aprotic 255 sulfolanesubstance is, sulfolane completely is miscible completely with miscible a wide range with ofa different wide range liquids, of different e.g., water, liquids, acetone, e.g., glycerol, water, 256 tolueneacetone, and glycerol, other toluene aromatic and solvents, other aromatic but it does solvents, not dissolve but it does in alkanes. not dissolve Sulfolane, in alkanes. in its pure Sulfolane form,, 257 isin almostits pure odorless, form, is colorlessalmost odorless and weakly, colorless to moderately and weakly basic; to moderately it forms a globularbasic; it forms molecule a globular with a 3 1/2 258 specificmolecule gravity with a of specific 1.26 g/cm gravityand of a high1.26 g/cm Hildebrand3 and a solubilityhigh Hildebrand parameter solubility (δ = 27.2 parameter [MPa] )(δ= which27.2 259 translates[MPa]1/2) intowhich high translates solvency power into high for reactions solvency containing power polarizable for reactions intermediates containing [20 polarizable]. Basically, 260 sulfolaneintermediates is thermally [20]. Basically, stable, therefore sulfolane it is thethermally solvent stable, of choice therefore for different it is the acid-catalyzed solvent of choice reactions for ◦ 261 atdifferent elevated acid temperatures,-catalyzed reactions but at 220 at C,elevated it decomposes temperatures, slowly but with at the 220°C formation, it decomposes of sulfur dioxideslowly with and 262 athe polymeric formation material of sulf [ur11 ].dioxide With its and high a polymeric stability against material strong [11]. acids With and its high bases, stability sulfolane against can operate strong 263 withinacids and a wide bases range, sulfolane of reaction can operate types/conditions within a wide including range of formation reaction types/ of variousconditions fluoroaromatic including 264 compounds,formation of oxidations, various nitrations, fluoroaromatic rearrangements, compounds, phosphonylations oxidations, nitrations, and condensation rearrangements, reactions, 265 respectively.phosphonylations Some ofand its othercondensation more important reactions, physicochemical respectively. propertiesSome of its are other provided more in importantTable1. 266 physicochemical properties are provided in Table I.

267 Table 1. Chemical and physical properties of sulfolane.

Property Value Units Reference Molecular formula C4H8SO2 - [40]

Sustainability 2018, 10, 3677 8 of 21

Table 1. Chemical and physical properties of sulfolane.

Property Value Units Reference

Molecular formula C4H8SO2 -[40] Molecular weight 120.17 g × cm−1 [41] Freezing point 27 ◦C[27] Melting point 28.5 ◦C[40] Flashpoint 165–178 ◦C[40] Autoignition point 528 ◦C[27] Decomposition (at pressure 99.3 kPa) 283 ◦C[42] Density at 15 ◦C 1.276 g × cm−1 [40] Viscosity at 30 ◦C 0.0987 P [43] Vapour pressure 27.6 ◦C 0.0062 mm Hg [42] 200 ◦C 85.23 mm Hg [44] n-Octanol-water partition coefficient −0.77 log [42] (Kow) Henry’s law constant 4.6 × 10−10 atm × m−3 × mol−1 [42] Solubility in water 25 ◦C 379, miscible g × L−1 [45] 30 ◦C miscible g × L−1 [46] Sorption by aquifer material <1 L × kg−1 [47] Hildebrand solubility parameter 27.2 MPa1/2 [9] PKa 12.9 −logK [48] Heat capacity 30 ◦C 181.5 J × mol−1 × K−1 [49] 80 ◦C 197.8 J × mol−1 × K−1 [49] Dielectric constant 43.3 - [38] Refractive index 30 ◦C 1.47 - [50] Dipole moment (in benzene) 4.69 D [50] −1 Dermal permeability coefficient (Kp) 0.2 mg × hour [9]

Owning to its combination of chemical and/or physical properties, sulfolane has been employed in a variety of new applications as an extraction distillation solvent, polymer solvent, polymer plasticizer, polymerization solvent as well as in the electronic/electrical applications.

2.3. Sulfolane-Induced Corrosion The liquid-liquid extraction of benzene/toluene/xylenes (BTX) from naphtha and gasoline fractions using sulfolane is usually achieved at temperatures in the range of 180–200 ◦C in a typical, extractor-stripper-regenerator configuration (see Figure3). Under standard operating conditions pure sulfolane is considered to be a stable compound (decomposition rate is 0.009%/h) [11] and non-aggressive towards carbon/stainless steel, but at temperatures about 200 ◦C process of sulfolane decomposition spikes, followed by generation of SO2 and formation of corrosive H2SO3 according to the generic Equation (2).

C4H8O2S + O2 + H2O → H2SO3 + C3H7CO2H (2) As a matter of fact, sulfolane should be stored under a nitrogen blanket and out of contact with oxygen and strong oxidizing agents such as chlorates, nitrates and peroxides, that can cause degradation. The presence of impurities such as oxygen and/or chlorides or water can accelerate sulfolane ‘breakdown’ process as well. In fact, some general correlations between oxygen/chloride and water concentrations are described, leading to enhanced sulfolane corrosivity, that causes noticeable issues of the refining installation as illustrated schematically in Figure3. Moreover, oxygen-degraded sulfolane has a lower pH with lower extractive power, a higher acid number and a darker color compared to pure sulfolane as shown in Figure4. Sustainability 2018, 10, x FOR PEER REVIEW 9 of 22

C4H8O2S + O2 + H2O → H2SO3 + C3H7CO2H (2) 279 As a matter of fact, sulfolane should be stored under a nitrogen blanket and out of contact with 280 oxygen and strong oxidizing agents such as chlorates, nitrates and peroxides, that can cause 281 degradation. The presence of impurities such as oxygen and/or chlorides or water can accelerate 282 sulfolane ‘breakdown’ process as well. In fact, some general correlations between oxygen/chloride 283 and water concentrations are d 284 Sustainability 2018, 10, 3677 9 of 21 285 286

287 Figure 4. Samples of sulfolane, from the left:left: pure sulfolane, sulfolane after the experiment at 95 ◦°CC 288 (details inin thethe experimentalexperimental section)section) withwith thethe additionaddition ofof 2%,2%, 4%4% andand 6%6% vol.vol. water,water, respectively.respectively.

289 Chlorides react react with with organic organic acidic acid moleculesic molecules and possibly and possibly sulfolane sulfo itselflan toe create itself precipitates. to create 290 Unfortunately,precipitates. Unfortunately, there is a ‘gap’ there in theis a detailed‘gap’ in the and detailed quantitative and quantitative evaluation ofevaluation individual of impurities,individual 291 theirimpurities, limits and their interactions limits and on interactions corrosion rate on corrosion of carbon rate and alloyedof carbon steels and [ 25 alloyed]. Electrochemical steels [25]. 292 corrosionElectrochemical monitoring corrosion techniques monitoring such as techniques Linear Polarization such as Resistance Linear Polarization (LPR), Electrochemical Resistance (LPR), Noise 293 (ECN)Electrochemical or Harmonic Noise Analysis (ECN) or are Harmonic broadly Analysis recognized are broadly for their recognized responsiveness for their and responsiveness accuracy for 294 specificationand accuracy offor instantaneous specification corrosionof instantaneous rate; however, corrosion application rate; however, field hasapplication been basically field has limited been 295 bybasically the necessity limited for by a the conductive/aqueous necessity for a conductive/ process environment.aqueous process This limitation environment. stems This from limitation the fact, 296 thatstems electrochemical from the fact, systemsthat electrochemical cannot provide systems reliable cannot readings provide in media reliable at conductance readings in levels media less at 297 thanconductance 10µS/cm, levels due less to significant than 10μS/cm impact, due of to solution significan resistancet impact on of measuredsolution resistance values of on Polarization measured 298 Resistancevalues of (PR). Polarization To overcome Resistance this limitation (PR). T ao noticeable overcome progress this limitation in multi-technique a noticeable electrochemical progress in 299 systems,multi-technique involving electrochemical low frequency systems impedance, involving and harmonic low frequency distortion analysis, impedance has andbeen harmonic achieved. 300 Corrosiondistortion analysis control and, has monitoring been achieved in sulfolane. Corrosion units control does and not monitoring find prominent in sulfolane presence units in published does not 301 literaturefind prominent and what presence little is in available published focuses literature on indirect, and what qualitative little assessments, is available focuses e.g., color on of indirect, process 302 sulfolane,qualitative acid assessments, number or e.g., pH, color rather of than process direct sulfolane, measurement acid ofnumber corrosion or pH, rates. rather Evidently, than direct those 303 qualitativemeasurement approaches of corrosion provide rates. an Evidently, approximately those qualitative cumulative approaches picture of sulfolaneprovide an physicochemical approximately 304 quality,cumulativ thate does picture not necessarily of sulfolane assist physico in quantifyingchemical propensity quality, that for sulfolanedoes not corrosion necessarily in real assist systems. in 305 quantifyingElectrochemical propensity techniques for sulfolane such corrosion as Linear in Polarization real systems. Resistance (LPR)—capable for corrosion 306 measurementElectrochemical in terms techniques of minutes/hours such as may Linear not Polarization work efficiently Resistance in a very (LPR) low-conductive - capable for (3–10 corrosionµS/cm) 307 environmentmeasurement of insulfolane-hydrocarbon terms of minutes/hours mixture may due to not high work IR-drop efficiently between electrodes. in a very Other low-conductive techniques, 308 including(3-10μS/cm) electrochemical environment impedance of sulfolane spectroscopy-hydrocarbon (EIS) mixture or Potentiodynamic/Galvanodynamic due to high IR-drop between 309 Polarization,electrodes. Other are mostly techniques suited, for including laboratory electrochemical application and cannot impedance be utilized spectroscopy for field operation. (EIS) or 310 MorePotentiodynamic/Galvanodynamic functional space can be given Polarization to systems, incorporatingare mostly suited multi-electrochemical for laboratory application approach and by 311 combiningcannot be utilized low frequency for field operation. impedance More measurements functional space (LFI), can harmonic be given distortion to systems analysis incorporating (HDA) 312 andmulti electrochemical-electrochemical noise approach (ECN), by that combining may reduce, low to frequency a reasonable impedance degree, the measurements impact of IR-drop. (LFI), 313 Withharmonic proper distortion adjustment analysis of electrode (HDA) surface and electrochemical area and maintenance noise (ECN) of, close that proximity may reduce, between to a 314 electrodes,reasonable it degree, is possible the impact to accurately of IR-drop. measure With corrosion proper rates, adjustment even at of very electrode low conductance surface area levels and 315 (5maintenanceµS/cm or less) of close [25]. Theproximity mechanism between for detectingelectrodes, the it result is possible of corrosion to accurately is to monitor measure the equipment corrosion 316 inrates contact, even with at very the solventlow conductance as described levels in the (5 experimentalμS/cm or less) (case [25]. study) The mechanism section. for detecting the 317 result of corrosion is to monitor the equipment in contact with the solvent as described in the 318 2.4.experimental Biotransformation (case study) and Biodegradation section. There is no routine environmental monitoring protocols and sufficient data available, that 319 investigate2.4. Biotransformation the environmental and Biodegradation fate including adsorption, (bio)transformation and excretion of sulfolane. In fact, sulfolane has been detected in contaminated groundwater samples and identified in the edible plant tissues, taken from nearby gardens surrounding industrial installations [12]. Unfortunately, very little information is accessible on the toxicity and possible irritancy of sulfolane. In general, this substance appeared to be completely free from skin irritating due to low skin penetration (skin permeability of only 0.2 mg/(m2/h)) and sensitizing properties with only very mild irritancy to the eyes [11]. Although the human health impact is unknown, toxicity effects were examined extensively in rats exposed to sulfolane [50], e.g., influence of sulfolane on behavioral and autonomic thermoregulation was studied in rats (and also in mice) [51]. Aerosolized sulfolane is known to be acutely toxic at higher doses (over 200 mg/kg) with convulsions, vomiting, leukopenia and Sustainability 2018, 10, 3677 10 of 21 death in exposed guinea pigs, squirrel monkeys and dogs reported - none of these toxic effects were observed at exposures to lower concentrations (20 mg×m-3)[52]. Moreover, sulfolane produces toxic signs indicative of central nervous system (CNS) stimulation or dose-dependent depression at acute concentrations in mammals [53,54]. Available values of lethal dose, at which 50% percentage of subjects will die (LD50) for various animals species are summarized in Table2.

Table 2. LD50 values for various species and route of administration.

Property Species Administration Route Dosage Units References Rats 1.7–2.7 g × kg−1 (body weight) [50] Mice oral 1.9–2.5 no study [55] Human no study LD Mice, rats and guinea subcutaneously 50 0.6–1.5 g × kg−1 (body) [50] pigs injected intraperitoneal Rats 1.6 g × kg−1 [56] injection

The profound analysis of sulfolane toxicity is not the primary objective of this paper, but additional hazard data are presented in Supplementary Materials (Table S1). In the last decades, the indirect impact of air/water/soil contamination on human health and the surrounding environment has been extensively scrutinized and is better understood now [3,57]. Traditionally, the industrial waste/sludge have been treated by physicochemical methods, which are costly, use lots of chemicals and generate waste products. On the other hand, the biopurification of the contaminated air/water/soil is gaining particular attention due to high cost-effectiveness (low investment and operating expenses), treatment efficiency and environmental acceptability. Microbiological methods are becoming attractive alternatives to the conventional physicochemical removal/cleaning procedures as they are based on the natural ability of microorganisms to degrade odorous/toxic contaminants from industrial/municipal airstreams and operate under mild conditions (at around ambient temperature and atmospheric pressure) [1,2]. In other words, organic contaminants can be attenuated by microbial degradation; however the rates of volatilization, diffusion and leaching are basically dependent on the sorption mechanisms (sorbate-sorbent interactions) of organic pollutants onto soil constituents [22]. In fact, these microbial ‘predators’ use the pollutants as nutritional source to draw energy and carbon for growth (biomass production) and to maintain biological activity under aerobic or anaerobic conditions, whereas other nutrients (minerals and trace elements) need to be supplied. In a broader sense, a single/mixed microbial population or consortium might be regarded as the biocatalysts in the degradation process, which continually evolve to meet the changes in pollutant feed and environmental conditions transforming pollutants into biomass and harmless (by-)products [7]. Since natural processes are not effective in sulfolane removal, several treatment methods have been suggested, including adsorption on biologically activated carbon [58] and aerobic or anaerobic microcosm procedures to assess the biological and abiotic sulfolane degradation potential in the contaminated ground water and sediment. The mixed population of microbes was used to bioremediate the sulfolane-containing wastewater from refinery under aerobic conditions [59]. The biodegradation potential of sulfolane by planktonic microbes residing in groundwater alone was examined as well [51]. Several important factors limiting sulfolane biodegradation in the contaminated subarctic aquifer substrate were scrutinized extensively, including the impact of the nutrient limitations (nitrate, sulphate or iron-reduced conditions) as well as the hydrocarbon co-contamination on the sulfolane-contaminated samples under the oxic and anoxic conditions, respectively. Obviously, oxygen was specified as an important biodegradation stimulator, since anaerobic conditions do not appear to support sulfolane bioremediation. Basically, in situ nutrient amendments (mineral or organic) and hydrocarbon co-contamination are likely to naturally control the biodegradation rate, especially at the bacterial incubation stage. In order to augment the biodegradation of sulfolane in soil, the addition of nitrogen and, to lesser extent phosphate, is valid as was suggested by Greene et al. [56–62]. Sustainability 2018, 10, 3677 11 of 21

Since there have been some studies reporting on the biodegradation of sulfolane in water, there is some knowledge ‘gap’ that hampers the proper understanding and assessment of the advanced oxidative processed (AOPs), employed in the treatment of sulfolane in an aqueous medium [12]. Almost four decades ago, the vacuum UV photolysis of sulfolane was proposed with SO2, cyclobutane, 1-butene and ethylene as the reaction products [63]; however preliminary experiments on sulfolane degradation using H2O2/UV in the contaminated groundwaters were reported in 2005 [53]. The application of UV-based AOPs to treat sulfolane in an aqueous medium (synthetic water and ‘real’ sulfolane-containing groundwater) with suitable photoactive oxidants (H2O2/TiO2/O3) was reported recently, revealing a ‘synergetic effect’ observed in the combined (H2O2/O3) photolysis [12]. Moreover, the effective method for mineralization of sulfolane-contaminated (ground)water by calcium peroxide (CaO2) and calcium oxide (CaO) conjugated with ozone was proposed recently as well [23]. Hence, the laboratory-scale experimental program was prepared to test the sulfolane bioremediation rate and specify the suitable working conditions [54]. It was shown, that combination of high pH and CaO/CaO2 system seems to enhance the sulfolane mineralization effectiveness. In fact, Fenton’s catalyst with the mixture of nitrilotriacetic acid (NTA), Fe(III) and H2O2 was also successfully employed for sulfolane degradation in groundwater [26]. Removal of sulfur-based molecules from waste gases or groundwater is necessary for reasons of health, safety and also anti-corrosion protection due to corrosive potential of these compounds to mild steel or concrete during emission. Luckily, most odorous inorganic compounds are readily degraded in the desulfurization process by sulfur-oxidizing bacterial populations or microbial consortia [64].

3. Materials and Methods

Design and Operational Configuration of Laboratory Scale Aparature The following corrosion study were performed by using multi-technique electrochemical monitoring system SmartCET® by Honeywell (Morris Plains, NJ, USA), that is based on a combination of Low Frequency Impedance (LFI), Harmonic Distortion Analysis (HDA) and Electrochemical Noise (ECN). The industrial, wired transmitter, model CET5500 was used in this study. It is a remotely controlled transmitter designed to conduct corrosion measurements and deliver results through combined analogue-digital communication utilizing 4–20 mA and HART (Highway Addressable Remote Transducer) system. The HART protocol enables two-way communication and transfer of information to and from the field device. The corrosion parameters were measured at 30 s cycles, recorded at 60 s intervals (average) and displayed on the computer in the form of real time graphs. Chemically pure (≥99%; ≤0.2% H2O) sulfolane was used as test fluid. Flat-coupon style electrodes made of stainless steel AISI 304L (dimensions 89 × 20 × 2 mm) were used. For the purpose of the proposed investigations a dedicated testing vessel was designed and constructed. Electrodes were supported on a special, grooved glass frame to ensure their stability during the experiment and maintain contact with the solvent. Electrodes were separated by additional glass spacers. The electrode’s mounting rack is shown in Figure5. Sustainability 2018, 10, x FOR PEER REVIEW 12 of 22 Sustainability 2018, 10, x FOR PEER REVIEW 12 of 22 408 Chemically pure (≥ 99%; ≤ 0.2% H2O) sulfolane was used as test fluid. Flat-coupon style

408409 electrodesChemically made pureof stainless (≥ 99%; steel ≤ 0.2% AISI H 304L2O) sulfolane(dimensions was 89used × 2 0as × test2mm) fluid. were Flat-coupon used. For style the 409410 electrodespurpose of made the proposed of stainless investigations steel AISI a304L dedicated (dimensions testing 89 vessel × 20 was× 2mm) designed were and used. constructed. For the 410411 purposeSustainabilityElectrodes of 2018 were the, 10 proposed , supported 3677 investigations on a special, a gr dedicated testing vessel was designed and constructed.12 of 21 411412 Electrodes were supported on a special, gr 412413 413

414 Figure 5. Glass fram framee with coupon-ty coupon-typepe electrodes. 414 Figure 5. Glass frame with coupon-type electrodes. 415 Electrodes were immersed in a round bottom, thick-walled reactor vessel of 500 cm 33 capacity,capacity, 415416 madeElectrodes of heat-resistant were immersed glass. Test in vessel a round was bottom, also equipped thick-walled with mercury reactor vessel thermometer of 500 cm and 3 capacity, a reflux reflux 416417 madecondenser of heat-resistant with a moisture glass. adsorber Test vessel filledfilled was withwith also CaClCaCl equipped22. The The withoverall overall mercury corrosion-test thermometer setup setup andis is shown a reflux i inn 417418 condenserFigure 66.. ToTo with limitlimit a moisture thethe contactcontact adsorber ofof liquidliquid filled withwith with air,air, CaCl specialspecial2. The silicone-typesilicone-type overall corrosion-test greasegrease waswas setup usedused onison shown allall glassglass in 418419 Figureconnections. 6. To limitFor For additional additional the contact protection protection of liquid against againstwith air, air air special ingress, ingress, silicone-type each each test was grease conducted was used under on inertall glass gas 419420 connections.(99.995% Argon) Argon) For blanket.additional blanket. The protection The test test vessel vesselagainst was placed was air ingress, in a heating each test bowl was and conducted heated to under the appropriate inert gas 420421 (99.995%temperature. Argon) During blanket. experiment The test(ca. 96 vessel h) sulfolane was solution was continuously stirred (ca. 1000 rmp, 421422 1 cm magnetic stirrer) to simulate flowing conditions. 422

423 Figure 6. The complete laboratory setup for sulfolane corrosivity tests. 423 FigureFigure 6 6.. TheThe complete complete laboratory laboratory setup setup for for sulfolane sulfolane corrosivity tests tests.. 424 The corrosion parameters such as general corrosion rate (PV – primary variable) expressed in 424425 mils Theper yearcorrosion (mpy), parameters localized corrosionsuch such as as general general potential corrosion corrosion (dimensionless rate rate (PV (PV—primary Pitting – primary Factor), variable) actual expressed Stern-Geary in 425426 milscoefficient per year – B (mpy), value (mV)localized and corrosion a capacitance po potentialtential-related (dimensionless Corrosion Mechanism Pitting Factor), Indicator actual (dimensionless Stern Stern-Geary-Geary 426427 coefficientcoefficient—BCMI) were – recorded B value (mV) . Detailed and a description capacitance capacitance-related of-related measured Corrosion parameters Mechanism is given Indicator elsewhere ( (dimensionlessdimensionless (Table S2 in 427428 CMI)Supplementary were recorded recorded. Materials). Detailed Detailed. Scheme description description 2 shows of of genericmeasured measured experimental parameters parameters approach, is is given given elsewhere elsewherekey-process (Table (Table variables S2 in 428429 SupSupplementaryandplementary their limits. Materials).Materials). SchemeScheme2 shows2 shows generic generic experimental experimental approach, approach, key-process key-process variables variables and 429 andtheir their limits. limits. Sustainability 2018, 10, 3677 13 of 21 Sustainability 2018, 10, x FOR PEER REVIEW 13 of 22 Sustainability 2018, 10, x FOR PEER REVIEW 13 of 22

430 Scheme 2.2. ExperimentalExperimental design diagram.diagram. 430 Scheme 2. Experimental design diagram. 4. Results and Discussion 431431 44. .Results Results and and Discussion Discussion The data collected by the SmartCET 5500 HART Corrosion Transmitter was processed by a 432 The data collected by the SmartCET 5500 HART Corrosion Transmitter was processed by a 432 computerThe data program collected and presentedby the SmartCET in numerical 5500 form HART in Corrosiona text file. In Transmitter order to visualize was processed and interpret by a 433 computer program and presented in numerical form in a text file. In order to visualize and interpret 433 computerthe results program obtained and from presented the experiments, in numerical two form software in a text scripts file. were In order implemented to visualize in and the MATLABinterpret 434 the results obtained from the experiments, two software scripts were implemented in the MATLAB 434 theenvironment results obtained for downloading from the experiments, and analyzing two data software from a scripts text file. were As aimplemented data pre-processing in the MATLAB technique 435 environment for downloading and analyzing data from a text file. As a data pre-processing 435 environmdata binningent forwas downloading used. Data binning and analy or bucketingzing datais from a method a text in which file. the As original a data [continuous] pre-processing data 436 technique data binning was used. Data binning or bucketing is a method in which the original 436 techniquevalues which data fall into binning a given was small used. interval, Data a bin, binning is replaced or bucketing by a value representativeis a method in of that which interval the original[65,66]. 437 [continuous] data values which fall into a given small interval, a bin, is replaced by a value representative of 437 [continuous]This mathematical data values operation which wasfall into carried a given out small in order interval, to show a bin, more is repla accuratelyced by a the value trends representative of measured of 438 that interval [65,66]. This mathematical operation was carried out in order to show more accurately 438 thatvalues. interval The [ average65,66]. This value mathematical was calculated operation from each was interval. carried An out overview in order of to the show data more binning accurately process 439 the trends of measured values. The average value was calculated from each interval. An overview 439 theis presented trends of belowmeasured (Figure values.7). The average value was calculated from each interval. An overview 440440 ofof the the datadata binningbinning proc processess isis presented presented below below (Figure (Figure 7 7).).

Figure 7. Data binning process. 441 Figure 7. Data binning process. 441 Figure 7. Data binning process. 4.1. Temperature 442442 4.1. Temperature 4.1. TemperatureTemperature is one of the most important parameters impacting sulfolane corrosivity. 443 Temperature is one of the most important parameters impacting sulfolane corrosivity. 443 DecompositionTemperature of sulfolaneis one of with the releasing most important of SO2 and parameters subsequent impacting formation sulfolane of H2SO 3 corrosivity.accelerates 444 Decomposition of sulfolane with ◦releasing of SO2 and subsequent formation of H2SO3 accelerates at 444 Decompositionat temperatures of abovesulfolane 180–200 with releasingC, which of couldSO2 and lead subsequent to higher formation corrosivity. of H Figure2SO3 accelerates8a–d shows at 445445 temperaturesimpacttemperatures of temperatures aboveabove 180180--200ºC,200ºC, on the whichwhich corrosion couldcould processes leadlead toto higherhigher on AISI corrosivity.corrosivity. 304L stainless FigureFigure steel 88aa--dd in showsshows pure impact sulfolane.impact ofof 446446 temperaturesGeneraltemperatures corrosion on on the the rate corrosion corrosion increases processes processes with temperature, on on AISI AISI 304L304L but stainless stainless even at steel230 steel ◦ inC in the pure pure maximum sulfolane. sulfolane. corrosion General General 447447 corrosionratecorrosion doesn’t raterate exceed increasesincreases 1 mpy withwith as temperature,temperature, illustrated in butbut Figure eveneven8a. atat Hence, 230ºC230ºC the itthe is maximummaximum evident that corrosioncorrosion 304L stainless raterate doesn’tdoesn’t steel 448448 exceedisexceed mostly 1mpy 1mpy immune asas illustrated illustrated in pure sulfolane in in Figure Figure solution 8 8aa. . Hence, Hence, irrespectively it it is is evident evident to that tested that 304L 304L temperature stainless stainless steel (up steel to is is 230 mostly mostly◦C). 449449 immuneTheimmune Pitting inin pure Factorpure sulfolanesulfolane parameter solutionsolution which irrespectively isirrespectively used for assessment toto testedtested of temperaturetemperature propensity (up to(up localized toto 230ºC).230ºC). corrosion TheThe PittingPitting was 450450 FactorbelowFactor par 0.15,parameterameter that indicates whichwhich isis dominationusedused forfor assessmentassessment of uniform ofof propensity (general)propensity corrosion toto localizedlocalized processes corrosioncorrosion (Figure waswas 8 belowbelowb). Pitting 0.150.15, , 451451 thatFactorthat indicatesindicates above 0.2–0.3 dominationdomination is usually ofof uniformuniform taken as (genera(genera localizedl)l) corrosioncorrosion corrosion processesprocesses dominant (Figure(Figure regime. 88bb The).). PittingPitting B value FactorFactor (Figure aboveabove8c) 452452 0.2is0.2- the-0.30.3 ismostis usuallyusually stable takentaken at 95 asas ◦ localizedlocalizedC. For higher corrosioncorrosion temperatures, dominantdominant regimeregime this parameter. . TThehe BB valuevalue exhibits (Figure(Figure some 88cc)) fluctuations isis thethe mostmost 453453 stablestable at at 95 95°C.°C. For For higher higher temperatures, temperatures, this this parameter parameter exhibits exhibits some some fluctuations fluctuations (the (the processes processes 454454 occurringoccurring on on the the electrodes electrodes show show high high intensity). intensity). The The high high values values of of B B (<50mV) (<50mV) suggest suggest Sustainability 2018, 10, 3677 14 of 21

(theSustainability processes 2018 occurring, 10, x FOR onPEER the REVIEW electrodes show high intensity). The high values of B (<5014 mV) of 22 suggest cathodic-reaction limiting process (limited access of oxygen for cathodic process on electrode). 455 Figurecathodic8d shows-reaction changes limiting of CMI-parameter, process (limited which access is related of oxygen to surface for cathodic capacitance. process During on all electrode). tests, 456 theFigure CMI was8d shows below of changes 60 units, of which CMI-parameter is typical, ofwhich passive is related materials to like surface austenitic capacitance. stainless During steels. all 457 Lowtests, values the CMI of CMI was (<100) below suggest,of 60 units, that which no additional is typical deposition of passive occurs materials on electrodes,like austenitic e.g., stainless from 458 carbonicsteels. productsLow values of sulfolaneof CMI (<100) decomposition. suggest, that no additional deposition occurs on electrodes, e.g., 459 from carbonic products of sulfolane decomposition.

460 Figure 8. a b c 461 Figure General8. General corrosion corrosion rate rate ( ), (a), localized localized corrosion corrosion potential potentia ( l), (b), dynamic dynamic B-value B-value ( ), (c), corrosion corrosion mechanism indicator (d) measured in sulfolane at temperature 95 ◦C, 190 ◦C and 230 ◦C. 462 mechanism indicator (d) measured in sulfolane at temperature 95°C, 190°C and 230°C. Figure9 shows exemplary electrodes before and after the experiment carried out at 230 ◦C 463 Figure 9 shows exemplary electrodes before and after the experiment carried out at 230ºC (Figure9a,b). Pictures of the electrode surface (after cleaning) taken by a microscope are shown in 464 (Figure 9a, b). Pictures of the electrode surface (after cleaning) taken by a microscope are shown in Figure9c,d as well. 465 Figure 9c, d as well. Sustainability 2018, 10, 3677 15 of 21 Sustainability 2018, 10, x FOR PEER REVIEW 15 of 22

Figure 9. Electrodes before (a) and after (b) the experiment in pure sulfolane at 230 ◦C (after etching). 466 FigureElectrode 9. Electrodes surface before before (c) and (a) and after after (d) experiments (b) the experiment under a microscope in pure sulfolane (×500). at 230 °C (after 467 etching). Electrode surface before (c) and after (d) experiments under a microscope (x500). The surface of electrodes does not show any significant signs of corrosion. No localized 468 phenomenonThe surface (pitting) of electrodes was observed. does not It is show in line any with significant earlier observation signs of of corrosion. the Pitting No Factor localized values 469 phenomenon(<0.15), that (pitting) indicated was general observed. corrosion It is mode.in line with earlier observation of the Pitting Factor values 470 (<0.15), that indicated general corrosion mode. 4.2. Water 471 4.2. Water Pure sulfolane (analytical grade) used for experiments contained less than 0.2% vol. of water. 472 It isPure expected, sulfolane that (analytical under tested grade) conditions used for experiments (atmospheric contained pressure) less water than likely 0.2% evaporatedvol. of water. once It 473 istemperature expected, that exceeded under 100 tested◦C. conditions Hence, series (atmospheric of experiments pressure) with increasingwater likely water evaporated contamination once 474 temperaturewere carried exceeded out at the 100ºC. temperature Hence, of series 95 ◦C of to experiments prevent excessive with increasingwater evaporation water contamination (Figure 10a–d). 475 wereAs shown carried in out Figure at the 10 a,temperature general corrosion of 95°C rate to increasesprevent excessive with the amount water evaporatio of water inn sulfolane (Figure 10 solution.a-d). 476 AsIt isshown interesting, in Figure that 10a, presence general of water corrosion at concentration rate increases of about with 6% the vol., amount at 95 of◦C, water generated in sulfolane the level 477 solution.of corrosion It is interesting (ca. 0.15–0.18, that mpy), presence that of was water close at toconcentration the one observed of about at 6%vol 190 ◦C., withat 95ºC, pure generated sulfolane 478 the(ca. level 0.2 mpy). of corrosion It is evident, (c.a. 0.15 that-0.18 watermpy) has, athat strong was impact close to on the sulfolane one observed corrosivity. at 190ºC with pure 479 sulfolaneFor (c.a. all experiments 0.2mpy). It is with evident water-contained, that water sulfolanehas a strong the impact localized on corrosion sulfolane potential, corrosivity. expressed by 480 PittingFor all Factor, experiments was at very with low water level-contained (<0.1) indicating sulfolane domination the localized of generalcorrosion corrosion potential, mechanism expressed as 481 bydepicted Pitting in Factor Figure, 10wasb. Theat very B value low (see level Figure (<<0.1) 10c) is indicating quite stable domination during the experimentof general irrespective corrosion 482 mechanismof water content. as depicted The CMI in Figure parameter 10b. for The experiments B value (see with Figure water 10c) addition is quite did stable not exceed during 25 units the 483 experiment(see Figure irrespective 10d), which of means water that content. no surface The CMI layer parameter was formed for during experiment the measurement.s with water addition 484 did notAn exceed interesting 25 units observation (see Figure was 10d), made which regarding means that sulfolane no surface color layer during was the formed experiments during the with 485 measurement.water-sulfolane. As illustrated in Figure4, the more water in the solution the more black sediment 486 wasAn present. interesting It is interesting,observation because was made such regarding experiments sulfolane were performedcolor during at the about experiments 95 ◦C, that with is far 487 waterbelow-sulfolane sulfolane. As decomposition illustrated in temperatureFigure 4, the (200–230 more water◦C). in Apparently, the solution presence the more of waterblack acceleratessediment 488 wasdecomposition present. It is reactions, interesting even, because at relatively such lowexperiments temperatures, were causing performed more at suspended about 95ºC solids,, thatbut is far also 489 belowincreasing sulfolane the corrosiondecomposition level. temperature (200–230ºC). Apparently, presence of water accelerates 490 decomposition reactions, even at relatively low temperatures, causing more suspended solids, but 491 also increasing the corrosion level. Sustainability 2018, 10, 3677 16 of 21 Sustainability 2018, 10, x FOR PEER REVIEW 16 of 22

492 Figure 10. General corrosion rate (a), localized corrosion potential (b), dynamic B-value (c), corrosion 493 Figure 10. General corrosion rate (a), localized corrosion potential (b), dynamic B-value (c), mechanism indicator (d) measured in sulfolane solutions with the addition of 2%, 4% and 6% vol. 494 corrosion mechanism indicator (d) measured in sulfolane solutions with the addition of 2%, 4% and water at 95 ◦C. 495 6% vol. water at 95°C. 4.3. Oxygen and Chlorides 496 4.3. Oxygen and Chlorides Contamination of sulfolane by oxygen and presence of chlorides are usually recognized as 497 corrosion-leadingContamination factors of in sulfolane high temperature by oxygen sulfolane and presence process of (aromatic chlorides extraction). are usually Hence, recognized several as 498 testscorrosion were carried-leading out factorsto reveal in a quantified high temperature effect of ingress sulfolane (contamination) process (aromatic of oxygen, extraction). chlorides Hence, and 499 waterseveral to pure tests sulfolane. were carried During out oxygen-contamination to reveal a quantified tests, effect the air of ingresswas introduced (contamination) under the of surface oxygen, 500 of thechlorides sulfolane and solution water to at pure quantities sulfolane. of 5 cmDuring3/1st oxygen injection,-contamination 50 cm3/2nd injection,tests, the 500air was cm3 /3rdintroduandced 3 3 501 5000under cm3 /4th,the surface respectively, of the atsulfolane intervals solution of about at 24 quantities h. The impact of 5cm of chlorides/1st injection, was simulated50cm /2nd through injection, 3 3 502 addition500cm of/3rd 50 ppm and 5000cm of Cl− as/4th, NaCl. respectively, To minimize at intervalsexcessive of water about evaporation, 24h. The impact all experiments of chlorides were was 503 conductedsimulated at through temp. 95 addition◦C (Figure of 50ppm 11a–d). of Cl Figure¯ as NaCl. 11a shows To minimize general excessive corrosion water trends evaporation, for various all 504 configurationsexperiments of were contaminants. conducted It at istemp shown,. 95ºC that (Figure ingress 11 ofa– oxygend). Figure does 1 not1a shows increase general the sulfolane corrosion 505 corrosivity.trends for Addition various of configurations chlorides clearly of contaminants. increases the corrosion It is shown rate,, that when ingress compared of oxygen to water does and not 506 water/oxygenincrease the conditions. sulfolane corrosivity. The average Addition Pitting Factor of chlorides value observed clearly duringincreases all experiments,the corrosion was rate below, when 507 0.05compare as illustratedd to water in Figure and water/oxygen 11b. Addition conditions. of oxygen The and average chlorides Pitting does Factor not change value theobserved dominant, during 508 uniformall experiments degradation, was mode. below It 0.05 is interesting, as illustrat becauseed in Figure presence 11b. ofAddition chlorides of oxygen is usually and considered chlorides asdoes 509 promotingnot change generation the dominant, of pitting corrosion—especiallyuniform degradation on mode. steels likeIt is 304L. interesting The B value, because (see Figure presence 11c) of 510 remainschlorides fairly is stableusually throught considered the experiment as promoting regardless generation of oxygen of pitting or chloride corrosion addition, – especially respectively. on steels 511 like 304L. The B value (see Figure 11c) remains fairly stable throught the experiment regardless of 512 oxygen or chloride addition, respectively. Corrosion mechanism indicator is comparable for the Sustainability 2018, 10, 3677 17 of 21

CorrosionSustainability mechanism 2018, 10, x FOR indicator PEER REVIEW is comparable for the experiment with water and water/oxygen.17 of 22 For the experiment with NaCl the values of CMI do not exceed 45 units, so no film layer on the surface 513 wasexperiment created (Figure with water11d). and water/oxygen. For the experiment with NaCl the values of CMI do not 514 exceed 45 units, so no film layer on the surface was created (Figure 11d).

515 Figure 11. General corrosion rate (a), localized corrosion potential (b), dynamic B-value (c), corrosion 516 mechanismFigure 11 indicator. General ( d corrosion) measured rate in sulfolane(a), localized solutions corrosion with thepotential addition (b), of dynamic 6% vol. B of-value water, (c), 517 watercorrosion and oxygen, mechanism and water, indicator oxygen (d) andmeasured NaCl at in 95 sulfolane◦C. solutions with the addition of 6% vol. of 518 water, water and oxygen, and water, oxygen and NaCl at 95°C . 5. Conclusions 519 5. ConclusionsA worldwide rise of atmospheric pollution due to the expansion of industrial/agricultural areas 520 and urbanA worldwide settlements rise is a of peculiar atmospheric ‘landmark’ pollution of the due modern to the civilization. expansion The of industrial/agricultural global ecosystem is 521 confrontedareas and with urban volatile settlements organic compounds is a peculiar (VOCs) ‘landmark’ as well as of inorganic the modern odorous civilization. compounds The (VICs), global 522 thatecosystem pose hazards is confronted to health with of human volatile beings organic and compounds plants vegetation, (VOCs) forming as well a as significant inorganic part odorous of 523 indoors/outdoorscompounds (VICs) pollution., that pose The hazards sulfur-containing to health derivativesof human beings and their and metabolites, plants vegetation regarded, forming as ‘old a 524 devilssignificant of green’ part chemistry, of indo constituteors/outdoors a relevant pollution. class of air/water/soilThe sulfur-containing contaminants derivatives with increasingly and their 525 detrimentalmetabolites, impact regarded on human as ‘old health devils following of green’ long-term chemistry, exposure. constitute In a over-polluted relevant class world of air/water/soil modern 526 chemistrycontaminants plays invariablywith increasingly pivotal role detrimental in development impact sustainable,on human health atom-economical following long and- operationallyterm exposure. 527 simpleIn over protocols-polluted to provideworld modern eco-friendly chemistry (by-)products; plays invariably however pivotal some industrially-engineered role in development sustainable, solvents 528 haveatom become-economical environmentally and operationally unfavorable. simple Pollutants protocols emitted to from provide certain eco solids-friendly or liquids (by originate-)products; 529 fromhowever anthropogenic some industrially activity or- theengineered biogenic emissionssolvents have of certain become reactive environmentally hydrocarbon derivatives unfavorable. 530 formedPollutants as chemical emitted (by-)products from certain of solids natural or transformations, liquids originate respectively. from anthropogenic The increasing activity public or the 531 biogenic emissions of certain reactive hydrocarbon derivatives formed as chemical (by-)products of 532 natural transformations, respectively. The increasing public awareness of necessity for 533 environmental protection is the main driving force for the increasingly stringent regulations Sustainability 2018, 10, 3677 18 of 21 awareness of necessity for environmental protection is the main driving force for the increasingly stringent regulations governing release of hazardous pollutants and reduced sulfur compounds. Hence, environmental legislations are constantly pushing industry for developing/optimizing of cost-effective ‘green’ manufacturing technologies, that impose less burden on the ecosystem An attractive alternative to commonly used industrial extractive liquids is sulfolane (C4H8SO2), a versatile dipolar aprotic solvent. In the present review we mainly focus on the sulfolane synthesis, application of sulfolane as an extractive solvent due to its ‘unique’ physicochemical properties as well as the potential of sulfolane to cause equipment corrosion and subsequent spills. For the purpose of the proposed investigations a dedicated testing vessel was designed and constructed. The potential risk for groundwater contamination, danger for human health and ways of sulfolane biodegradation were reviewed briefly as well. Moreover, the primary goal of the presented case study was to verify applicability of industrial, multi-electrochemical technique for reliable detection of corrosion in low conductive process fluids. Several aspects of corrosion measurement of general and localized corrosion modes were investigated. Incipient attempts to quantify impact of process parameters (temperature) and impurities (oxygen and chlorides) on stainless steel corrosion in pure sulfolane provided reasonably meaningful. A number of experiments to assess the effect of various process parameters on the corrosion behavior of AISI 304L steel in sulfolane were carried out. To summarize the experimental section: • general corrosion rate during all experiments did not exceed of 1 mpy, which is expected behavior of austenitic stainless steel; • clear impact of temperature on corrosion rate was observed with its maximum at 230 ◦C; • general (uniform) degradation mode was dominant as confirmed by both Pitting Factor values (<0.1) and surface microscopic examination; • strong impact of water (even at low temperature) on sulfolane corrosivity was observed—increase of water concentration accelerates sulfolane degradation as indicated by elevated corrosion rate and increase of suspended dark deposits; • no impact of oxygen on general corrosion and localized corrosion potential was revealed; • chlorides generate the most severe corrosive environment (at test temperature of 95 ◦C) in terms of general corrosion, but do not enhance localized corrosion phenomenon; • no significant deposition was observed during all tests—CMI parameter was below 100 units. It should be emphasized, that there is a clear absence of detailed and quantitative evaluation of individual impurities, their limits and interactions on corrosion rate of carbon and alloyed steels; therefore more detailed control of experiments, a wider range of solution conditions, a wider range of temperatures, and with this the corresponding surface analysis of the electrodes must be performed in the future.

Supplementary Materials: The following are available online at http://www.mdpi.com/2071-1050/10/10/3677/ s1, Table S1: Hazard statement for sulfolane. Table S2: Description of recorded corrosion parameters. Author Contributions: A.B., V.K.—conceptualization, investigation, formal analysis, article writing and editing. S.K.—formal analysis and article writing. P.D.—article writing and editing, A.S.—investigation and data analysis. J.J.—article writing and editing. Funding: This research received no external funding. Acknowledgments: We would like to profoundly acknowledge Honeywell Process Solutions for any help. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

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