Cyclic Sequential Removal of Alizarin Red S Dye and Cr(VI) Ions Using Wool As a Low-Cost Adsorbent

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Article Cyclic Sequential Removal of Alizarin Red S Dye and Cr(VI) Ions Using Wool as a Low-Cost Adsorbent

Mustafa I. Khamis 1, Taleb H. Ibrahim 2,*, Fawwaz H. Jumean 1, Ziad A. Sara 1 and Baraa A. Atallah 1

1 Department of Biology, Chemistry and Environmental Sciences, American University of Sharjah, Sharjah 26666, UAE; [email protected] (M.I.K.); [email protected] (F.H.J.); [email protected] (Z.A.S.); [email protected] (B.A.A.) 2 Department of Chemical Engineering, American University of Sharjah, Sharjah 26666, UAE * Correspondence: [email protected]; Tel.: +971-507769239

Received: 22 April 2020; Accepted: 5 May 2020; Published: 9 May 2020

Abstract: Alizarin red S (ARS) removal from wastewater using sheep wool as adsorbent was investigated. The influence of contact time, pH, adsorbent dosage, initial ARS concentration and temperature was studied. Optimum values were: pH = 2.0, contact time = 90 min, adsorbent dosage = 8.0 g/L. Removal of ARS under these conditions was 93.2%. Adsorption data at 25.0 ◦C and 90 min contact time were fitted to the Freundlich and Langmuir isotherms. R2 values were 0.9943 and 0.9662, respectively. Raising the temperature to 50.0 ◦C had no effect on ARS removal. Free wool and wool loaded with ARS were characterized by Fourier Transform Infrared Spectroscopy (FTIR). ARS loaded wool was used as adsorbent for removal of Cr(VI) from industrial wastewater. ARS adsorbed on wool underwent oxidation, accompanied by a simultaneous reduction of Cr(VI) to Cr(III). The results hold promise for wool as adsorbent of organic pollutants from wastewater, in addition to substantial self-regeneration through reduction of toxic Cr(VI) to Cr(III). Sequential batch reactor studies involving three cycles showed no significant decline in removal efficiencies of both chromium and ARS.

Keywords: Alizarin Red S; wool; adsorption isotherms; dyes; chromium; cyclic process

1. Introduction Large amounts of dye-contaminated wastewaters are released yearly from leather, cosmetics, as well as the pharmaceutical, plastics and textile industries. Most of these dyes are skin irritants, mutagenic or carcinogenic [1,2]. Dye-polluted water decreases photosynthesis since light penetration is inhibited [3]. These dyes have complex aromatic structures, which give them thermal, optical and physicochemical stability, and thus, they could not be easily biodegraded by natural substances [4–6]. Alizarin red S (ARS) (Figure1), or 1,2-dihydroxy-9,10-anthra-quinonesulfonic acid sodium salt, is a water soluble anthraquinone dye originally derived from the root of the madder plant [7]. It has been extensively employed since ancient times in dyeing textiles [8]. It is a strong oxidizing agent, and hence, must be stored away from moisture and heat [9].

Processes 2020, 8, 556; doi:10.3390/pr8050556 www.mdpi.com/journal/processes Processes 2019, 7, x FOR PEER REVIEW 2 of 14

solid phase, thereby decreasing effluent volume. Native mustard husk has been used in ARS removal from aqueous solutions, with thermodynamic studies indicating that the process is spontaneous and endothermic [14]. Pentaerythritol modified multi-walled carbon nanotubes (ox-MWCNT-PER) has been shown to be highly efficient in ARS removal [15]. Removal of ARS from wastewater using alumina as adsorbent has been investigated at optimum conditions [16]. Moreover, activated carbon Processesis a good2020 adsorbent, 8, 556 for removal of ARS from wastewater, as indicated by favorable thermodynamic2 of 15 parameters [17].

Figure 1. AlizarinAlizarin red S (ARS).(ARS).

AHeavy wide metals, range ofincluding physical chromium, and chemical are widely treatment distributed technologies in the haveenvironment been investigated as a result forof removingnumerous dyes industrial from wastewater. applications. They These include include coagulation, electroplating, precipitation, chromate adsorption, manufacture, membrane wood filtration,preservation, electrochemical galvanization, techniques steel industry, and ozonation paint, textile [10]. production, Among these, ox adsorptionidative dyeing, is the cooling most widely water employed,towers, leather due tanning, to its high corrosion efficiency, inhibitors non-toxicity, and batteries readily [18]. available As a result, adsorbents, heavy metals low cost,are foun eased ofin recoverymany industrial and environmental wastewaters sustainability[19–21]. Chromium [11–13 is]. a Applicability metallic element largely listed depends by the on Environmental the cost and eProtectionfficiency of Agency adsorbent. (EPA) Currently, as one of various 129 priority potential pollutants adsorbents [22,23 are]. It used is found for removal in the air, of specificwater and organic soil, compoundsand occurs in from several wastewater. oxidation The states adsorption ranging of from dyes Cr(II) transfers to Cr(VI), them fromwith waterthe trivalent effluent and to solidhexaval phase,ent therebystates being decreasing the most effl uentstable volume. and common Native mustard[1-3]. In husknatural has waters, been used chro inmium ARS removal is present from in aqueous several solutions,forms, the with most thermodynamic common of which studies are indicatingCr(0), Cr(III), that and the processCr(VI). Cr(III) is spontaneous has very andlow endothermic solubility and [14 is]. Pentaerythritolrelatively stable, modified whereas multi-walled Cr(VI) is environmentally carbon nanotubes mobile (ox-MWCNT-PER) and highly toxic. has Exposure been shown to Cr(VI) to be highlycauses e variousfficient in health ARS problemremovals [,15 including]. Removal skin of ARS and fromstomach wastewater irritation, using dermatitis, alumina liver as adsorbent damage, haskidney been circulation investigated and at nerve optimum and conditionstissue damage [16]. [24 Moreover,–26]. Numerous activated technologies carbon is a goodhave adsorbentalready been for removalapplied in of the ARS removal from wastewater, of Cr(VI) from as indicated aqueous bysolutions. favorable These thermodynamic include adsorption, parameters biosorpti [17].on, ion- exchange,Heavy foam metals, flotation, including electrolysis, chromium, surface are widely adsorption distributed precipitation, in the environment reverse osmosis, as a result sand offiltration, numerous chemical industrial reduction/oxidation, applications. These electrochemical include electroplating, precipitation, chromate membrane manufacture, filtration wood and preservation,solvent extraction galvanization, [27–30]. steel industry, paint, textile production, oxidative dyeing, cooling water towers,In previous leather tanning, studies corrosion by our group, inhibitors sheep and wool batteries has been [18 ].found As a to result, be efficient heavy metals in Cr(VI) are removal, found in manywith subsequent industrial wastewatersreduction to Cr(III) [19–21 ].[31,32 Chromium]. This study is a metallic reports on element the results listed of by using the Environmentalsheep wool for Protectionthe removal Agency of ARS (EPA) from aswastewater one of 129 and priority the possibility pollutants of [22 regenerating,23]. It is found the incontamin the air,ated water wool. and soil,The andoutcome occurs of in the several research oxidation could states be utilized ranging in from a sequential Cr(II) to Cr(VI), batch with reactor the trivalentfor removal and ofhexavalent the two statespollutants being from the wastewaters. most stable and common [1–3]. In natural waters, chromium is present in several forms, the most common of which are Cr(0), Cr(III), and Cr(VI). Cr(III) has very low solubility and is2. relativelyMaterials stable,and Methods whereas Cr(VI) is environmentally mobile and highly toxic. Exposure to Cr(VI) causes various health problems, including skin and stomach irritation, dermatitis, liver damage, kidney circulation2.1. Materials and nerve and tissue damage [24–26]. Numerous technologies have already been applied in the removal of Cr(VI) from aqueous solutions. These include adsorption, biosorption, ion-exchange, All chemicals were of analytical grade and used without further purification. Solutions were foam flotation, electrolysis, surface adsorption precipitation, reverse osmosis, sand filtration, chemical prepared using distilled deionized water (DDW) and their concentrations determined reduction/oxidation, electrochemical precipitation, membrane filtration and solvent extraction [27–30]. spectrophotometrically. ARS was from BDH (Turkey). Potassium dichromate was from Riedel De- In previous studies by our group, sheep wool has been found to be efficient in Cr(VI) removal, Haen (Germany). Acetone and 5-di-phenyl carbazide were from Sigma Aldrich (USA). Sheep wool with subsequent reduction to Cr(III) [31,32]. This study reports on the results of using sheep wool (Sharjah animal market) was trimmed and riffled, then washed with water and detergent for two for the removal of ARS from wastewater and the possibility of regenerating the contaminated wool. days. The outcome of the research could be utilized in a sequential batch reactor for removal of the two pollutants from wastewaters. Processes 2020, 8, 556 3 of 15

2. Materials and Methods

2.1. Materials All chemicals were of analytical grade and used without further puriﬁcation. Solutions were prepared using distilled deionized water (DDW) and their concentrations determined spectrophotometrically. ARS was from BDH (Turkey). Potassium dichromate was from Riedel De-Haen (Germany). Acetone and 5-di-phenyl carbazide were from Sigma Aldrich (USA). Sheep wool (Sharjah animal market) was trimmed and riﬄed, then washed with water and detergent for two days.

2.2. Instrumentation ARS and Cr(VI) concentrations were determined spectrophotometrically using Cary 50 (Varian, Australia). Total chromium was determined using Spectra AA220FS (Varian, Australia). Samples were shaken using Edmund Buhler shaker (KS-15/TH-15, Bodelshausen, Germany) at 25.0 0.1 C. pH was ± ◦ measured on a Thermo-Orion 210A + pH meter (USA) equipped with a combined glass electrode. IR spectra were obtained on a Spectrum One FTIR (Perkin Elmer, Waltham, MA, USA). ARS oxidation byproducts were detected using HPLC (Shimadzu, LC-2040C, Kyoto, Japan).

2.3. Methods

2.3.1. Reagent Preparation A 1000 mg/L stock ARS solution was prepared at pH 2.0 and used to prepare standard solutions in the range 1–20 mg/L. Stock solutions containing 1000 mg/L Cr(VI) were prepared by dissolving potassium dichromate in DDW. Stock ligand solutions were prepared by mixing 0.20 mL of 0.05% of 1,5 diphenyl carbazide (in acetone) with 2 drops of 6.0 M sulfuric acid and 0.10 mL of sample. Standard Cr(VI) solutions were in the range 1–10 mg/L.

2.3.2. Determination of Cr ions Concentration Total Cr concentration was determined by atomic absorption spectroscopy (AAS). The 1,5-diphenyl carbazide method could not be used in the determination of Cr(VI) alone, because Cr(VI) removal by wool-ARS complex was accompanied by formation of ARS oxidation by-products. ARS degradation by oxidation involves the cleavage of dye-chromophore components [33,34]. FTIR analysis of residual 1 products from dye oxidation gave IR bands at 1717, 1623, 1387, 1105 and 1045 cm− , attributed to 2 >C=O (carbonyl), >C=C< (alkenes),-C-C-C (alkanes), SO4 − and –C-O-C- groups, respectively. These by-products interfere with analysis of Cr(VI) by 1,5-diphenyl carbazide. Hence, only total Cr could be analyzed by AAS. The highest removal of Cr(VI) was 93%, as detected by AAS. However, previous studies by our group revealed that, at this short term contact, slight amounts of Cr(III) were released into solution [31]. This process causes incomplete removal of Cr(VI).

2.3.3. Adsorption Studies Batch adsorption studies were carried out in flasks containing 50 mL of test solutions at the desired initial ARS concentrations. λmax for dye solutions was 261 nm. Batch adsorption studies were then carried out. Wool loaded with ARS was placed in flasks containing the desired initial Cr(VI) concentration and the contents shaken. Shaking conditions were: 1.5 h, 25.0 ◦C and 175 rpm. AAS was used to measure total Cr concentrations. Oxidation by-products of ARS by Cr(VI) were detected using HPLC under the following conditions: mobile phase 40:60% methanol:phosphate buffer, Pinnacle D8 C8 5 µm (RESTEK, Bellefonte, PA, USA), analytical wavelength 540 nm and 1.5 mL/min flow rate.

2.3.4. Regeneration In-Place Studies Wool loaded with ARS was prepared by shaking wool with 100 mg/L ARS solution at optimum conditions. The resulting wool-ARS was washed with DDW and dried. The remaining ARS solution Processes 2020, 8, 556 4 of 15 was analyzed spectrophotometrically. ARS removal was 88%. Wool-ARS was shaken with 50 mg/L Cr(VI) solution at optimum conditions for 4 days. Subsequently, the wool-ARS-Cr was collected, washed with DDW and dried. The remaining Cr(VI) solution was analyzed by AAS, giving a removal eﬃciency of 86.8%. The cycle was repeated three times.

3. Results and Discussion

3.1. Adsorption Studies

ProcessesRemoval 2019, 7, x of FOR ARS PEER was REVIEW calculated using 4 of 14

% Removal = ((Co Ce)/Co) 100 (1) % Removal = ((Co−-Ce)/Co)*100∗ (1) where Co and Ce areare initial initial and equilibrium equilibrium ARS concentrations (mg/L), (mg/L), respectively.

3.1.1. Optimization Studies on ARS Removal by Wool Eﬀect of Contact Time Effect of Contact Time Figure2 shows the e ﬀect of contact time on ARS removal. At all three pH values used, removal Figure 2 shows the effect of contact time on ARS removal. At all three pH values used, removal increases with increasing contact time until a maximum is reached at ca. 90 min, selected as the increases with increasing contact time until a maximum is reached at ca. 90 min, selected as the optimum time. optimum time.

100 90 80 70 60 50 40 30 % Removal % ARS of Removal 20 10 0 0 50 100 150 200 Time (min)

pH 2.0 pH 3.2 pH 5.7

Figure 2. Effect of contact time on ARS removal. Adsorbent dosage = 8.0 g/L, T = 25.0 °C, shaking Figure 2. Eﬀect of contact time on ARS removal. Adsorbent dosage = 8.0 g/L, T = 25.0 ◦C, shaking speed = 175175 rpm. rpm.

EffectEﬀect of pH pH ARS is employed as an indicator in acid-baseacid-base titration titration,, and changes color in the pH range 4.0 (yellow)-6.0(yellow)-6.0 (red) [[3535].]. At pH < 4.6,4.6, the acidic form predominates, whereas at pH >> 6.0, it is mostly in the the the basic basic form. form. Figure Figure3 shows 3 shows the etheﬀect effect of pH of on pH ARS on removal. ARS removal. As pH increases,As pH increases, removal decreasesremoval decreasesfrom 90% from at pH 90% 2.0 reachingat pH 2.0 a reaching minimum a minimum of <20% at of pH <20%> 5.0. at HencepH > 5.0. the Hence acidic the form acidic ARS form is the ARS one ismore the favorablyone more adsorbed favorably by adsorbed wool. pH by 2.0 wool. was thuspH 2.0 selected was thus as optimum selected in as subsequent optimum measurements.in subsequent measurements.

100 80 60 40 20 % Removal % ARS of Removal 0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 pH Figure 3. Effect of pH on ARS removal by wool. Adsorbent dosage = 8.0 g/L, initial [ALS] = 100 mg/L, T = 25.0 °C, contact time = 90 min, shaking speed = 175 rpm.

Processes 2019, 7, x FOR PEER REVIEW 4 of 14

% Removal = ((Co-Ce)/Co)*100 (1) where Co and Ce are initial and equilibrium ARS concentrations (mg/L), respectively.

3.1.1. Optimization Studies on ARS Removal by Wool

Effect of Contact Time Figure 2 shows the effect of contact time on ARS removal. At all three pH values used, removal increases with increasing contact time until a maximum is reached at ca. 90 min, selected as the optimum time.

100 90 80 70 60 50 40 30 % Removal % ARS of Removal 20 10 0 0 50 100 150 200 Time (min)

pH 2.0 pH 3.2 pH 5.7

Figure 2. Effect of contact time on ARS removal. Adsorbent dosage = 8.0 g/L, T = 25.0 °C, shaking speed = 175 rpm.

Effect of pH ARS is employed as an indicator in acid-base titration, and changes color in the pH range 4.0 (yellow)-6.0 (red) [35]. At pH < 4.6, the acidic form predominates, whereas at pH > 6.0, it is mostly in the the basic form. Figure 3 shows the effect of pH on ARS removal. As pH increases, removal decreases from 90% at pH 2.0 reaching a minimum of <20% at pH > 5.0. Hence the acidic form ARS is theProcesses one2020 more, 8, 556 favorably adsorbed by wool. pH 2.0 was thus selected as optimum in subsequent5 of 15 measurements.

100 80 60 40 20 % Removal % ARS of Removal 0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 pH

FigureFigure 3. 3.EffectEﬀect of ofpH pH on on ARS ARS removal removal by by wool. wool. Adsorbent Adsorbent dosagedosage= = 8.08.0 gg/L,/L, initial initial [ALS] [ALS]= =100 100 mg mg/L,/L,

ProcessesT =T 2019 25.0= 25.0, 7°,C, x◦ C,FORcontact contact PEER time timeREVIEW = =9090 min, min, shaking shaking speed speed == 175175 rpm. rpm. 5 of 14

EffectEﬀect of ofWool Wool Dosage Dosage FigureFigure 44 shows shows the the effect e ﬀect of of wool wool dosage dosage on ARS removalremoval at at optimum optimum conditions. conditions. ARS ARS removal removal increasesincreases with with increasing increasing wool wool dosage dosage until until a a maximum 8.08.0 gg/L./L. This This dosage dosage was was selected selected as as optimal optimal and used in further experiments. and used in further experiments.

120

100

40 % Removal % ARS of Removal

0 0.0 2.0 4.0 6.0 8.0 Wool dosage (g/L)

FigureFigure 4. 4.EffectEﬀect of of adsorbent adsorbent dosage dosage o onn removal removal of ARSARS byby woolwool at at di differentﬀerent wool wool dosages. dosages. Initial Initial

[ARS][ARS] = 100= 100 mg/L, mg/L, pH pH = =2.0,2.0, contact contact time time == 9090 min, min, T T == 25.025.0 °◦C,C, shaking shaking speed = 175 rpm.

EffectEﬀect of ofTemperature Temperature ForFor initial initial ARS ARS concentrations concentrations ofof 60 60 mg mg/L/L, 80, 80 mg mg/L/L and and 100 mg100/L, mg/L, increasing increasing the temperature the temperature from from25.0 25.0 to 50.0 to ◦ 50.C0 has °C little has eﬀ littleect on effect ARS on removal ARS removal (Figure5). (Figure The temperature 5). The temperature of 25.0 ◦C was of selected25.0 °C as was selectedoptimal as in optimal adsorption in adsorption isotherms. isotherms.

100

% Removal % ARS of Removal 60

50 25 30 35 40 45 50 T (o C)

100 ppm 80 ppm 60 ppm

Figure 5. Effect of temperature and initial ARS concentration on removal of ARS by wool. Adsorbent dosage = 8.0 g/L, contact time = 90 min, pH = 2.0, shaking speed = 175 rpm.

3.1.2. Adsorption Isotherms

The adsorption capacity (qe) for ARS removal was evaluated using

qe = (Co−Ce) × V/m (2)

Processes 2019, 7, x FOR PEER REVIEW 5 of 14

Effect of Wool Dosage Figure 4 shows the effect of wool dosage on ARS removal at optimum conditions. ARS removal increases with increasing wool dosage until a maximum 8.0 g/L. This dosage was selected as optimal and used in further experiments.

120

100

40 % Removal % ARS of Removal

0 0.0 2.0 4.0 6.0 8.0 Wool dosage (g/L)

Figure 4. Effect of adsorbent dosage on removal of ARS by wool at different wool dosages. Initial [ARS] = 100 mg/L, pH = 2.0, contact time = 90 min, T = 25.0 °C, shaking speed = 175 rpm.

Effect of Temperature For initial ARS concentrations of 60 mg/L, 80 mg/L and 100 mg/L, increasing the temperature fromProcesses 25.0 to2020 50., 8,0 556 °C has little effect on ARS removal (Figure 5). The temperature of 25.06 of°C 15 was selected as optimal in adsorption isotherms.

100

% Removal % ARS of Removal 60

50 25 30 35 40 45 50 T (o C)

100 ppm 80 ppm 60 ppm

FigureFigure 5. Effect 5. Eﬀ ofect temperature of temperature and and initial initial ARS ARS concentration concentration on on removal removal of of ARS ARS by by wool. wool. Adsorbent Adsorbent dosagedosage = 8.0= g/L,8.0 gcontact/L, contact time time = 90= 90min, min, pH pH = =2.0,2.0, shaking shaking speed speed == 175 rpm.rpm.

3.1.2.3.1.2. Adsorption Adsorption Isotherms Isotherms

The adsorption capacity (qe) for ARS removal was evaluated using The adsorption capacity (qe) for ARS removal was evaluated using

qe = (Co Ce) V/m (2) qe = (Co−Ce) ×× V/m (2)

where qe is the equilibrium adsorption capacity (mg adsorbate/g adsorbent), V is the volume of solution (L) and m is the mass of adsorbent (g). At optimum adsorption parameters, qe was evaluated at several initial concentrations and the results ﬁtted to the linearized forms of the Langmuir (Equation (3)) and Freundlich (Equation (4)) isotherms [36]. These are

Ce/qe = 1/Qb + Ce/Q (3)

1/n qe = Kf Ce (4)

where Ce is the equilibrium concentration (mg/L), qe is the amount adsorbed at equilibrium, in mg/g adsorbent, Q (mg/g) and b (L/mg) are the Langmuir constants, representing adsorption capacity and energy, respectively. Kf and n are the Freundlich constants. The poor fit to the Langmuir isotherm in Figure6a indicates that adsorption does not follow this isotherm. This may be attributed to the nonequivalence adsorption sites on wool. Figure6b shows the data fitted to the Freundlich isotherm. The good linearity of this plot clearly reveal that adsorption best fits the Freundlich isotherm, indicating that adsorption sites are not equivalent. The Freundlich constants, Kf and n, were 3.38 and 1.70, respectively. Processes 2019, 7, x FOR PEER REVIEW 6 of 14 where qe is the equilibrium adsorption capacity (mg adsorbate/g adsorbent), V is the volume of solution (L) and m is the mass of adsorbent (g). At optimum adsorption parameters, qe was evaluated at several initial concentrations and the results fitted to the linearized forms of the Langmuir (Equation (3)) and Freundlich (Equation (4)) isotherms [36]. These are

Ce/qe = 1/Qb + Ce/Q (3)

qe = Kf Ce1/n (4) where Ce is the equilibrium concentration (mg/L), qe is the amount adsorbed at equilibrium, in mg/g adsorbent, Q (mg/g) and b (L/mg) are the Langmuir constants, representing adsorption capacity and energy, respectively. Kf and n are the Freundlich constants. The poor fit to the Langmuir isotherm in Figure 6a indicates that adsorption does not follow this isotherm. This may be attributed to the nonequivalence adsorption sites on wool. Figure 6b shows the data fitted to the Freundlich isotherm. The good linearity of this plot clearly reveal that adsorption best fits the Freundlich isotherm, indicatingProcesses 2020 that, 8, 556 adsorption sites are not equivalent. The Freundlich constants, Kf and n, were 3.387 and of 15 1.70, respectively.

a 3.5

3.0

2.5

e 2.0 /q e

C 1.5

1.0

0.5

0.0 0 50 100 150 200 250 300

Ce, mg/L

b 2.5 y = 0.5333x + 0.6431 2.0 R² = 0.9943

1.5 e

log q 1.0

0.5

0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0

log Ce

Figure 6. (a) Langmuir and (b) Freundlich plots for adsorption of ARS by wool. Adsorbent dosage = Figure 6. (a) Langmuir and (b) Freundlich plots for adsorption of ARS by wool. Adsorbent dosage = 8.0 g/L, T = 25.0 °C, contact time = 90 min, pH = 2.0, shaking speed = 175 rpm. 8.0 g/L, T = 25.0 ◦C, contact time = 90 min, pH = 2.0, shaking speed = 175 rpm.

3.1.3. Adsorption Kinetics The kinetics of adsorption of ARS by wool was studies at pH 2 and were ﬁtted to both Lagergren pseudo ﬁrst order model (Equation (5)) and pseudo second order model (Equation (6)) [37,38].

ln (qe qt) = ln qe k t (5) − − t 1 t = + (6) 2 qt k2qe qe Our data was found to best ﬁt the pseudo second order model with R2 = 1 yielding a value for the 1 1 second order rate constant (k2) of 0.134 g mg− min− at 25.0 ◦C. Processes 2019, 7, x FOR PEER REVIEW 7 of 14

3.1.3. Adsorption Kinetics The kinetics of adsorption of ARS by wool was studies at pH 2 and were fitted to both Lagergren pseudo first order model (Equation (5)) and pseudo second order model (Equation (6)) [37,38].

ln ( e− t ) = ln e− k t (5)

= + (6) t 2 e e Our data was found to best fit the pseudo second order model with R2 = 1 yielding a value for −1 −1 Processesthe second2020, 8 order, 556 rate constant (k2) of 0.134 g mg min at 25.0 °C. 8 of 15

3.2. Removal of ARS by Wool and by Wool Loaded with Cr(VI) 3.2. Removal of ARS by Wool and by Wool Loaded with Cr(VI) 3.2.1. FTIR spectra 3.2.1. FTIR Spectra An earlier publication by our group reported on Electron dispersive X-ray spectroscopy (EDS) andAn FTIR earlier characterization publication by of our wool group and reported wool on loaded Electron with dispersive Cr(VI) [31 X-ray]. IR spectroscopy spectra of (EDS) ARS andwere FTIRsubsequently characterization reported of wool[33]. andIn this wool work, loaded FTIR with was Cr(VI) used [31 to]. monitor IR spectra rem ofoval ARS of were ARS subsequently by wool, In reportedFigure 7 [a33, the]. In peak this work,at 3459 FTIR cm−1 was is due used to tothe monitor –OH group, removal whereas of ARS peaks by wool, at 3094 In Figure cm−1 and7a, the2926 peak cm−1 1 1 1 atarise 3459 from cm− =Cis-H due groups to the in –OH aromatic group, rings. whereas Peaks peaks at 2400 at 3094cm−1, cm 1666− andcm−1 2926, 1634 cm cm−−1 ariseand 1588 from cm=C-H−1 are 1 1 1 1 groupscharacteristic in aromatic of anthraquinone rings. Peaks at molecules, 2400 cm− ,whereas 1666 cm −those, 1634 at cm1155− andcm−1 1588 and cm728− cmare−1 characteristicare for sulfonate of 1 1 anthraquinonegroups. Figure molecules, 7b shows whereasspectra of those free atwool 1155 wit cmh− featuresand 728 described cm− are earlier for sulfonate [31]. Upon groups. complexation Figure7b showswith spectraARS, changes of free wool were with observed features (Figure described 7c). earlier Specifically, [31]. Upon the broadcomplexation peak at with 4000 ARS,–2800 changes cm−1 is 1 weresharpened observed, and (Figure a new7 bandc). Speciﬁcally, at 2400 cm the−1 appears. broad peak Changes at 4000–2800 in the broad cm −peakis sharpened,between 160 and0–800 a new cm−1 1 1 bandare also at 2400 observed. cm− appears. These changes Changes indicate in the that broad ARS peak is essentially between 1600–800 taken up cm by− wool.are also Adsorption observed. of TheseCr(VI) changes by wool indicate loaded that with ARS ARS is essentiallyresults in further taken up FTIR by wool.changes Adsorption (Figure 7 ofd), Cr(VI) primarily by wool in the loaded 1200– 1 with800 ARScm−1 range. results This in further observation FTIR changesindicates (Figure that ARS7d), on primarily wool undergoes in the 1200–800 oxidation cm by− Cr(VI)range. to This form observationaromatic byproducts. indicates that ARS on wool undergoes oxidation by Cr(VI) to form aromatic byproducts.

FigureFigure 7. 7.FTIR FTIR spectra spectra of of ( a()a) ARS, ARS, ( b(b)) free free wool, wool, (c ()c wool-ARS,) wool-ARS, (d ()d wool-ARS) wool-ARS loaded loaded with with Cr(VI). Cr(VI).

3.2.2. Oxidation Byproducts

Upon adsorption of Cr(VI) by wool loaded with ARS, oxidation by-products of ARS were formed (Figure3.2.2. Oxidation8), indicating Byproducts that adsorbed Cr(VI) undergoes reduction to Cr (III). These by-products interfere with spectrophotometric determination of Cr(VI), with the result that concentrations of individual Cr species could not be determined. AAS, which gives total Cr, was hence used to follow the uptake of Cr(VI) by wool-ARS using short term contact time, so that Cr(III) has minimum interference [31]. The eﬀect of contact time, pH, dosage and temperature were studied and optimized. Processes 2019, 7, x FOR PEER REVIEW 8 of 14

Upon adsorption of Cr(VI) by wool loaded with ARS, oxidation by-products of ARS were formed (Figure 8), indicating that adsorbed Cr(VI) undergoes reduction to Cr (III). These by-products interfere with spectrophotometric determination of Cr(VI), with the result that concentrations of individual Cr species could not be determined. AAS, which gives total Cr, was hence used to follow Processesthe uptake2020, 8 of, 556 Cr(VI) by wool-ARS using short term contact time, so that Cr(III) has minimum9 of 15 interference [31]. The effect of contact time, pH, dosage and temperature were studied and optimized.

49000

39000

29000

19000 d

9000 c b -1000 a 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Figure 8. HPLC chromatogram ofof aqueousaqueous solutionssolutions inin equilibriumequilibrium with with ARS ARS at at pH pH 2.0. 2.0. ((aa))ARS ARS atat pHpH 2 (green),(green), (b(b)) Wool Wool loaded loaded with with ARS ARS (grey), (grey), (c) ( ARSc) ARS+ Cr(VI) + Cr(VI) without without wool wool (blue), (blue), (d) wool (d) wool loaded loaded with ARSwith afterARS after adsorption adsorption of Cr(VI) of Cr(VI) (yellow). (yellow). Initial Initial [ARS] [ARS]= 1.0 = 1.0 mg mg/L,/L, pH pH= =2.0; 2.0; contact contact time time 120 120 h, h,T T= = 25.0 °C, shaking speed = 175 rpm. Mobile phase is 40% methanol and 60% phosphate buffer, flow rate 25.0 ◦C, shaking speed = 175 rpm. Mobile phase is 40% methanol and 60% phosphate buﬀer, ﬂow rate = 1.5 mL/min. mL/min.

High performance liquid chromatographychromatography (HPLC)(HPLC) waswas usedused toto analyseanalyse solutions inin contact with wool,wool, ARS or both, in in presence presence or or absence absence of of Cr(VI). Cr(VI). In In Figure Figure 88aa (green (green),), the the ARS ARS peak peak appears appears at at15 15min min retention retention time. time. Upon Upon removal removal by by wool, wool, this this peak peak disappears, disappears, due due to to ARS ARS adsorption by wool (Figure(Figure8 8bb (grey)). (grey)). This This peak peak also also disappears disappears upon upon mixing mixing Cr(VI) Cr(VI) with with ARS ARS in in solution solution without without wool, wool, a processa process that that results results in the in appearancethe appearance of a newof a peaknew atpeak 4 min at retention4 min retention time, among time,other among low other intensity low peaksintensity that peaks are not that apparent are not inapparent this chromatogram in this chromatogram at this scale at (Figurethis scale8c (Figure (blue)). 8 However,c (blue)). However, exposing woolexposing loaded wool with loaded ARS towith Cr(VI) ARS causes to Cr(VI) not causes only the not disappearance only the disappea of ARSrance peak of at ARS 15 min, peak but at also15 min, the appearancebut also the ofappearance otherwise lowof otherwise intensity peakslow intensity in ARS-Cr(VI) peaks in without ARS-Cr(VI) wool(Figure without8c) wool at retention (Figure times8c) at ofretention 4.0, 4.4, times 4.7, 5.5, of 4.0, 6.9 and4.4, 4.7, 9.0 min5.5, 6.9 (Figure and 89.0d (yellow).min (Figure These 8d (yellow) peaks indicate. These thatpeaks the indicate mechanism that the of ARSmechanism oxidation of ARS on wool oxida istion similar on wool to that is insimilar aqueous to that solution. in aqueous These solution. peaks have These been peaks attributed have been to a varietyattributed of oxidationto a variety by-products of oxidation [34 ,by35].-products Thus adsorption [34,35]. Thus of Cr(VI) adsorption by wool of loaded Cr(VI) with by wool ARS loaded results inwith ARS ARS oxidation results within ARS simultaneous oxidation with release simultaneous of oxidation release by-products of oxidation to solution. by-produc Thists process to solution. frees andThis regeneratesprocess frees wool and from regenerates ARS for wool reuse. from ARS for reuse.

3.2.3. OptimizationOptimization Studies on Cr(VI) Removal by Wool LoadedLoaded withwith ARSARS

EEffectﬀect ofof ContactContact TimeTime Figure9 9shows shows the the e ﬀ ect effect of contact of contact time on time Cr(VI) on removal Cr(VI) removalby wool loaded by wool with loaded ARS. Equilibrium with ARS. removalEquilibrium is reached removal after is reached 40 min. Thisafter can40 bemin. compared This can with be compared the 90 min with needed the for90 removalmin needed of ARS for aloneremoval (Figure of ARS3). However,alone (Figure in a previous3). However, study, in the a previous optimum study, time for the removal optimum of Cr(VI)time for by removal free wool of underCr(VI) shortby free term wool conditions under shor wast term 60 min conditions [31]. Hence, was adsorption60 min [31]. is He speedednce, adsorption by the presence is speeded of ARS by onthewool. presence of ARS on wool.

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90 80

80 70

70 60

% Removal of Cr(VI) 50

% Removal of Cr(VI) 0 20 40 60 80 100 120 140 160 180 50 0 20 40 60Time,80 min 100 120 140 160 180 Time, min

Figure 9. 9.E ﬀEffectect of contactof contact time time on Cr(VI) on Cr(VI) removal removal by wool byloaded wool with loaded ARS. Initial with [Cr(VI)] ARS. Initial concentration [Cr(VI)] concentrationFigure 9. Effect is 100 of mg/L, contact adsorbent time on dosage Cr(VI) = removal 8.0 g/L, bypH wool= 6.0, loadedT = 25.0 with°C, shaking ARS. Initial speed [Cr(VI)]= 175 rpm . is 100 mg/L, adsorbent dosage = 8.0 g/L, pH = 6.0, T = 25.0 ◦C, shaking speed = 175 rpm. concentration is 100 mg/L, adsorbent dosage = 8.0 g/L, pH = 6.0, T = 25.0 °C, shaking speed = 175 rpm. EEffectﬀect ofof pHpH Effect of pH pH is an important factor that controls uptake of Cr(VI Cr(VI).). Figure 10 reveals that removal of Cr(VI) by woolpH loaded is an important with ARS factor reaches that controls a maximum uptake of of 77.8% Cr(VI at). Figure pH 2.0,2.0 10, andandreveals remainsremains that removal almostalmost of constantconstant Cr(VI) at higherby wool pH. loaded In previous with ARS studies reaches [[3939 –a 41maximum], the optimumoptim of 77.8%um pH at for pH Cr(VI) 2.0, and removal remains by almost free constantwool was at 2.0, indicatinghigher pH. that In reductionprevious studies of Cr(VI)Cr(VI) [39 is–41 catalyzed], the optim by um hydrogen pH for ions.Cr(VI) In removal this this study, study, by free removal wool ofwas Cr(VI) 2.0, by indicating that reduction of Cr(VI) is catalyzed by hydrogen ions. In this study, removal of Cr(VI) by wool loaded with ARS is essentially independent of pH. pH 2.0 was selected as optimum for removal wool loaded with ARS is essentially independent of pH. pH 2.0 was selected as optimum for removal of Cr(VI)Cr(VI) by wool loaded withwith ARS.ARS. of Cr(VI) by wool loaded with ARS.

90 90

8080

7070

6060

5050

4040 % Removal Removal % of Cr(VI) % Removal Removal % of Cr(VI) 3030

2020 1.01.0 2.02.0 3.03.0 4.04.0 5.05.0 6.0 6.0 pHpH

Figure 10. Effect of pH on removal of Cr(VI) by wool loaded with ARS. Initial [Cr(VI)] concentration = Figure 10. EffectEﬀect of pH on removal of Cr(VI) byby woolwool loadedloaded withwith ARS.ARS. InitialInitial [Cr(VI)][Cr(VI)] concentrationconcentration == 100 mg/L, adsorbent dosage = 8.0 g/L, contact time = 120 min., T = 25.0 °C, shaking speed = 175 rpm. 100100 mgmg/L,/L, adsorbentadsorbent dosagedosage == 8.0 gg/L,/L, contact time = 120 min., T = 25.025.0 °◦C,C, shaking shaking speed speed == 175175 rpm. rpm.

E Effectect of of Adsorbent Adsorbent Dosage Dosage Effectﬀ of Adsorbent Dosage The removal of Cr(VI) by wool loaded with ARS was studied at several adsorbent dosages The removalremoval of Cr(VI) Cr(VI) by wool wool loaded loaded with with ARS ARS waswas studied studied at at several several adsorbent adsorbent dosages dosages (Figure(Figure 11 11).). As As the the dosage dosage increases, increases, Cr(VI)Cr(VI) removalremoval increasesincreases until it reaches ca. 90.3 90.3%,%, at at a a dosage dosage of (Figure 11). As the dosage increases, Cr(VI) removal increases until it reaches ca. 90.3%, at a dosage 10.0of 10.0 g/L. g/L. This This is due is due to the to increasethe increase of free of free active active sites sites on on wool wool and and the the increase increase in in the the number number of of sites ofsites 10.0 loaded g/L. This with is ARS. due to the increase of free active sites on wool and the increase in the number of loadedsites loaded with with ARS. ARS.

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100

% Removalof Cr(VI) 20 % Removalof Cr(VI) 20

0 0.0 2.0 4.0 6.0 8.0 10.0 wool loaded with ARS dosage (g/L)

Figure 11. EEffectﬀect of adsorbent dosage on removal of Cr(VI) by wool loaded with ARS. Initial [Cr(VI)] concentration = 100 mg/L, pH = 2.0, T = 25.0 °C, contact time = 120 min, shaking speed = 175 rpm. concentrationconcentration == 100 mg/L, mg/L, pH = 2.0,2.0, T T == 25.025.0 °◦C,C, contact contact time time == 120120 min, min, shaking shaking speed speed == 175175 rpm. rpm.

EEffectffect ofof TemperatureTemperature Figure 12 showsshows the eeffectffect ofof temperaturetemperature onon removalremoval ofof Cr(VI)Cr(VI) byby woolwool loadedloaded withwith ARS.ARS. AtAt constantconstant temperature, removalremoval variesvaries withwith initialinitial concentrationconcentration inin aa semi-randomsemi-random trend.trend. It increases as the initial concentrationconcentration rises from 25 to 100 mgmg/L,/L, thenthen drops atat 200 mgmg/L./L. TheThe expectationexpectation isis thatthat removalremoval oughtought toto decreasedecrease withwith increasingincreasing initialinitial concentration.concentration. The opposing trendstrends withinwithin thethe twotwo simultaneoussimultaneous processes thatthat accompany removal maymay explainexplain this apparentapparent anomaly. The firstfirst isis thethe adsorption of Cr(VI) by free sites on woolwool and the secondsecond is thethe oxidationoxidation of ARSARS byby Cr(VI).Cr(VI). InIn thethe first,first, removalremoval decreasesdecreases withwith increasingincreasing initialinitial concentration,concentration, whereas in thethe second,second, thethe oppositeopposite eeffectffect isis observed.observed. Figure 12 also reveals that at constant initial Cr(VI) concentration, Cr(VI) removal decreases with increasingincreasing temperature.temperature. This observation indicates that the principal adsorption step isis exothermic.exothermic.

100 90 80 70 60 50 40 30 % Removal of Cr(VI) % Removal of Cr(VI) 20 10 0 25 30 35 40 45 50 55 60 65 T, ˚C

25ppm 50ppm 100ppm 200ppm

Figure 12. Effect of temperature and Cr(VI) concentration on removal of Cr(VI) by wool loaded with Figure 12.12. EEffectﬀect of temperature and Cr(VI)Cr(VI) concentrationconcentration on removal of Cr(VI) by woolwool loaded with ARS. Adsorbent dosage = 8.0 g/L, pH = 2.0, contact time = 120 min, shaking speed = 175 rpm. ARS.ARS. AdsorbentAdsorbent dosagedosage == 8.0 gg/L,/L, pH = 2.0, contact time = 120120 min, min, shaking shaking speed speed == 175175 rpm. rpm.

3.3. Adsorption Mechanism Based on the above results, Figure 13 shows a proposed two-step mechanism for removal of Cr(VI) by wool loaded with ARS (ARS-W). The second step is similar to that proposed in previous

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Processes3.3. Adsorption 2019, 7, x FOR Mechanism PEER REVIEW 11 of 14 reportsProcessesBased for removalon2019 the, 7, x above FOR of PEERCr(VI) results, REVIEW by Figure other natural13 shows adsorbents, a proposed but two-step with ARS mechanism yielding for oxidation removal11 ofproducts of 14 Cr(VI) [by39,41 wool]. loaded with ARS (ARS-W). The second step is similar to that proposed in previous reports for removalreports of Cr(VI)for removal by other of Cr(VI) natural by other adsorbents, natural adsorbents, but with ARS but with yielding ARS yielding oxidation oxidation products products [39,41 ]. [39,41].

Figure 13. Proposed mechanism for removal of Cr(VI) by wool loaded with ARS. Figure 13. ProposedProposed mechanism mechanism for for removal removal of Cr(VI) Cr(VI) by wool loaded with ARS ARS..

3.4. Isotherm3.4. Isotherm Analysis Analysis 3.4. Isotherm Analysis AdsorptionAdsorption data data were were ﬁtted fitted to to both both thethe LangmuirLangmuir and and Freundlich Freundlich models. models. Figure Figure 14 shows 14 shows the the Adsorptiondependence ofdata adsorption were fitted capacity to both on the Cr(VI) Langmuir concentration and Freundlich after a 120 models. min contact, Figure using 14 shows wool the dependence of adsorption capacity on Cr(VI) concentration after a 120 min contact, using wool loaded dependenceloaded with of adsorptionARS as adsor capacitybent. The dependence on Cr(VI) concentrationdoes not resemble after any a known 120 min adsorption contact, behavior, using wool loadedwithindicating ARS with as adsorbent.ARS that as equilibrium adsor Thebent. dependence was The not dependence achieved does notwithin does resemble this not time. resemble any This known is becauseany adsorption known removal adsorption behavior, by adsorption indicatingbehavior, indicatingthat equilibriumis followed that equilibriumby was a time not dependent achieved was not withinredox achieved process. this within time. Hence, This this the istime. isotherm because This removalcouldis because not bybe removal adsorptionfitted to anyby adsorption isof followedthe isby followed a timemodels dependent bythat a predicatetime redoxdependent equilibrium process. redox between Hence, process. theadsorbate isothermHence, and the could adsorbent. isotherm not be could ﬁtted not to anybe fitte of thed to models any of thatthe modelspredicate that equilibrium predicate betweenequilibrium adsorbate between and adsorbate adsorbent. and adsorbent. 18

18 16 14 16 12 14 10 12 8 10 (mg/g) q 6 8 4 q (mg/g) q 6 2 4 0 0 10 20 30 40 50 60 70 80 2 Ce (mg/L) 0 0 10 20 30 40 50 60 70 80 Figure 14. Adsorption capacity (q) for the removal of Cr(VI) by wool loaded with ARS as function of Cr(VI) concentration. Contact time = 120 min, adsorbentCe (mg/L) dosage = 8.0 g/L, pH = 2.0, T = 25.0 °C, shaking speed = 175 rpm. Figure 14. Adsorption capacity (q) for the removal of Cr(VI) by wool loaded with ARS as function of Figure 14. Adsorption capacity (q) for the removal of Cr(VI) by wool loaded with ARS as function Cr(VI) concentration. Contact time = 120 min, adsorbent dosage = 8.0 g/L, pH = 2.0, T = 25.0 °C, shaking of Cr(VI) concentration. Contact time = 120 min, adsorbent dosage = 8.0 g/L, pH = 2.0, T = 25.0 ◦C, speedshaking = 175 speed rpm.= 175 rpm.

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3.5. Regeneration Studies 3.5. Regeneration Studies Wool loaded with ARS has been used to investigate its regeneration by the simultaneous removal Wool loaded with ARS has been used to investigate its regeneration by the simultaneous and reduction of Cr(VI) from industrial wastewater, followed by ARS oxidation. For this purpose, removal and reduction of Cr(VI) from industrial wastewater, followed by ARS oxidation. For this wool loaded with ARS was prepared by shaking wool with 100 mg/L ARS solution at optimum purpose, wool loaded with ARS was prepared by shaking wool with 100 mg/L ARS solution at conditions. The resulting wool-ARS was washed with DDW and dried. Spectrophotometric analysis optimum conditions. The resulting wool-ARS was washed with DDW and dried. Spectrophotometric of the equilibrium solution gave ARS removal of 93.2%. Wool-ARS was then shaken with 50 mg/L analysis of the equilibrium solution gave ARS removal of 93.2%. Wool-ARS was then shaken with 50 Cr(VI) solution at optimum conditions for 4 days. Wool-ARS-Cr was washed with DDW and dried for mg/L Cr(VI) solution at optimum conditions for 4 days. Wool-ARS-Cr was washed with DDW and further studies. AAS analysis of the equilibrium solution gave 86.8% Cr(VI) removal. The cycle was dried for further studies. AAS analysis of the equilibrium solution gave 86.8% Cr(VI) removal. The repeated four times. Figure 15 shows removal of ARS and Cr(VI) after each cycle. Removal of both cycle was repeated four times. Figure 15 shows removal of ARS and Cr(VI) after each cycle. Removal ARS and Cr(VI) by wool varies in the range 60%–90%, even at the end of the fourth cycle. This ﬁnding of both ARS and Cr(VI) by wool varies in the range 60%–90%, even at the end of the fourth cycle. suggests promising prospects for using a sequential batch reactor for the simultaneous removal of This finding suggests promising prospects for using a sequential batch reactor for the simultaneous organic pollutants and Cr(VI), which is self-regenerated under ﬁeld conditions. removal of organic pollutants and Cr(VI), which is self-regenerated under field conditions.

120

100

% Removal % 40

0 1 2 3 4 Cycle number

ARS Cr

FigureFigure 15.15. EffectEﬀect of different diﬀerent cycles cycles on on removal removal of of Cr(VI) Cr(VI) and and ARS ARS by by wool. wool. Adsorbent Adsorbent.. dosage dosage = 8.0= g/L, pH = 2.0, T = 25.0 °C, shaking speed = 175 rpm. 8.0 g/L, pH = 2.0, T = 25.0 ◦C, shaking speed = 175 rpm.

4.4. ConclusionsConclusions RemovalRemoval e efficiencyfficiency of of ARS ARS by by wool wool depends depends on on contact contact time, time, adsorbent adsorbent dosage, dosage, initial initial ARS ARS concentrationconcentration and and pH. pH. At At 25.0 25.◦C,0 The°C, Freundlich The Freundlich adsorption adsorption isotherm isotherm gave a good gave fit a to good the adsorption fit to the data.adsorption ARS undergoes data. ARS slowundergoes oxidization slow oxidization by Cr(VI) at by pH Cr(VI) 2.0. This at pH oxidation 2.0. This and oxidation the ability and ofthe wool ability to adsorbof wool Cr(VI) to adsorb combine Cr(VI) to combine form the to basis form of the an basis effective of an method effective for method sequential for sequential removal of removal ARS and of Cr(VI),ARS and resulting Cr(VI), inresulting partial in self-regeneration partial self-regeneration of wool from of wool ARS. from Wool ARS. loaded Wool with loaded ARS with was ARS efficient was inefficient removing in removin Cr(VI) withg Cr(VI) the simultaneous with the simultaneous appearance appearance of oxidation of ARS oxidation by-products. ARS by A-products. sequential A batchsequential reactor, batch designed reactor, for designed four cycles, for four indicated cycles, indicated no significant no significant reduction reduction in the ability in the of ability wool toof removewool to ARS,remove followed ARS, followed by Cr(VI). by A Cr(VI). two-step A two mechanism-step mechanism for this removal for this hasremoval been has proposed. been proposed. The first involvesThe first fastinvolves adsorption fast adsorption of Cr(VI) onof woolCr(VI) loaded on wool with loaded ARS, andwith the ARS second, and involvesthe second an oxidationinvolves an of ARS,oxidation followed of ARS by, desorption followed by of desorption oxidation by-products of oxidation into by- solution.products Severalinto solution. of these Several by-products of these were by- detectedproducts by were HPLC. detected The surface by HPLC. of wool The beforesurface and of afterwool adsorptionbefore and of after ARS, adsorption followed byof theARS, adsorption followed ofby Cr(VI),the adsorption which was of characterizedCr(VI), which bywas FTIR. characterized The results by support FTIR. The the suggestedresults support mechanism. the suggested These findingsmechanism. form These a basis findings for the form design a basis of batch for the sequential design of reactor batch forsequential removal reactor of ARS for and removal Cr(VI), of under ARS fieldand Cr(VI), conditions under and field with conditions zero liquid and discharge. with zero liquid discharge.

Author Contributions: Ziad Sara and Baraa Atalla performed most of the experiments. This research was led by Ibrahim Taleb. Mustafa Khamis and Fawwaz Jumean were instrumental in the initiation, conceptualization and planning of this work.

Funding: It was solely supported by the American University of Sharjah under grant number FRG17-R-17.

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Author Contributions: Z.A.S. and B.A.A. performed most of the experiments. This research was led by T.H.I., M.I.K. and F.H.J. were instrumental in the initiation, conceptualization and planning of this work. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by the American University of Sharjah under grant number FRG17-R-17. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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