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Home , Carbonic acid, Gold mining, Hydrogen cyanide

REMEDIATION: CURRENT AND PAST TECHNOLOGIES C.A. Young§ and T.S. Jordan, Department of Metallurgical , Montana Tech, Butte, MT 59701

ABSTRACT Cyanide (CN-) is a toxic species that is found predominantly in industrial effluents generated by metallurgical operations. Cyanide's strong affinity for metals makes it favorable as an agent for metal finishing and treatment and as a lixivant for metal leaching, particularly . These technologies are environmentally sound but require safeguards to prevent accidental spills from contaminating soils as well as surface and ground . Various methods of cyanide remediation by separation and oxidation are therefore reviewed. Reaction mechanisms are given throughout. The methods are compared in regard to their effectiveness in treating various cyanide species: free cyanide, , weak- dissociables and strong-acid dissociables.

KEY WORDS cyanide, metal-cyanide complex, thiocyanate, oxidation, separation

INTRODUCTION ent on the of these heavy metals through their tissues, cyanide is very toxic. Waste waters from industrial operations The mean lethal dose to the human adult is transport many chemicals that have ad- between 50 and 200 mg [2]. U.S. EPA verse effects on the environment. Various standards for drinking and aquatic-biota chemicals leach heavy metals which would waters regarding total cyanide are 200 and otherwise remain immobile. The chemicals 50 ppb, respectively, where total cyanide and heavy metals may be toxic and thus refers to free and metal-complexed cya- cause aquatic and land biota to sicken or nides [3]. According to RCRA, all cyanide species are considered to be acute haz- die. Most waste- processing tech- ardous materials and have therefore been nologies that are currently available or are designated as P-Class hazardous wastes being developed emphasize the removal of when being disposed of. P-Class hazard- the chemicals or heavy metals as cations. ous wastes are the most regulated waste in However, the anions associated with heavy regard to amounts accumulated in a given metal cations can be equally as toxic but year as compared to the other class desig- are largely ignored. In this regard, the nations: F, K and U. remediation of cyanide has been consid- ered paramount [1]. Metal-complexed are classified according to the strength of the metal- Cyanide is a singly-charged anion contain- cyanide bond. Free cyanide refers to the ing unimolar amounts of and nitro- most toxic forms of cyanide: cyanide anion gen atoms triply-bonded together: C≡N- or - and cyanide. Weak-acid disso- CN . It is a strong ligand, capable of com- ciables (WADs) refer to cyanide complexes plexing at low concentrations with virtually with metals such as cadmium, , any heavy metal. Because the health and nickel and . Although thiocyanate survival of plants and animals are depend-

§ Corresponding author.

104 Proceedings of the 10th Annual Conference on Research (SCN-) is a WAD, it is often considered in ferred to as oxidation methods. These its own category. Strong-acid dissociables physical, adsorption, complexation and oxi- (SADs) refer to cyanide-complexes with dation methods are described in the ensu- metals such as cobalt, gold, and . ing discussions. This nonspecific, complexing makes cyanide attractive for (1) extracting metals, Physical methods particularly gold, from ores via various Physical methods for cyanide treatment leaching operations; (2) finishing/treating can be accomplished using dilution, mem- metal products and their surfaces, and (3) branes, electrowinning and hydroly- preventing certain particles from becoming sis/distillation. hydrophobic in the flotation separation of minerals. Cyanide concentrations for Dilution leaching and finishing/treating are several orders of magnitude higher than those en- Dilution is the only treatment method which countered in flotation whereas cyanide so- does not separate or destroy cyanide. This lutions are more voluminous in leaching as method involves combining a toxic cyanide compared in finishing/treating. Over a bil- waste with an effluent that is low in or free lion tons of gold ore are leached each year of cyanide to yield a waste water below with cyanide. Consequently, in order to discharge limits. Consequently, dilution is prevent surface and ground water contami- simple and cheap and is often used as a nation, procedures for the safe and proper stand-alone or back-up method to insure treatment, storage and handling of effluents that discharge limits are satisfied [3]. Dilu- are of primary concern for cyanide leaching tion is usually considered to be unaccept- operations [4, 5]. able since the total amount of cyanide dis- charge is not altered and since naturally- In this study, current and past remediation occurring processes such as adsorption methods for waste and process waters and precipitation can attenuate and thereby containing cyanide are reviewed. Emphasis concentrate the cyanide in ground and is placed on the oxidation methods which surface waters. actually destroy the cyanide. Membranes CYANIDE REMEDIATION Cyanide can be separated from water us- Various procedures exist for treating cya- ing membranes with either electrodialysis nide and are comprised of several physical, or reverse osmosis. In electrodialysis, a adsorption, complexation and/or oxidation potential is applied across two electrodes methods. These methods predominantly separated by a membrane permeable to involve separation or destruction processes cyanide. The cyanide requiring and may occur naturally. Separation proc- purification is placed in the half-cell contain- esses include the physical, adsorption and ing the cathode or negative electrode. complexation methods that are used to Cyanide, because it is negatively charged, concentrate and thereby recover cyanide will diffuse through the membrane and for recycling. On the other hand, destruc- concentrate in the half-cell containing the tion processes are used to sever the car- anode or positive electrode. In reverse os- bon- thereby destroying mosis, pressure is applied to a cyanide so- the cyanide and producing non-toxic or lution needing treatment but, in this case, less-toxic species. Because the carbon water is forced through a membrane im- and/or nitrogen atoms in the cyanide mole- permeable to cyanide. Both of these meth- cule undergo changes in oxidation state, ods have been shown to be applicable to destruction processes are commonly re-

Proceedings of the 10th Annual Conference on Hazardous Waste Research 105 containing free and metal- Progress is continuing to make electrowin- complexed cyanide [6-11]. ning technology economically viable to di- lute solutions. Direct applications to cya- Electrowinning nide remediation may then be possible. SADs and WADs can be reduced to corre- Hydrolysis/distillation sponding metals by applying a potential across two electrodes immersed in the Free cyanide naturally hydrolyzes in water same solution: to produce aqueous [HCN(aq)]: y-x - o - M(CN)x + ye → M + xCN (1) CN- + H+ → HCN(aq) (4) Thiocyanate does not respond. Free cya- nide is liberated which makes solutions The aqueous hydrogen cyanide can then more amenable to other recovery and volatilize as hydrocyanic gas [HCN(g)]: remediation processes. Because this elec- trowinning reaction involves the reduction HCN(aq) → HCN(g) (5) of an anion at the cathode, a negatively- charged electrode, metal recoveries and Because hydrocyanic gas has a vapor current efficiencies are low. This is com- pressure of 100 kPa at 26ºC, which is pensated for by using steel wools as high- above that of water (34 kPa at 26ºC), and a surface area cathodes, increasing agitation point of 79ºC, which is below that of to further increase mass transport, increas- water (100ºC), cyanide separation can be ing solution temperatures, using appropri- enhanced at elevated temperatures and/or ate solution and conductivities, in- reduced pressures [14-16]. Distillation rates creasing metal concentrations if possible, can also be increased by increasing the and redesigning the electrowinning cell of agitation rate, the air/solution ratio, and the which four designs have been developed surface area at the air/solution interface. for gold processing: Zadra, AARL, NIM Hydrocyanic gas can be captured and con- graphite-chip and MINTEK parallel plate centrated for recycling in conventional ab- cells [3, 12]. Furthermore, Reaction 1 must sorption-scrubbing towers. It can also be be accompanied by a corresponding oxida- vented to the open and has tion reaction, usually evolution, at been noted to occur naturally in the anode with an equivalent number of ponds, especially in warm and arid envi- electrons, e-, being donated: ronments [17]. In such cases, it is para- mount that environmental regulations be - - y/4.{4OH → 2H2O + O2 + 4e } (2a) satisfied. Thiocyanate, WADs and SADs are not affected. + - y/4.{2H2O → O2 + 4H + 4e } (2b) Complexation methods Electrowinning is predominantly used for gold processing; however, it has been used Cyanide treatment can also be done with for cyanide regeneration, in which case it is several complexation methods such as often referred to as the Celec or HSA proc- acidification/volatilization, metal addition, ess [3, 13]. Electrowinning performs well in flotation and solvent extraction. concentrated solutions; at dilute concentra- tions, hydrogen evolution predominates, Acidification/volatilization possibly masking Reaction 1 completely: Because Reaction 4 is in equilibrium at pH 9.3, cyanide predominates above this pH + - → 2H + 2e H2(g) (3) value and aqueous hydrogen cyanide pre- dominates below. Consequently, cyanide

106 Proceedings of the 10th Annual Conference on Hazardous Waste Research will remain in solution at pH values greater Acidification/volatilization (AV) and reneu- than approximately 11, which explains why tralization (R) processes are collectively operations maintain cyanide solutions referred to as the AVR or Mills-Crowe above this pH. On the other hand, cyanide process that was originally developed circa will volatilize at pH values less than ap- 1930 at the Flin Flon operation in proximately 8; the lower the pH, the higher [2, 3, 22]. Different versions were imple- the rate of volatilization. If the pH is lowered mented later at the Real del Monte mine in below pH 2, hydrocyanic gas will also be Mexico, the CANMET facility in Canada, evolved from WADs: and the Golconda CRP venture in Australia [23-25]. A similar technology, the Cyani- y-x + → y+ M(CN)x + xH xHCN(g) + M (6) sorb™ process, has recently been tested at the Golden Cross operation in New Zea- Thiocyanate and SADs react similarly but land and the De Lamar mine in Idaho and solutions must typically be adjusted below found to be more efficient: (1) acidification pH 0 and consequently are only moderately was conducted at pH 6 using better de- affected [18]. Since this would increase signed packed towers, and (2) absorption acid consumption beyond reason, typical was conducted using caustic to prevent acidification/volatilization operations use a fouling from gypsum precipitation [26, 27]. pH between 1.5 and 2. Volatilization rates It is apparent that the AVR and Cyani- can also be increased in the same manner sorb™ processes consume tremendous as distillation. In addition, volatilization has amounts of and bases but are pre- been observed naturally due to the dissolu- ferred over the hydrolysis/distillation proc- tion of and subsequent for- ess due to reduced energy consumption mation of [15, 16, 19]. and increased volatilization rates.

After acidification, the solution is predomi- Metal addition nantly cyanide-free but must be reneutral- ized so it can be discharged or recycled for Cyanide can be rendered unreactive by the further use. Reneutralization causes the addition of various metals and metal metals which were liberated via Reaction 6 cations to promote the formation of metal- to be precipitated as : cyanide complexes or precipitates. For ex- ample, in the Merrill-Crowe process devel- y+ - M + yOH = M(OH)y (s) (7) oped in 1890 for gold recovery, zinc is added to gold cyanide solutions resulting in If lime [Ca(OH)2] is used for reneutralization the precipitation or cementation of the gold and (H2SO4) is used for acidifi- [28]: cation, gypsum (CaSO4-2H2O) will also be - o → o - precipitated: 2Au(CN)2 + Zn 2Au + Zn(CN)4 (9)

2+ 2- In essence, a gold SAD-complex is ex- Ca + SO4 = CaSO4-2H2O (s) (8) changed in solution for a zinc WAD- complex thus making the solution more Sludges of gypsum and metal hydroxides amenable to other remediation methods. are often difficult to separate from water Cementation of gold and other metals in [20], which is a problem that can be mini- cyanide solutions has also been accom- mized by using other acids such as nitric, plished with aluminum, copper and iron [29- HNO [21], or other bases such as caustic, 3 33]. NaOH [22]. Nitric and caustic are more ex- pensive but savings can be realized in In another process first employed in 1909 sludge separation and treatment. 2+ 3+ [34], ferrous (Fe ) or ferric (Fe ) cations

Proceedings of the 10th Annual Conference on Hazardous Waste Research 107 are added to cyanide solutions to form the dition but can be removed via substitution 4- respective SAD-complexes Fe(CN)6 and for cyanide in the products [46]. Clearly, 3- Fe(CN)6 . These ferrohexacyanide and metal addition is usually not a stand-alone ferrihexacyanide anions are considered to method for cyanide treatment. be essentially nontoxic due to their high stability; however, concerns regarding their Flotation photodecomposition (see photolysis meth- Flotation was first implemented in 1880 for ods), and therefore their long term stability, concentrating minerals from ores [47, 48] have been raised [2]. Nevertheless, proc- and has since been adapted for remedia- esses have been developed where these tion. In regards to cyanide remediation, ion anions are precipitated as double salts. For and conventional flotation practices have example, ferrohexacyanide anions precipi- been used to separate SAD-complexes tate as upon addition of ferric and precipitates formed naturally or by cations [35-37]: metal addition. Flotation is usually con-

4- 3+ ducted quickly to prevent these species 3Fe(CN) + 4Fe → Fe [Fe(CN) ] (s) (10) 6 4 6 3 from decomposing and redissolving [18, Prussian blue precipitation is commonly 49-52]. In ion flotation, a heteropolar surfac- observed in process circuits. Ferrohexacy- tant, usually a cationic such as trica- anide anions can also be precipitated as prylmethyl chloride (R4NCl), is added to react with the anionic SAD- Cu2Fe(CN)6, Ni2Fe(CN)6 and Zn2Fe(CN)6 upon addition of cupric, nickel and zinc complex to precipitate as an organic (R) cations, respectively [3, 37, 38]: double :

y-x 4- 2+ (y-x)R4NCl + M(CN)x → Fe(CN)6 + 2M → M2Fe(CN)6(s) (11) - (R4N)y-xM(CN)6(s) + (y-x)Cl (13) Similarly, ferrihexacyanide anions can be Usually, the double salt precipitate will nu- precipitated as Prussian Brown with ferrous cleate to form colloids or larger-sized parti- cations or as Prussian Green with ferric cles. Ion flotation works well for separating cations [39-41]: SADs but only partially for WADs. The fate 3- 2+ of thiocyanate is unknown. By comparison, 2Fe(CN) + 3Fe → Fe [Fe(CN) ] (s) (12a) 6 3 6 2 the same or different surfactant is added in 3- 3+ conventional flotation to adsorb at the pre- Fe(CN)6 + Fe → Fe2(CN)6(s) (12b) cipitate surface. In both cases, hydrophobic In other precipitation processes, cuprous species are created that can be separated and argentous cations have been added in into a froth phase upon injection of air excess to form CuCN and AgCN single bubbles into the system. Various chemicals salts from free cyanide [42-44]. Silver pre- such as frothers and modifiers can be cipitation is used commonly as a titration added to maximize the flotation response. technique for determining free cyanide concentrations [45]. Solvent extraction In solvent extraction (SX), an organic sol- Each of the cemented metals, SAD- vent or diluent is used to solubilize at least complexes and cyanide precipitates formed one of many organic extractants of solvat- by metal addition processes is stable, but it ing, chelating or ion-exchange capabilities. is paramount that the pH be maintained The diluent must be immiscible in and less during and after their formation to prevent dense than water, the aqueous phase, and their decomposition and dissolution. Thio- the extractant must remain in the diluent has limited response to metal ad- and have selectivity for the aqueous spe-

108 Proceedings of the 10th Annual Conference on Hazardous Waste Research cies being remediated. The diluent and ex- the material is separated from the solution tractant are collectively referred to as the by, for example, screening, gravity separa- organic phase. When the organic and tion or flotation. The material is then placed aqueous phases are mixed, the aqueous into another vessel where the cyanide is species is stripped from the aqueous phase desorbed into a low volume solution and by the extractant in reactions similar to Re- thus concentrated. Finally, the material is action 13 above. The species has loaded separated again and, in most cases, reacti- into the organic phase. Various chemicals vated and recycled for further use. termed modifiers can be added to the or- ganic and aqueous phases to maximize the Minerals extraction. Mixing is ceased at the appro- Soils, wastes and ores containing minerals priate time, and the stripped water and such as ilmenite (FeTiO3), hematite loaded organic are allowed to disengage, (Fe O ), bauxite [AlO.OH/Al(OH) ] and py- thus effecting a separation of the species. 2 3 3 rite (FeS2), as well as mineral-groups such The process is repeated to transfer the as feldspars, zeolites and clays have been species from the loaded organic to a sec- shown to effectively adsorb free and metal- ond aqueous phase of lower volume and complexed cyanides [62-66]. Depending on appropriate chemistry. As a result, the con- the mineral, cyanide adsorption is usually a centration of the species is increased and combination of two mechanisms: ion ex- the organic phase is stripped and recycled. change, precipitation or coulombic interac- Recycling is necessary to minimize ex- tion. Cyanide attenuation by such mineral penses due to organic loss and is possible commodities purifies ground and surface because the extraction is reversible. waters but increases cyanide consumption in leaching operations. Solvent extraction technology was originally developed for uranium processing in the Activated carbon mid-1950s [53] and received world-wide attention when the technology was suc- Active or activated are typically cessfully applied to copper processing ten prepared by partial thermochemical de- years later [54, 55]. Applications have since composition of carbonaceous materials— been developed for selectively extracting predominantly wood, peat, and coco- gold cyanide using amine and phosphorous nut shells. Adsorption characteristics solvent extractants [56-58]. Other change with preparation method and mate- applications of solvent extraction to cyanide rial. Because activated carbons have high are the analytical detection procedures porosity and high surface area, adsorption used in colorimetry [59-61] and, conse- capacities and rates are high. Adsorption, quently, of industrial interest for possible however, is not very selective; cations, ani- adaptation to future processing and reme- ons, and neutral species can be adsorbed diation. In this regard, very little is known simultaneously at various sites via ion ex- about the use of solvent extraction for the change, solvation, chelation and coulombic remediation of nearly all cyanide species. interactions. Activated carbons are com- monly used in packed-bed systems for Adsorption methods treating waste waters and gases. Although the use of activated carbons dates back to Minerals, activated carbons and resins ad- ancient and , the first commer- sorb cyanide from solution. Several types cial application was in gas masks circa of contact vessels can be used for this pur- 1915 [67]. Applications to cyanide waste pose and include elutriation columns, agi- waters have been reported with packed- tation cells, packed-bed columns and bed systems and shown to be applicable at loops, etc. Once the cyanide is adsorbed, dilute cyanide concentrations [68, 69] with

Proceedings of the 10th Annual Conference on Hazardous Waste Research 109 increased adsorption of WADs and SADs regenerate more efficiently, have longer with copper or silver pretreatment [18, 70, , and desorb faster. At present, the only 71]. resin being used in for cyanide re- covery is the chelation resin, Vitrokele V912 In gold leaching with cyanide, packed-bed [79]. systems are usually avoided and have given rise to the use of continuous opera- Oxidation methods tions exemplified by carbon-in-column (CIC), carbon-in-leach (CIL) and carbon-in- Except for the dilution process, the physi- pulp (CIP) processes. Although these proc- cal, complexation and adsorption methods esses have been designed for gold-cyanide yield a high concentration product that still adsorption, it does not preclude applying must be treated prior to discharge. The them for the remediation of free or other toxic cyanide species remain. Conse- metal-cyanide complexes as implied by quently, these methods are used predomi- Muir et al. [15]. High adsorption capacities nantly for separation, recovery and recy- and rates as well as strong ability for reacti- cling. In order to destroy the cyanide, sub- vation are important parameters for all acti- sequent oxidation methods are needed. vated carbons but, when used in CIC, CIL Cyanide oxidation can be conducted using and CIP processes, they must also have biological, catalytic, electrolytic, chemical high mechanical strength, high wear resis- and photolytic methods. tance, and a consistent particle size. Bio-oxidation Resins Various species of , fungi, algae, Resins are usually polymeric beads con- yeasts and plants, along with their associ- taining a variety of surface functional ated enzymes and amino acids, are known groups with either chelation or ion- to oxidize cyanide naturally [80-91]. The exchange capabilities, somewhat similar to predominant mechanism of bio-oxidation is the metabolic conversion of cyanide to cy- solvent extraction processes discussed - earlier. They can be selective and have anate, OCN , a species less toxic than high adsorption capacities depending on cyanide: whether the chelating or ion-exchange CN- + ½O (aq) → OCN- (14) properties are strong or weak. Resins that 2 are deposited on substrates as thin films Oxygen is usually considered to be aque- are predominantly used in packed-bed ous but may be present as a gas. Several systems, whereas resins that do not require intermediate species are formed during this a substrate are mostly used in continuous reaction depending on which biomass is processes like those used with activated used, what cyanide species are present, carbons for gold (i.e., RIC and RIP). Gold- and what the solution conditions are regard- blatt [72] developed the first resin column ing cyanide concentration, pH, and tem- for cyanide recovery. Metal-cyanide com- perature. Once the cyanate is produced plexes have since been found to adsorb and released, it will hydrolyze to ammo- more strongly but this adsorption is de- nium and ions: pendent on which resin is being used and - + - - how the solution and/or resin are pretreated OCN + 3H2O → NH4 + HCO3 + OH (15) [73-77]. Thiocyanate appears to adsorb weakly. In a comparison of resins to acti- Reaction 15 is favorable under conditions vated carbons, Fleming and Cromberge less than approximately pH 7 at room tem- [78] showed that resins can be more cost peratures [92, 93]. Ammonium is also con- effective since they resist organic fouling, sidered toxic and must also be treated prior

110 Proceedings of the 10th Annual Conference on Hazardous Waste Research to discharge, usually by nitrification or deni- on ore from the Summitville Mine, a Super- trification processes [94]. fund SITE in Colorado, proved superior to peroxide rinse tests and, as a result, field Of the various biomasses examined, bac- tests are underway at the mine site. Fur- teria are perhaps the best known. Bacterial thermore, the technology is being adapted oxidation of cyanide has been developed for use in fluidized bed reactors for treating for effluent treatment at the Homestake effluents [97]. In other processes, anaero- gold processing plant in Lead, South Da- bic bacteria have been used to oxidize kota [89, 90]. A biofilm composed primarily cyanide in oxygen-starved conditions [98- of paucimobilis bacteria on 100] and could therefore be applied to rotating biological contactors is used to ground waters and stagnant tailings ponds. metabolize cyanide in water from tailings In these cases, Reactions 14, 16 and 17 and underground mines. Unlike most other would occur with water and not oxygen. biomasses, this strain of bacteria can treat metal cyanide complexes including the SAD species: As noted previously, activated carbon is a y-x strong adsorbent for cyanide. However, M(CN)x + 3xH2O + x/2O2(aq) → y+ + - - M + xNH4 + xHCO3 + xOH (16) when adsorption takes place in the pres- ence of oxygen, the activated carbon will Bacterial accumulation of the metal cations catalytically oxidize cyanide to cyanate as (My+) occurs by absorption, adsorption or shown in Reaction 14 [15, 18, 68, 71, 101- precipitation (see Reaction 7). 104]. When copper is present either in so- The metal-loaded bacteria continually lution or preadsorbed on the activated car- slough off the contactors and are removed bon, the rate of catalysis increases, a phe- by clarification. In addition, thiocyanate is nomenon attributed to the increased ad- remediated: sorption of cyanide. If the copper co- catalyst is in excess, the cyanate hydrolysis - SCN + 3H2O + 2O2(aq) → shown as Reaction 15 is enhanced. Gold 2- + - + SO4 + NH4 + HCO3 + H (17) processing plants therefore minimize oxy- gen input into their carbon adsorption cir- Since Reactions 15 and 16 produce hydrox- cuits. Although catalysis by activated car- ides, thiocyanate bio-oxidation can help bon is effective for treating free cyanide buffer the system. Ammonium produced by and WADs as well as some SADs, it has Reactions 15-17 is then treated by bacterial not been developed into a cyanide destruc- nitrification with a biofilm of aerobic, autot- - tion process. The fate of thiocyanate is un- rophic bacteria to produce (NO2 ) known. A similar system using a PbO2 which, in the presence of oxygen, quickly - catalyst has been favorably tested [105]. oxidizes to (NO3 ). Lime is added to precipitate the sulfate and bicarbonate as Electrolysis gypsum and calcite (CaCO3), respectively (see Reaction 8). The water is clarified and Principles of electrowinning are applicable discharged into a stream. to electrolysis and, as a result, operating parameters are basically the same. How- Other bacterial remediation processes are ever, cyanide remediation occurs as an being developed. In one process, alginate- oxidation of free cyanide at the anode encapsulated Pseudomonas putida bacte- rather than a reduction of a metal-cyanide ria is used to treat decommissioned heap complex at the cathode. Furthermore, be- leach pads and other sources of ground cause cyanide and the anode are oppo- water contamination [95, 96]. Column tests sitely charged, current efficiencies are much better. Cyanate is usually produced

Proceedings of the 10th Annual Conference on Hazardous Waste Research 111 using one of two techniques: electro- Natural degradation of cyanide can occur oxidation and electro-chlorination. In elec- via reactions with gaseous and aqueous tro-oxidation, the cyanide reaction occurs oxygen (see Reaction 14). The reaction directly [13, 18, 106-108]: occurs quickly in agitated slurries, slowly in tailings ponds, and is catalyzed in the pres- - - → - - CN + 2OH OCN + H2O + 2e (18) ence of activated carbon and certain aero- bic bacteria as previously noted. It is whereas, in electro-chlorination, the reac- somewhat effective for thiocyanate and tion is the same overall but occurs via a WADs but ineffective for SADs. These re- reaction sequence [18, 109]: actions are often referred to as atmospheric

- - oxidation reactions [2, 3]. 2Cl → Cl2(g) + 2e (19a) Ozonation for cyanide destruction has been - → - CN + Cl2(g) CNCl(aq) + Cl (19b) examined extensively because it is a supe- - - - rior oxidant to oxygen [110-116]. Like oxy- CNCl(aq) + 2OH → OCN + H O + Cl (19c) 2 gen, reacts with cyanide to produce Chlorocyanogen (CNCl), or , is cyanate; however two mechanisms have formed as an intermediate and chloride been proposed: anions are regenerated and therefore serve CN- + O (aq) → OCN- + O (aq) (21a) as a catalyst. The reaction should be car- 3 2 ried out between 40ºC and 50ºC to mini- - → - - 3CN + O3(aq) 3OCN (21b) mize the formation of chlorate (O3Cl ). Un- - der these conditions, hypochlorite (OCl ) is which are referred to as simple and cata- produced [108]: lytic ozonation, respectively. Simple ozona- tion yields oxygen which can further oxidize - → - + - Cl + H2O OCl + 2H + 2e (20) cyanide. On the other hand, catalytic ozonation represents a high reaction effi- which aids in chemical oxidation as dis- ciency and has been observed at high ad- cussed in the next section (see alkaline dition rates, although rarely. Continued chlorination). Hypochlorite is toxic, espe- addition of ozone will convert cyanate to cially to fish, and must be controlled prior to and nitrogen gas: discharge. Since hydroxide anions are con- sumed in both reaction processes, pH con- - 2OCN + 3O3(aq) + H2O → - trol above 11 is necessary to prevent hy- 2HCO3 + N2(g) + 3O2(aq) (22a) drocyanic and chlorocyanic gases from - - volatilizing. Electro-oxidation does not per- 2OCN + O3(aq) + H2O → 2HCO3 + N2(g) (22b) form as well as electro-chlorination. Both are effective against thiocyanate and Because continued ozonation prevents cy- WADs but ineffective against SADs. anate hydrolysis via Reaction 15 and does not oxidize the cyanate to nitrite or nitrate, Chemical addition neither nitrification nor denitrification are needed. Furthermore, ozonation is highly The most popular method for cyanide de- destructive towards thiocyanate and WADs struction is by addition of oxidants. Oxi- but is not destructive towards SADs. In dants have high electron affinity and addition, because hydroxide decomposes therefore strip cyanide of electrons pre- ozone, it becomes less efficient at pH val- dominantly yielding cyanate as a reaction ues greater than approximately 11. This is product. Oxygen, ozone, hydrogen perox- a “Catch 22” situation since cyanide solu- ide, , hypochlorite and diox- tions should be maintained above pH 11 to ide are the most common oxidants. prevent hydrocyanic gas volatilization.

112 Proceedings of the 10th Annual Conference on Hazardous Waste Research - Other disadvantages of ozonation include CN + 2HOCH(aq) + H2O → - its high expense for generating ozone. HOCH2CN(aq) + OH (24a) → In the Degussa process, HOCH2CN(aq) + H2O HOCH CONH (aq) (24b) is used to chemically oxidize cyanide [117]. 2 2 Hydrogen peroxide is an oxidant stronger Cyanate and ammonium ions are also ob- than oxygen but weaker than ozone. It is tained as by-products. (3) When sulfuric often considered better because it is rela- acid and hydrogen peroxide are mixed, a tively cheap, water soluble, and easy to highly exothermic reaction occurs, forming handle and store. The Degussa process persulfuric or Caro's acid, H2SO5. Caro's has been examined in several research acid readily decomposes to oxygen and efforts, especially for comparing to other sulfuric acid and, therefore, must be made processes [118-125]. Results of these on site at the time of use. It reacts with studies show that hydrogen peroxide reacts cyanide to produce cyanate [123, 129]: with cyanide to produce cyanate and, when added in excess, nitrite and carbonate and, - - CN + H2SO5(aq) → OCN + H2SO4(aq) (25) eventually, nitrate form: Similar reactions occur for thiocyanate and - → - CN + H2O2 OCN + H2O (23a) WADs; however, SADs do not react. Test

- work has also shown that hydrocyanic gas OCN + 3H2O2 → - 2- + volatilization is negligible in spite of the fact NO2 + CO3 + 2H2O + 2H (23b) that an acid is being added and produced. - - NO2 + H2O2 → NO3 + H2O (23c) This is attributed to fast reaction rates and the inability of SADs to volatilize since equi- Otherwise, the cyanate will hydrolyze ac- librium pH is established between 6.5 and cording to Reaction 15. The Degussa proc- 8 (see Reaction 6). Reactions similar to ess is successful at oxidizing most WADs Caro's acid were also noted with persulfate, 2- but has little or no effect on thiocyanate S2O8 . and SADs. Even so, it has been incorpo- rated into numerous operations in Alkaline chlorination has been practiced Canada and the U.S. as a primary cyanide ever since cyanide leaching of gold was destruction process as well as a stand-by commercially developed in 1889 and, con- process for emergency situations. sequently, has been the most commonly applied technique for cyanide destruction Other chemicals can be added to increase [18, 28, 109, 130-132]. As will be ex- reaction rates and efficiencies of hydrogen plained, alkaline chlorination removes toxic peroxide: cupric cation, formalde- WAD and free cyanide species completely, hyde/Kastone reagent and sulfuric acid [1]. quickly and economically. However, it suf- Cupric cations act as catalysts for Reac- fers from high reagent costs regarding al- tions 23a-c [2, 3] but can be consumed by kaline pH control and chlorine precipitating ferrohexacyanide anions (see gas/hypochlorite consumption, high dis- Reaction 11) [2]. Cyanide reacts with for- charge concentrations of chloride and hy- maldehyde (HOCH) in the presence of pochlorite anions of which both are toxic, DuPont's proprietary Kastone reagent to and inability to remediate SADs. form glycolnitrile (HOCH2CN) which, in turn, hydrolyses to glycolic acid Alkaline chlorination can be accomplished (HOCH2CONH2) in the presence of hydro- using chlorine gas as discussed earlier for gen peroxide [126-128]: electro-chlorination (see Reactions 19b and 19c):

Proceedings of the 10th Annual Conference on Hazardous Waste Research 113 ------CN + Cl2(g) + 2OH → OCN + H2O + 2Cl (26) SCN + 4OCl + 2OH → - 2- - OCN + SO4 + 4Cl + H2O (31a) Chlorine gas also reacts with thiocyanate y-x - - M(CN)x + xOCl + yOH → and metal-complexed cyanides to produce - - cyanate: xOCN + xCl + M(OH)y (31b)

- - As with chlorine gas reactions, only WADs SCN + 4Cl2(g) + 10OH → - 2- - OCN + SO4 + 8Cl + 5H2O (27a) react in this manner; SADs are inert. Fi- nally, when in excess, hypochlorite can y-x - M(CN)x + xCl2(g) + (2x+y)OH → also react with cyanate to produce nitrogen - - xOCN + 2xCl + M(OH)y + xH2O (27b) and carbon dioxide:

- - Due to the high pH, sulfate and metal hy- 2OCN + 3OCl + H2O → - - droxide precipitates are formed; however, N2(g) + 2CO2(g) + 3Cl + 2OH (32) only WADs react according to Reaction 27b. SADs are inert. When in excess, chlo- Clearly, hydroxide consumption is minimal rine gas can also react with cyanate to pro- and only becomes high when concentra- duce nitrogen and carbon dioxide: tions of thiocyanate and WADs are high, as indicated by Reactions 31a and 31b. To- - - 2OCN + 3Cl2(g) + 4OH → day, hypochlorite is predominantly added - N2(g) + 2CO2(g) + 6Cl + 2H2O (28) as or calcium salts since these metal cations have natural buffering ten- In such a case, chlorine and hydroxide dencies. consumptions become excessive, only in- creasing the difficulty in maintaining alkalin- The INCO process for cyanide destruction ity above pH 10 to avoid the volatilization of involves mixing and air with hydrocyanic and chlorocyanic gases. free and metal-complexed cyanide as well as thiocyanate to yield cyanate [133-137]: To compensate for this high base con- - sumption, the hypochlorite alkaline chlori- CN + SO2(g) + H2O + O2(g) → - nation process was adapted due, in princi- OCN + H2SO4(aq) (33a) ple, to observations that the hypochlorite y-x M(CN)x + xSO2(g) + xH2O + xO2(g) → that was produced by the hydrolysis of - y+ chlorine gas also remediated cyanide: xOCN + xH2SO4(aq) + M (33b)

- - - - SCN + 4 SO2(g) + 5H2O + 4O2(g) → Cl2(g) + 2OH → Cl + OCl + H2O (29) - OCN + 5H2SO4(aq) (33c)

In this regard, hypochlorite reactions are Reactions 33a-c can be catalyzed with cu- similar to those depicted for chlorine gas pric or nickel cations; however, SADs do (see electro-chlorination). For example, hy- not react. Since the reactions generate pochlorite and cyanide react, producing sulfuric acid and are most efficient near pH chlorocyanogen which, in turn, hydrolyses 9, lime is added for pH control. This gen- to cyanate (see Reaction 19c): erates sludge due to metal hydroxide and gypsum precipitation which, as noted ear- CN- + OCl- + H O → CNCl(aq) + 2OH- (30a) 2 lier, can be difficult to clarify (see Reactions - - - 7 and 8). The INCO process has been in- CNCl(aq) + 2OH → OCN + Cl + H2O (30b) corporated into numerous mining opera- Cyanate is also formed via hypochlorite re- tions in Canada and the U.S. since the actions involving thiocyanate and metal- technology was commissioned in 1983. complexed cyanides:

114 Proceedings of the 10th Annual Conference on Hazardous Waste Research Other remediation processes have been light can be used but applications are lim- considered for cyanide oxidation. These ited [141]. Photolytic reactions can be in- include reacting cyanicides, a term used for duced directly if the absorbing compound is species which consume cyanide in leach the species being remediated or indirectly if circuits, with cyanide to produce less-toxic the absorbing compound is available for species. For example, solid polysulfide transferring the photon-energy to the spe- minerals such as elemental sulfur (S8) and cies being remediated. Photolysis can, pyrrhotite (Fe1-xS) and aqueous sulfur-oxy therefore, be conducted in the absence of 2- anions such as polythionate (S2O6 ) and photosensitizers (direct photolysis) or in 2- thiosulfate (S2O3 ) are known to react natu- their presence as aqueous species rally with cyanide to produce thiocyanate (homogeneous photolysis and photocata- [3, 18, 138, 139]. However thiocyanate is lysis) or solid semi-conductors difficult to remediate, as already discussed, (heterogeneous photocatalysis). These and is also a good lixivant for gold [43, photolytic methods are relatively new tech- 140]. Consequently, cyanide remediation nologies and are thus referred to as ad- via thiocyanate formation is usually not vanced oxidation processes. Recently, they considered viable. have been reviewed but with primary em- phasis on oxidation of organic compounds Photolysis [142, 143]. Photolysis can enhance reduction/oxidation Direct photolysis is not applicable to free (redox) reactions by providing energy from cyanides but does occur with some WADs electromagnetic radiation to catalyze elec- and SADs, particularly the ferric and ferrous tron transfer processes. Electromagnetic hexacyanide complexes [124, 125, 144- radiation is absorbed, causing an electron 146]. Although subject to considerable de- in the absorbing compound to pass from bate, the following reaction mechanism the ground state to an excited state, possi- was proposed for ferrihexacyanide: bly separating paired electrons from one another. The electrons become more sus- 3- hυ Fe(CN)6 + H2O → ceptible to the chemical environment and 2- - [Fe(CN)5H2O] + CN (34a) are therefore more apt to participate in re- 2- hυ dox reactions. Photoreduction occurs when [Fe(CN)5H2O] + 2H2O → - + the absorbing compound donates the ex- Fe(OH)3(s) + 5CN + 3H (34b) cited electron to another species. Photo- oxidation occurs when the absorbing com- These reactions are reversible; however, pound accepts an electron from another the ferrihydroxide [Fe(OH)3] can react fur- species in order to fill its electron vacancy. ther with free cyanide and ferrihexacyanide The excited electron will eventually relax to to form Prussian Blue precipitates, similar the ground state of the reaction products. to Reaction 10. Reactions 34a and b are referred to as photo-aquation reactions and Energy needed to promote electrons from have also been observed for cobalthexacy- 3- the ground state to the excited state is anide, Co(CN)6 , the strongest SAD [146, usually equivalent to that possessed by ul- 147]. Furthermore, ferrihexacyanide can be traviolet (UV) radiation. Many redox reac- produced via the direct photolysis of ferro- tions can be catalyzed upon exposure to hexacyanide: artificial sources such as arcs, lamps and 4- hυ 3- - lasers or to natural sunlight, which is free Fe(CN)6 → Fe(CN)6 + e (35) but not necessarily available 24 hours a day. Higher energies such as gamma ra- or the homogeneous photolysis of ferro- diation and lower energies such as visible hexacyanide with oxygen [148]:

Proceedings of the 10th Annual Conference on Hazardous Waste Research 115 4- hυ - . - 4Fe(CN)6 + 2H2O + O2 → OCN + 3OH → HCO3 + ½N2(g) + H2O (38a) 3- - 4Fe(CN)6 + 4OH (36) OCN- + 6OH. → - - + Combinations of Reactions 34a-36 liberate HCO3 + NO2 + H + 2H2O (38b) cyanide at UV-wavelengths less than ap- OCN- + 8OH. → proximately 420 nm, explaining why fish - - + are commonly killed during the day and not HCO3 + NO3 + H + 3H2O (38c) the night as well as why adding ferric and Cyanide destruction by photolytic ozonation ferrous cations to cyanide for detoxification requires 1 mole of ozone for every mole of is highly suspect (see metal addition meth- cyanide (i.e., 1:1) to form cyanate but this ods) in spite of the fact that UV-radiation ratio increases to 5:1 when nitrate is pro- does not significantly penetrate water [2]. duced. Such high reagent consumptions, Clearly, direct photolysis can remediate when equated with high temperatures and WADs and SADs but dangerously liberates high UV intensities, can make photolytic free cyanide during the reaction. ozonation costly. Homogeneous photolysis has also been Free cyanide, thiocyanate, WADs and used in combination with several of the SADs can also be oxidized by the homo- chemical oxidation methods discussed ear- geneous photolysis of hydrogen peroxide lier in order to increase reaction efficien- [122-125, 152, 153]. The photolytic peroxi- cies, thereby minimizing reagent consump- dation reaction also yields hydroxyl radi- tion and making SADs more amenable to cals: destruction. For example, homogeneous photolysis with ozone has been shown to hυ . H2O2 → 2OH (39) remediate free cyanide, WADs and SADs to concentrations below 0.1 ppm from con- and, as a result, Reactions 15, 37b, and centrations ranging between 1 and 100,000 38a, b and c can be observed depending ppm [112, 149-151]. Increases in UV in- on the amount of excess hydrogen perox- tensities, ozone concentrations (i.e., ide that is present. Remediation of free flowrates), and solution temperatures im- cyanide, thiocyanate, WADs and SADs is prove reaction kinetics. Reactions 21a, b near 100%, independent of concentration. and 22 occur but to a much lesser extent Because Reaction 39 occurs quickly, Re- due to the production of hydroxyl radicals . actions 23a, b and c do not occur. Like (OH ) and their subsequent reaction with photolytic ozonation, increasing UV intensi- cyanide: ties, hydrogen peroxide concentrations, and solution temperatures enhance reac- hυ → . H2O + O3(aq) 2OH + O2(aq) (37a) tion rates. Furthermore, reaction stoi- chiometries are 1:1 when cyanate forms CN- + 2OH. → OCN- + H O (37b) 2 and 5:1 when nitrate forms. Nevertheless, Because hydroxyl radicals are devoid of hydrogen peroxide is more attractive due to charge and have a high affinity for elec- its relative inexpense and ease of storage trons, they can quickly strip virtually any and handling. chemical of electrons including thiocyanate, Homogeneous photolysis with ozone, hy- WADs and SADs, thus causing their oxida- drogen peroxide, and other aqueous pho- tion. Resulting cyanate will hydrolyze ac- tosensitizers is disadvantageous because cording to Reaction 15 or will react with the photosensitizers are consumed and continued photolytic ozonation to produce often added in excess to ensure complete bicarbonate and either nitrogen gas (see oxidation occurs. As already noted, photo- Reaction 22a), nitrite or nitrate: sensitizer consumption and costs are

116 Proceedings of the 10th Annual Conference on Hazardous Waste Research + hυ . + therefore high. In addition, the resulting ef- H2O + h → OH + H (41a) fluent must be processed to treat the ex- cess photosensitizer. In regards to ozone OH- + h+ → OH. (41b) and hydrogen peroxide, this can be ac- complished by giving the hydroxyl radicals On the other hand, hydroxyl radicals are time to decompose to water and oxygen in produced in the second mechanism by re- a holding pond; however, such a simple acting dissolved oxygen with excited elec- solution is not always available for other trons in the conduction band through a su- - photosensitizers. These problems can be peroxide (O2 ) intermediate: alleviated using catalytic photosensitizers - → - that can be recycled. Homogeneous photo- O2(aq) + e O2 (42a) catalysis with aqueous photosensitizers - + → . and heterogeneous photocatalysis with 2O2 + 2H 2OH + O2(aq) (42b) solid photosensitizers result. Homogeneous photocatalysis, however, is rarely practiced Both reaction mechanisms are concluded due to the difficulty in separating aqueous with cyanide oxidizing to cyanate, nitrogen species for recycling. On the other hand, gas, nitrite and/or nitrate by the hydroxyl heterogeneous photocatalysis is more vi- radicals (see Reactions 37b and 38a, b, c) able because standard solid-liquid separa- and are also effective for remediating thio- tion processes or fixed-bed systems can be cyanate, WADs and SADs. Early work had used. indicated that bicarbonate and were produced by cyanate hydrolysis ac- The solid photosensitizers used in hetero- cording to Reaction 15; however, ammonia geneous photocatalysis are usually cheap, is readily oxidized in these systems. The nontoxic and inert semiconductors. UV- third mechanism involves reducing cyanide absorbance promotes electrons in the va- by holes in the valence band to produce . lence band across the bandgap and into cyanyl radicals, CN : the conductance band of the semiconduc- - + → . tor which creates a hole, h+, in the valence CN + h CN (43) band and an excited electron, e-, in the Hydroxyl radicals produced according to conductance band: Reactions 41a, b and/or 42a, b then react

υ - + with the cyanyl radicals: semiconductor h → (e h ) (40) CN. + OH. → OCN- + H+ (44) However, the impingent UV radiation must have energy equal to or greater than the Resulting cyanate further reacts with hy- bandgap of the semiconductor. The elec- - + droxyl radicals as depicted earlier in Reac- tron-hole (e h ) pair can then induce redox tions 38a, b and c. reactions provided that the reacting species are adsorbed at the semiconductor surface Spectroscopic evidence supports these and that the reaction products are inert to three reaction mechanisms and is believed the reverse reaction and/or desorb quickly. to be independent of the semiconductor photocatalysts that were tested—TiO2, Heterogeneous photocatalysis has been ZnO, FeTiO3, SiO2, Fe2O3, ZnS and CdS, to examined for cyanide oxidation and three name a few. The metal oxides worked best mechanisms have been proposed [122- because of their chemical stability; how- 125, 152-161]. The first mechanism pro- ever, anatase, a polymorph of TiO2, was duces hydroxyl radicals via the reduction of preferred due to its high quantum efficiency either adsorbed water or adsorbed hydrox- for photoconversion, its stable formation of ide by holes in the valence band:

Proceedings of the 10th Annual Conference on Hazardous Waste Research 117 electron-hole pairs, and its large bandgap hand, the other processes use heteroge- of 3.2 eV. This band gap equates to a neous photocatalysis with TiO2. In these maximum wavelength of approximately systems, the UV-radiation is artificial or so- 387.5 nm which implies that virtually all lar and the TiO2 is either suspended or portions of the UV spectrum can be used. fixed on one of several supports (glass In addition, electron-hole pairs were also beads or meshes); however, the supports made more stable by depositing metal is- can decrease photoconversion efficiencies lands of, for example, and rho- due to reflection and refraction of the inci- dium onto the surfaces of the photocata- dent UV-radiation [169]. Although all six of lysts. these systems were principally designed for organic destruction, several have been Presently, photolysis is used in at least six demonstrated for cyanide oxidation. remediation systems, aside from those employed for disinfection. These include CONCLUSIONS the perox-pure™ process of Vulcan Per- oxidation Systems, Inc. [162], the Rayox® Cyanide remediation can be accomplished Process of Solarchem Environmental Sys- using numerous separation and oxidation tems, Inc. [163], the Ultrox™ Process of processes. Separation processes include Ultrox, Inc. [164], the PhotoReDox™ Sys- physical methods with membranes, elec- tem of ClearFlow, Inc. [165, 166], and the trowinning and hydrolysis/distillation; com- two systems developed at Sandia National plexation methods with acidifica- Laboratories in Albuquerque, NM [167], tion/volatilization, metal addition, flotation and National Renewable Energy Labora- and solvent extraction; and adsorption tory in Golden, CO [168]. The first three methods with various minerals, activated processes use homogeneous photolysis carbons and resins. These methods are with hydrogen peroxide and have also used to purify effluents for discharge by been adapted for use with ozone and other concentrating and recovering the cyanide aqueous photosensitizers. Differences be- for recycling. On the other hand, oxidation tween the systems are in cell design processes are used to destroy the cyanide (horizontal versus vertical) and artificial UV- and include various biological, catalytic, intensity (3 kW versus 60 W). On the other electrolytic, chemical and photolytic meth-

Table 1. Effectiveness of separation processes for cyanide remediation as discussed in this paper as modified and updated from Ingles and Scott [18].

118 Proceedings of the 10th Annual Conference on Hazardous Waste Research ods. Several investigations compared the paper was not subjected to MWTP peer effectiveness of separation and oxidation and administrative review and, therefore, processes and found them to be dependent may not necessarily reflect the views of the on the cyanide species that were present; program. No official endorsement should free cyanide (CN- and HCN), thiocyanate be inferred. (SCN-), weak-acid dissociables (WADs), and strong-acid dissociables (SADs) may REFERENCES be remediated by one process and not an- other. The advanced oxidation processes 1. M. Foote, C. Barry and J. Figueira, Is- associated with photolytic methods work sues Identification and Technology Pri- well for all cyanide species and conse- oritization Report: Cyanide, MWTP Ac- quently are the preferred choice for reme- tivity I, Volume 3, 1993. diation; however, if certain cyanide species are not present, remediation can be ac- 2. J.L. Huiatt et al., Workshop: Cyanide complished by more appropriate alterna- from Mineral Processing, Utah Mining tives. This is clearly illustrated in Tables 1 and Mineral Resources Institute, Salt and 2. Consequently, the proper choice for Lake City, UT, 1983. cyanide remediation must be made on a 3. J. Marsden and I. House, The Chemis- case-by-case basis; then and only then can try of , Ellis Horwood, a real decision be made regarding the need New York, NY, 1992. for further cyanide treatment. 4. R.B. Bhappu, J.H. Fair and J.R. Wright, ACKNOWLEDGMENT Waste Problems Relative to Mining and This work was conducted as part of the Milling of Molybdenum, Proc. 22nd Ind. Mine Waste Technology Program (MWTP) Waste Conf., Lafayette, IN, 1967, pp. under an interagency agreement, IAG ID 575-592. No. DW89935117-01-0, between the U.S. Environmental Protection Agency (EPA) 5. L.E. Towill et al., Reviews of the Envi- and the U.S. Department of Energy (DOE), ronmental Effects of : V. Contract No. DE-AC22-88ID12735. This Cyanides, Interagency Report, Oak

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128 Proceedings of the 10th Annual Conference on Hazardous Waste Research 165.R. Enzweiler, L. Wagg and J. Dong, Requirement, Design and Field Test of State-of-the-Art Industrial Wastewater Recycling System, Proc. Amer. Inst. Chem. Eng., Seattle, WA, 1993.

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Proceedings of the 10th Annual Conference on Hazardous Waste Research 129