Hydrothermal Treatment of Arsenic Sulfide Residues from Arsenic

International Journal of Environmental Research and Public Health

Article Hydrothermal Treatment of Arsenic Sulﬁde Residues from Arsenic-Bearing Acid Wastewater

Liwei Yao 1, Xiaobo Min 1,2,*, Hui Xu 1, Yong Ke 1,2,*, Yanjie Liang 1,2 and Kang Yang 3 ID

1 School of Metallurgy and Environment, Central South University, Changsha 410083, China; [email protected] (L.Y.); [email protected] (H.X.); [email protected] (Y.L.) 2 Chinese National Engineering Research Center for Control & Treatment of Heavy Metal Pollution, Changsha 410083, China 3 School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China; [email protected] * Correspondence: [email protected] (X.M.); [email protected] (Y.K.); Tel.: +86-731-88-830-511 (X.M. & Y.K.); Fax: +86-731-88-710-171 (X.M. & Y.K.) Received: 14 July 2018; Accepted: 18 August 2018; Published: 28 August 2018

Abstract: Arsenic sulfide residue (ASR), a by-product from the treatment of arsenic-bearing acidic wastewater, is abundantly generated but not properly disposed of in China. The utilization of such high-content arsenic waste residue is limited by the market. The traditional methods of stabilization/solidification (S/S) by lime cement or iron salt have a large mass/volume addition, high dumping cost and secondary pollution risk. In this paper, hydrothermal technology was used to treat three kinds of ASRs obtained from different smelters to minimize waste. The leaching toxicity and chemical speciation of the generated products was also evaluated by TCLP and BCR analyses. It was found that the hydrothermal treatment could greatly reduce the volume and moisture content of the ASRs. TCLP tests showed that the leachability of arsenic and heavy metals significantly decreased after the treatment. According to the BCR analysis, most of the unstable As, Cd and Cr transformed into a residual fraction. Finally, XRD, SEM, Raman and XPS techniques were carried out to reveal the mechanism. As a result, hydrothermal treatment can efficiently achieve the dehydration, volume reduction and stabilization/solidification of ASRs.

Keywords: arsenic sulﬁde residue; hydrothermal treatment; dehydration; volume reduction; stabilization/solidiﬁcation; arsenic-bearing wastewater

1. Introduction Arsenic-containing waste is widely produced due to anthropogenic activities [1–4]. It is universally believed that the extractive process of non-ferrous metals is one of the most important sources of arsenic [5,6]. In the smelting of copper or lead-zinc ore, larger amounts of acidic wastewater are discharged when gaseous emission containing SO2, As, Cu, Pb, Zn and other impurities are washed with diluted acid [7]. Moreover, wasted electrolyte with a high concentration of arsenic is produced abundantly in copper and lead-zinc electrolyte processes [8,9]. At present, there are many treatment methods for arsenic-containing wastewater, such as precipitation, adsorption, and biological methods [10–14]. However, such wastewater is usually treated with sodium sulfide, sodium hydrosulfide or hydrogen sulfide to precipitate arsenic, and the deposition of arsenic sulfide residue (ASR) is then engendered [15,16]. It is estimated that hundreds of thousands of tons of ASR are generated annually in China. As a typical arsenic-containing waste, ASR will lead to serious environmental pollution if disposed of improperly. In the past decades, many efforts have been devoted to this subject. One of the directions is to reclaim arsenic from ASR. A vacuum method was proposed to recover elemental sulfur, arsenic

Int. J. Environ. Res. Public Health 2018, 15, 1863; doi:10.3390/ijerph15091863 www.mdpi.com/journal/ijerph Int. J. Environ. Res. Public Health 2018, 15, 1863 2 of 15 trioxide and arsenic sulfide from ASR in a step by step fashion [17]. In addition, a technique in which the ASR was reacted with CuSO4 was developed for the recovery of As2O3 [7]. Despite these achievements, the application of these technologies is limited because the market for arsenic is shrinking. Therefore, stabilization/solidification (S/S) has drawn the attention of the related researchers. The S/S process was recognized as “the best demonstrated available technology” for the disposal of solid waste by the US Environmental Protection Agency (USEPA). It is proven that the leaching concentrations of arsenic and other heavy metals in the waste could be effectively decreased after it is stabilized/solidified with lime, cement, or smelting slag [18–21]. For the case of ASR, a significant drop in the mobility of arsenic was also confirmed after S/S [22,23]. However, it could be noted that, because of the involvement of the addition of other chemicals, the volume and/or weight would be increased, which could ultimately lead to an uneconomic disposal. Recent studies have shown that hydrothermal treatment can effectively achieve the stabilization of hazardous waste but also realize the dehydration of the waste and hence reduce the volume of waste [24]. Vinals et al. [25] found that arsenic in calcium arsenate waste from a copper smelter can be stabilized by hydrothermal treatment, generating the precipitation as arsenical natroalunite which is effective for long-term storage. Qiu et al. [26] indicated that hydrothermal treatment assisted by microwave heating is a feasible approach for the solidiﬁcation of heavy metals in ﬂy ash. Our previous studies [27–30] determined that stabilization of heavy-metal-containing neutralization sludge and dehydration of calcium sulfate dihydrate and calcium sulfate hemihydrate in the neutralization sludge occurred during hydrothermal treatment. To the best of our knowledge, no study concerning the stabilization, dehydration and volume reduction of ASR by hydrothermal treatment has been reported. Therefore, in this research, ASRs collected from various smelting companies were subjected to the hydrothermal treatment. It is expected that the leachability of arsenic and the moisture and volume in the ASRs will be greatly decreased.

2. Materials and Method

2.1. Materials Arsenic sulﬁde residues used in the experiments were obtained from three nonferrous metal smelting companies located in Hubei, Shandong and Anhui, China, which were marked as HB-ASR, SD-ASR and AH-ASR, respectively. The ASRs were generated during the treatment of acidic wastewater with H2S. The collected ASRs were stored in the companies for several months or years. The raw pH of ASR is measured in water suspensions with a mass ratio of ASR to solution of 1:10. The selected physicochemical properties of the raw ASRs are presented in Table1.

Table 1. The physicochemical properties of the raw ASRs.

Material Location Smelting Company Raw pH Moisture (%) As Content (%) HB-ASR Hubei Copper 1.41 62.9 35.1 SD-ASR Shandong Zinc-lead 1.22 54.4 34.2 AH-ASR Anhui Copper 0.93 39.0 25.7

2.2. Experimental Procedure The collected ASR (10 ± 0.01 g) without drying was added into a 20 mL autoclave, which was heated to different temperature and then maintained for various periods. Unless otherwise specified, the hydrothermal treatment experimental conditions of HB-ASR, SD-ASR and AH-ASR were performed at 160 ◦C for 4 h. Thereafter, the autoclave was cooled to room temperature under tap water. Finally, the resulting products were filtered with 1 µm filter papers. The treated residues were collected, washed with 10 mL deionized water, and then dried at 60 ◦C overnight in a vacuum oven. To evaluate the stability of the filter residue, the toxicity characteristic leaching procedure (TCLP) was Int. J. Environ. Res. Public Health 2018, 15, 1863 3 of 15 performed. The filtered hydrothermal fluid contained high concentrations of heavy metals, and the concentration of heavy metals were determined using ICE-AES.

2.3. Analysis

2.3.1. Determination of the Dehydration Ratio and Volume Reduction Ratio Moisture content was measured by drying the samples at 60 ◦C under vacuum to a constant weight (±0.01 g). The dehydration ratio was then calculated according to Equation (1). The volume of samples before and after hydrothermal treatment was measured by the method of water displacement [31]. The volume reduction ratio can then be calculated according to Equation (2):

M × ϕ − M0 × ϕ0 Dehydration ratio = × 100% (1) M × ϕ

V − V0 Volume reduction ratio = × 100% (2) V where M (g) is the raw weight of the ASR; φ (%) is the raw moisture of the ASR; M0 (g) is weight of the treated ASR; ϕ0 (%) is the moisture of the treated ASR; V (mL) is the raw volume of the ASR; V0 (mL) is the volume of the treated ASR.

2.3.2. Determination of Heavy Metal Content in ASRs and the Stabilization Ratio The contents of heavy metals such as As, Cd, Cr, Pb, Cu and Zn in the raw material and treated ASR were determined using ICP-AES. Prior to the ICP-AES tests, the sample was digested through microwave-assisted acid digestion according to the procedure described in our previous study [7]. The hydrothermal dehydration process produces a hydrothermal fluid, which results in the loss of some heavy metals. The stabilization ratio of heavy metals was used to evaluate the fixing efficiency of heavy metals in the treated ASR during the treatment, which was calculated according to Equation (3):

C × V Stabilization ratio = 1 − × 100% (3) M × (1 − ϕ) × w where C (g/mL) is the concentration of heavy metals in the hydrothermal solution after treatment; V (mL) is the volume of the hydrothermal solution; M (g) is the weight of the raw ASR; φ (%) is the raw moisture of the ASR; and w (%) is the heavy metal content of the raw ASR.

2.3.3. Leaching Test TCLP tests were performed using the USEPA method to determine the leachability of heavy metals in the samples. The detailed procedure of the TCLP tests was shown in our previous study [32].

2.3.4. Sequential Extraction A three-step extraction procedure was ﬁrst proposed by the Community Bureau Reference (BCR). In this work, Davidson’s three-stage BCR sequential extraction procedure was used to analyse the effective combination forms of heavy metals in the residues. The detailed procedure of BCR tests was shown in our previous study [33].

2.3.5. X-ray Photoelectron Spectroscopy (XPS) X-ray photoelectron spectroscopy (XPS) measurements were carried out on a K-Alpha 1063 system (Thermo Fisher Scientiﬁc, Waltham, Massachusetts, USA) using Al-Kα X-ray as the excitation source. The base pressure in the analysis chamber was on the order of 10−9 Torr. Peak shifts due to surface charging were taken into account by normalizing energies based on the adventitious carbon peak at 284.5 eV. Survey and narrow-scan XPS spectra were obtained using pass energies of 100 and Int. J. Environ. Res. Public Health 2018, 15, 1863 4 of 15

30 eV, respectively. Survey scans were used to determine the average composition of the surface. The semi-quantitative composition of the near-surface samples was calculated from the peak areas of the S(2p) and As(3d) peaks and normalized by their respective sensitivity factor [34]. Narrow-scan spectra were obtained in order to determine the S and As surface species.

Int. J. Environ. Res. Public Health 2018, 15, x 4 of 15 2.3.6. Others Analysis 2.3.6. Others Analysis The particle size of the raw ASR was analysed in water by a laser particle size analyser (LS-POP (6)). The crystallographicThe particle composition size of the raw of samplesASR was was analysed characterized in water by by a X-ray laser particle diffraction size (XRD,analyser D/max2550 (LS-POP VB + 18 KW)(6)). at The a speed crystallographic of 10◦ min− 1compositionin a 2θ range of fromsamples 10◦ wasto 80 characterized◦. Morphological by X- changeray diffraction of the samples (XRD, was D/max2550 VB + 18 KW) at a speed of 10° min−1 in a 2θ range from 10° to 80°. Morphological change observed through a scanning electron microscope (SEM-EDS, Nova NanoSEM 230, Brno, Czech Republic). of the samples was observed through a scanning electron microscope (SEM-EDS, Nova NanoSEM Raman spectra (LABRAM-HR 800 spectrometer, Renishaw inVia, Gloucestershire, UK) were recorded 230, Brno, Czech Republic). Raman spectra (LABRAM-HR 800 spectrometer, Renishaw inVia, with a 513-nm-wavelengthGloucestershire, UK) wer He-Lee recorded laser and with acquisition a 513-nm-wavelength time of 10 He s. -Le laser and acquisition time of 10 s. 3. Results and Discussion 3. Results and Discussion 3.1. Characterization of the Raw ASR 3.1. Characterization of the Raw ASR The characterization of the raw HB-ASR was selected as a representative to illustrate. The characterizationThe characterization of the raw of HB-ASR the raw is HB presented-ASR was in selected Figure1 .as As a shownrepresentative in Figure to 1illustrate.a,b,d, the The general characterization of the raw HB-ASR is presented in Figure 1. As shown in Figure 1a,b,d, the general particles were irregular in shape, and sulfur (S8) crystal structures were found. The amorphous particles particles were irregular in shape, and sulfur (S8) crystal structures were found. The amorphous agglomerate into large particles. The EDS results showed that the residue was mainly composed particles agglomerate into large particles. The EDS results showed that the residue was mainly of S andcomposed As, indicating of S and thatAs, indicating the amorphous that the particlesamorphous are particles As-S compounds. are As-S compounds. Figure1 Figured shows 1d that ◦ ◦ ◦ the threeshows broad that peaks the three occur broad at 2 θ peaksvalues occur near at 18 2θ , values 31 and near 57 18, similar°, 31° and to that57°, si ofmilar amorphous to that of As 2S3, as reportedamorphous [35]. As2S3, as reported [35].

FigureFigure 1. Characterization 1. Characterization of the of the raw raw HB-ASR: HB-ASR: ( a(a)) SEM;SEM; ( b)) EDS EDS;; (c ()c Particle) Particle size size distribution distribution;; and (d and) (d) X-ray diffractionX-ray diffraction pattern. pattern.

Figure 1c shows a wide range of particle size distribution from 0.5 μm to 60 μm. The size of the µ µ Figureparticles1c showswas distributed a wide range in four of concentrated particle size areas distribution of approximately from 0.5 0.52m μm, to 604.24 μm,m. The 16.11 size μm of the µ µ µ particlesand was 34.57 distributed μm. The median in four particle concentrated size (D50) areasof 6.23 of μm approximately indirectly reflected 0.52 thatm, most 4.24 smallm, 16.11particlesm and 34.57 µwerem. The agglome medianrated particle into large size particles. (D50) of A 6.23 leachingµm indirectlytest was performed reﬂected using that the most TCLP small method, particles and were agglomeratedthe results into indicated large particles.that the arsenic A leaching-leachate test concentration was performed was 300.54 using mg/L, the TCLPwhich is method, far greater and the resultsthan indicated the regulation that the limit arsenic-leachate of 5 mg/L for As. concentration was 300.54 mg/L, which is far greater than the regulation limit of 5 mg/L for As.

Int. J. Environ. Res. Public Health 2018, 15, 1863 5 of 15

Int. J. Int.Environ. J. Int.Environ. Res. J. Environ. Public Res. Public HealthRes. Public Health 2018, Health 2018,15, x 15,2018, x 15, x 5 of 155 of 155 of 15 Int. J. Int.Environ. J. Int.Environ. Res. J. Environ. Public Res. Public HealthRes. Public Health 2018, Health 2018,15, x 15,2018, x 15, x 5 of 155 of 155 of 15 3.2. Dehydration and Volume Reduction 3.2. Dehydration3.2. Dehydration3.2. Dehydration and Voluand meVolu and Reduction meVolu Reductionme Reduction 3.2. Dehydration3.2Table. Dehydration3.2. 2Dehydration shows and Voluand the meVolu photosand Reduction meVolu Reduction ofme ASRs Reduction before and after treatment. In terms of appearance, the yellow TableTable 2 showsTable 2 shows the2 shows photos the photos the of photos ASRs of ASRs before of ASRs before and before afterand after andtreatment. after treatment. treatment. 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Thisphenomenon indicated Thisphenomenon phenomenon the is reductionsimilar is similar is to similar inthe to band resultthe to gapresultthe result of Gibbsof Gibbs ofet Gibbsal .et [ 36al ].et [who36 al]. [who 36proposed] whoproposed proposed that thatthe variationthatthe variation the variation in appearance in appearance in appearance indicated indicated indicated the reductionthe reduction the reduction in band in band in band of resultingGibbsof Gibbs ofet Gibbsal from .et [ 36al the ].et [who36 al increase]. who[proposed36] whoproposed in pressure proposed that thatthe and variationthatthe temperature. variation the variationin appearance in Meanwhile, appearance in appearance indicated itindicated is obvious indicated the reductionthe that reduction the the reduction in muddy band in band or in band gap gapresulting gapresulting resultingfrom from the from increasethe increasethe increasein pressure in pressure in pressureand andtemperature. andtemperature. temperature. Meanwhile, Meanwhile, Meanwhile, it is itobvious is itobvious is thatobvious thatthe thatthe the gapﬁne gapresulting particles gapresulting resultingfrom of ASRsfrom the from changedincreasethe increasethe toincreasein densepressure in pressure smoothin pressureand solid andtemperature. blocks.andtemperature. temperature. Furthermore, Meanwhile, Meanwhile, Meanwhile, the it volumeis itobvious is itobvious shrinkageis thatobvious thatthe and thatthe the muddymuddy ormuddy fine or fineparti or fineparticles particlesof ASRs ofcles ASRs changedof ASRs changed changedto dense to dense tosmooth dense smooth solidsmooth solid blocks. solid blocks. Furthermore, blocks. Furthermore, Furthermore, the volumethe volumethe volume muddydewateringmuddy ormuddy fine or phenomenon fineparti or partifinecles particlesof ASRs can ofcles beASRs changedof preliminarily ASRs changed changedto dense to inferred dense tosmooth dense smooth by the solidsmooth appearance solid blocks. solid blocks. Furthermore, blocks. changes. Furthermore, Furthermore, the volumethe volumethe volume shrinkageshrinkageshrinkage and dewateringand dewateringand dewatering phenomenon phenomenon phenomenon can becan preliminarily becan preliminarily be preliminarily inferred inferred byinferred the by appearance the by appearance the appearance changes. changes. changes. shrinkageshrinkageshrinkage and dewateringand dewateringand dewatering phenomenon phenomenon phenomenon can becan preliminarily be can preliminarily be preliminarily inferred inferred byinferred the by appearance the by appearance the appearance changes. changes. changes. Table 2. Photos of the raw and treated ASRs. TableTable 2. PhotosTable 2. Photos of2. thePhotos of raw the of andraw the treatedand raw treated and ASRs treated ASRs. ASRs. . TableTable 2. PhotosTable 2. Photos of2. thePhotos of raw the of andraw the treatedand raw treated and ASRs treated ASRs. ASRs. . Classiﬁcation HB-ASR SD-ASR AH-ASR ClassificationClassificationClassification HB -ASRHB- ASRHB- ASR SD-ASRSD- ASRSD- ASR AH-ASRAH- ASRAH- ASR ClassificationClassificationClassification HB -ASRHB- ASRHB- ASR SD-ASRSD- ASRSD- ASR AH-ASRAH- ASRAH- ASR

BeforeBefore treatmentBefore treatmentBefore treatment treatment BeforeBefore treatmentBefore treatment treatment

AfterAfter treatmentAfter treatment treatment AfterAfter treatmentAfter treatmentAfter treatment treatment

TableTable 3 listsTable 3 liststhe 3 moisture, thelists moisture, the moisture, dehydration dehydration dehydration ratio ratio and ratio volumeand volumeand reduction volume reduction reduction ratio ratio for eachratio for each ASR.for eachASR. It is ASR. clearIt is clear It is clear TableTable 3 listsTable 3 liststhe 3 moisture, thelists moisture, the moisture, dehydration dehydration dehydration ratio ratio and ratiovolumeand volumeand reduction volume reduction reduction ratio ratio for eachratio for each ASR.for eachASR. It is ASR. clearIt is clear It is clear that thathydrothermalTable thathydrothermal3 hydrothermal lists thetreatment moisture, treatment treatment can dehydration effectivelycan effectivelycan effectively realize ratio realize and the realize volume dehydrationthe dehydration the reduction dehydration and andvolume ratio volumeand for reduction eachvolume reduction ASR. reduction for It iseach for clear each for each that thathydrothermal hydrothermalthat hydrothermal treatment treatment treatment can effectivelycan effectivelycan effectively realize realize the realize dehydrationthe dehydration the dehydration and andvolume volumeand reduction volume reduction reduction for each for each for each ASR.thatASR. The hydrothermalASR. Theraw rawTheASRs rawASRs have treatment ASRs have high have high cancontents high effectivelycontents contentsof moisture, of realize moisture, of moisture, theranging dehydrationranging fromranging from approximately andfrom approximately volume approximately reduction39.0% 39.0% to 39.0%62.9%. forto each62.9%. to 62.9%. ASR.ASR. TheASR. Theraw rawTheASRs rawASRs have ASRs have high have high contents highcontents contentsof moisture, of moisture, of moisture, ranging ranging fromranging from approximately from approximately approximately 39.0% 39.0% to 39.0%62.9%. to 62.9%. to 62.9%. Nevertheless,ASR.Nevertheless, TheNevertheless, raw the ASRs moisturethe moisturethe have moisturecontents high contents contents contentsfor allfor of the all moisture,for treatedthe all treatedthe rangingASRs treated ASRs were fromASRs were less approximately were lessthan lessthan 7%. than7%.The 39.0% 7%.Thecalculation Thecalculation to 62.9%. calculation Nevertheless,Nevertheless,Nevertheless, the moisturethe moisturethe moisturecontents contents contentsfor allfor the allfor treatedthe all treatedthe ASRs treated ASRs were ASRs were less were lessthan lessthan 7%. than7%.The The7%.calculation Thecalculation calculation resultsNevertheless,results ofresults the of dehydration the of the dehydration the moisture dehydration ratios contents ratios show ratios show forthat allshow thatmore the morethattreated than more than 89% ASRs than 89%moisture were 89%moisture less moisturein raw than in rawASRs 7%. in ASRsraw The can ASRs calculation becan dehydrated becan dehydrated be results dehydrated resultsresults ofresults the of dehydration the of dehydration the dehydration ratios ratios show ratios show that show thatmore morethat than more than 89% than 89%moisture 89%moisture moisturein raw in rawASRs in ASRsraw can ASRs becan dehydrated becan dehydrated be dehydrated viaof viahydrothermal the hydrothermalvia dehydration hydrothermal treatment. ratios treatment. showtreatment. Moreover, thatMoreover, moreMoreover, the than thevolume 89%thevolume moisturevolumeof allof ASRs all inof rawASRsall reduced ASRsASRs reduced can reduceddramatically be dramatically dehydrated dramatically after viaafter after via viahydrothermal hydrothermalvia hydrothermal treatment. treatment. treatment. Moreover, Moreover, Moreover, the thevolume thevolume volumeof allof ASRs allof ASRsall reduced ASRs reduced reduceddramatically dramatically dramatically after after after hydrothermalhydrothermalhydrothermalhydrothermal treatment. treatment. treatment. treatment. The Moreover, Thevolume Thevolume the reductionvolume volume reduction reduction ofratios all ratios ASRs exceeded ratios reducedexceeded exceeded 60% dramatically 60%(HB 60%-(HBASR -(HBASR 78.67%, after- ASR78.67%, hydrothermal SD78.67%,- ASRSD- ASRSD- ASR hydrothermalhydrothermalhydrothermal treatment. treatment. treatment. The Thevolume Thevolume reductionvolume reduction reduction ratios ratios exceeded ratios exceeded exceeded 60% 60%(HB 60%-(HBASR -(HBASR 78.67%,- ASR78.67%, SD78.67%,- ASRSD- ASRSD- ASR 71.42%treatment.71.42% and71.42% andAH The- ASRAHand volume- ASR AH60.10%).- ASRreduction60.10%). The60.10%). volumeThe ratios volumeThe exceeded reduction volume reduction 60% reduction ratio (HB-ASR ratio is higher ratio is higher 78.67%, iswhen higher when SD-ASR the when original the 71.42%original the ASRoriginal and ASR contains AH-ASR ASRcontains contains 71.42%71.42% and71.42% andAH- ASRAHand- ASR AH60.10%).- ASR60.10%). The60.10%). volumeThe volumeThe reduction volume reduction reduction ratio ratio is higher ratio is higher iswhen higher when the when original the original the ASRoriginal ASR contains ASRcontains contains more60.10%).more water.more water. The As water. volume a As result, a As result, reduction athe result, hydrothermalthe ratio hydrothermalthe hydrothermal is higher treatment whentreatment treatment theproduces original produces produces excellent ASR excellent contains excellentdehydration dehydration more dehydration water. and Asandvolume a result,andvolume volume moremore water.more water. As water. a As result, a As result, thea result, hydrothermalthe hydrothermalthe hydrothermal treatment treatment treatment produces produces produces excellent excellent excellentdehydration dehydration dehydration and andvolume andvolume volume reductionthereduction hydrothermalreduction of the of ASRs. the of treatmentASRs. the ASRs. produces excellent dehydration and volume reduction of the ASRs. reductionreductionreduction of the of ASRs. the of ASRs. the ASRs.

TableTableTable 3. The 3.Table 3The .comparison The comparison3 .comparison The comparison of moisture of of moisture, moisture of, moisturedehydration dehydration, dehydration, dehydration ratio ratio ratioand and volumeratioand volume volume and reduction volume reduction reduction reductionration ration ration for for eachration for each eachASR. for ASR. eachASR. ASR. TableTable 3. TheTable 3 .comparison The 3 .comparison The comparison of moisture of moisture of, dehydrationmoisture, dehydration, dehydration ratio ratioand volumeratioand volume and reduction volume reduction reductionration ration for eachration for eachASR. for ASR.each ASR. MoistureMoisture MoistureDehydration Dehydration Dehydration Ratio Ratio Volume Ratio Volume ReductionVolume Reduction Reduction Ratio Ratio Ratio ClassificationClassificationClassification Moisture MoistureMoisture MoistureDehydration Dehydration DehydrationDehydration Ratio Ratio Volume Ratio Ratio Volume VolumeReductionVolume Reduction Reduction Reduction Ratio Ratio Ratio Ratio ClassificationClassiﬁcationClassificationClassification (wt.%)(wt.%) (wt.%) (%) (%) (%) (%) (%) (%) (wt.%)(wt.%) (wt.%) (%) (%) (%) (%) (%) (%) BeforeBefore treatmentBefore treatment treatment 62.9 62.9(wt.%) 62.9 (%) (%) HB-ASRHB- ASRHBBefore- ASRBefore treatment Before treatment treatment 62.9 62.9 62.9 96.8096.80 96.80 78.6778.67 78.67 HB-ASRHB- ASRHBAfter- ASRBeforeAfter treatment treatmentAfter treatment treatment 4.9 4.9 62.9 4.9 96.8096.80 96.80 78.6778.67 78.67 HB-ASR AfterAfter treatmentAfter treatment treatment 4.9 4.9 4.9 96.80 78.67 BeforeAfterBefore treatment treatmentBefore treatment treatment 54.4 54.4 4.9 54.4 SD-ASRSD- ASRSDBefore- ASRBefore treatmentBefore treatment treatment 54.4 54.4 54.4 97.7997.79 97.79 71.4271.42 71.42 SD-ASRSD- ASRSDAfter- ASRBeforeAfter treatment treatmentAfter treatment treatment 2.6 2.6 54.4 2.6 97.7997.79 97.79 71.4271.42 71.42 SD-ASR AfterAfter treatmentAfter treatment treatment 2.6 2.6 2.6 97.79 71.42 BeforeAfterBefore treatment treatmentBefore treatment treatment 39.0 39.0 2.6 39.0 AH-ASRAH- ASRAHBefore- ASRBefore treatment Before treatment treatment 39.0 39.0 39.0 89.7489.74 89.74 60.1060.10 60.10 AH-ASRAH- ASRAHAfter- ASRBeforeAfter treatme treatmentAfter treatment treatment 6.5nt 6.5 39.0 6.5 89.7489.74 89.74 60.1060.10 60.10 AH-ASR 89.74 60.10 AfterAfterAfter treatme treatmentAfter treatment treatment 6.5nt 6.5 6.5 6.5 3.3. Variations3.3. Variations3.3. Variations in Heavy in Heavy Metalin Heavy Metal Contents Metal Contents Contents 3.3. Variations3.3. Variations3.3. Variations in Heavy in Heavy Metalin Heavy Metal Contents Metal Contents Contents 3.3. Variations in Heavy Metal Contents TableTable 4 Tableshows 4 shows 4the shows heavythe heavythe metal heavy metal contents metal contents contentsin eachin each inASR eachASR before ASRbefore and before andafter andafter treatment. after treatment. treatment. After After After TableTable 4 Tableshows 4 shows 4the shows heavythe heavythe metal heavy metal contents metal contents contentsin eachin each inASR eachASR before ASRbefore and before andafter andafter treatment. aftertreatment. treatment. After After After hydrothermalhydrothermalTablehydrothermal4 showstreatment, treatment, the treatment, heavy the contentsthe metal contentsthe contents contentsof As of inand As eachof andCu As ASR increasedandCu before increased Cu increased andsomewhat, aftersomewhat, treatment.somewhat, whereas whereas After whereasthe hydrothermalcontentsthe contentsthe contentsof of of hydrothermalhydrothermalhydrothermal treatment, treatment, treatment, the contentsthe contentsthe contentsof As of and As of andCu As increasedandCu increased Cu increased somewhat, somewhat, somewhat, whereas whereas whereasthe contentsthe contentsthe contentsof of of Cd,treatment, Cr,Cd, Pb Cr,Cd, and Pb theCr, Znand Pb contents were Znand were sliZn ofghtly were Asslightly and reduced. slightly Cu reduced. increased reduced.This Thismight somewhat, Thismight be mightattributed be attributed whereas be attributed to the the to different contentsthe to different the differentbehaviors of Cd, behaviors Cr, behaviors Pbof these and of these Zn of these Cd, Cr,Cd, Pb Cr,Cd, and Pb Cr, Znand Pb were Znand were sliZnghtly were slightly reduced. slightly reduced. reduced.This Thismight Thismight be mightattributed be attributed be attributed to the to different the to different the differentbehaviors behaviors behaviors of these of these of these heavywereheavy metals slightlyheavy metals during reduced.metals during the during This treatment.the might treatment.the betreatment. For attributed example,For example,For to theexample, some different some adsorbed some adsorbed behaviors adsorbed ions ions ofcould these ionscould be heavy couldwashed be metalswashed be off,washed during whileoff, whileoff, the while heavyheavy metalsheavy metals during metals during the during treatment.the treatment.the treatment. For example,For example,For example,some some adsorbed some adsorbed adsorbed ions ionscould ionscould be couldwashed be washed be off,washed whileoff, whileoff, while sometreatment.some sulfidessome sulfides For could sulfides example, could decompose could decompose some decompose adsorbed into intosoluble ionssolubleinto salts couldsoluble salts and be saltsvolatileand washed volatileand H off,volatile2S while[H372S]. [H37 some2S]. [37 sulfides]. could decompose somesome sulfidessome sulfides could sulfides could decompose could decompose decompose into intosoluble solubleinto salts soluble salts and saltsvolatileand volatileand H volatile2S [H372S]. [H372S]. [37]. into soluble salts and volatile H2S[37].

Int. J. Environ. Res. Public Health 2018, 15, 1863 6 of 15

Table 4. Heavy metal contents in the raw and treated ASRs.

Contents of Heavy Metals (%) Classiﬁcation As Cd Cr Pb Cu Zn Int. J. Environ. Res. Public Health 2018, 15, x 6 of 15 before treatment 35.1 0.0680 0.0003 0.2600 0.05 0.0820 HB-ASR afterTable treatment 4. Heavy metal 42.1 contents 0.0230 in the raw 0.0002 and treated 0.2500 ASRs. 0.06 0.0110

before treatment 34.2 0.0011Contents 0.0002of Heavy Metals 0.0660 (%) 0.31 0.0012 SD-ASR Classification after treatment 37.7As 0.0009Cd Cr 0.0001 Pb 0.0820Cu 0.38Zn 0.0011 before treatmentbefore treatmen25.7t 35.1 0.00560.0680 0.0003 0.0032 0.2600 0.0044 0.05 0.0820 0.71 0.0110 AH-ASR HB-ASR after treatmentafter treatment 43.0 42.1 0.00160.0230 0.0002 0.0010 0.2500 0.0042 0.06 0.0110 1.13 0.0026 before treatment 34.2 0.0011 0.0002 0.0660 0.31 0.0012 SD-ASR after treatment 37.7 0.0009 0.0001 0.0820 0.38 0.0011 Table5 shows the heavybefore metals treatment concentration 25.7 0.0056 of the 0.00 hydrothermal32 0.0044 0.71 solution 0.0110 during the treatment AH-ASR process. Clearly, it can be seenafter that treatment a large amount43.0 0.0016 of heavy 0.0010 metals 0.0042 were 1.13 dissolved 0.0026 in the hydrothermal process under high temperature and high pressure, thereby changing the content of heavy metals in the solidifiedTable body. 5 shows the heavy metals concentration of the hydrothermal solution during the treatment process. Clearly, it can be seen that a large amount of heavy metals were dissolved in the hydrothermalTable process 5. Concentration under high of temperature heavy metals and in thehigh hydrothermal pressure, thereby fluid ofchanging treated ASRs.the content of heavy metals in the solidified body. As Cd Cr Pb Cu Zn SampleTable 5. ConcentrationV (mL) of heavy metals in the hydrothermal fluid of treated ASRs. (mg/L) As Cd Cr Pb Cu Zn Sample V (mL) HB-ASR 6.1 3437.0 151.5(mg/L) 1.20 2.50 0.03 337.50 SD-ASRHB-ASR 5.3 6.1 2302.53437.0 151.5 2.0 1.20 0.53 2.500.98 0.03 0.90337.50 4.00 AH-ASRSD-ASR3.5 5.3 17,160.02302.5 46.02.0 0.53 33.00 0.981.00 0.90 0.044.00 124.00 AH-ASR 3.5 17,160.0 46.0 33.00 1.00 0.04 124.00 Since the hydrothermal fluid contained high concentrations of heavy metals, it was necessary to Since the hydrothermal fluid contained high concentrations of heavy metals, it was necessary to assessassess the stability the stability of heavy of metalsheavy metals in the hydrothermalin the hydrothermal process process and avoid and theavoid transfer the transfer of contaminants of into thecontaminants wastewater. into Figure the 2wastewater. shows the stabilizationFigure 2 shows ratio the for stabilization ASRs over theratio hydrothermal for ASRs over temperature the ◦ at 160hydrothermalC for 4 h. As temperature can be seen at 160 from °C Figurefor 4 h. 2As, the can stabilization be seen from Figure ratios 2 of, the heavy stabili metalszation ratios from of high to low wereheavy copper, metals lead, from arsenic, high to cadmium, low were chromium copper, lead, and arsenic, zinc, respectively. cadmium, chromium The stabilization and zinc, ratios of arsenic,respectively. copper and The lead stabilization were close ratios to 100%,of arsenic, while copper most and of thelead chromium were close to and 100%, zinc while were most dissolved of in the hydrothermalthe chromium fluidand zinc during were thedissolved dehydration in the hydrothe process.rmal It canfluid be during seen the that dehydration the amount process. of secondary It can be seen that the amount of secondary pollutants produced by the hydrothermal fluid was pollutants produced by the hydrothermal fluid was relatively small, and the impact on the quality of relatively small, and the impact on the quality of heavy metals in the solidified ASR block is not heavysignificant. metals in the solidified ASR block is not significant.

FigureFigur 2.e 2.Stabilization Stabilization ratiosratios of of heavy heavy metals metals for for thethe ASR ASRs.s.

3.4. Solidiﬁcation/Stabilization3.4. Solidification/Stabilization Effect Effect TableTable6 shows 6 shows the leachingthe leaching concentrations concentrations of of thethe ASRs before before and and after after treatment. treatment. The leaching The leaching concentrations of As, Cu, Cd, Cr and Zn declined significantly, especially for arsenic. According to concentrations of As, Cu, Cd, Cr and Zn declined signiﬁcantly, especially for arsenic. According to the Table 2, the muddy granular ASRs were changed into stabilized/solidified blocks under high

Int. J. Environ. Res. Public Health 2018, 15, 1863 7 of 15 the Table2, the muddy granular ASRs were changed into stabilized/solidified blocks under high temperature/pressure. This was favorable for reducing the leachability of these elements because of the decrease of the specific surface of the ASRs. In addition, some components of heavy metals were probably encapsulated in the densified structure during the condensation process.

Table 6. Leaching concentration of heavy metals in the raw and treated ASRs.

Leaching Concentration of Heavy Metals (mg/L) Classiﬁcation As Cd Cr Pb Cu Zn before treatment 300.54 11.20 0.13 2.45 0.05 0.08 HB-ASR after treatment 1.68 0.05 ND 1.49 0.01 0.03 before treatment 39.12 0.08 0.03 0.07 0.31 0.02 SD-ASR after treatment 1.11 0.02 ND 0.09 0.13 0.01 before treatment 3860.25 2.45 1.62 1.38 21.43 5.61 AH-ASR after treatment 126.30 0.15 0.66 0.53 0.32 0.64

3.5. Chemical Speciation of Heavy Metals Three-staged BCR sequential extraction is conducted to assess environmental activity and potential ecological risks. Generally, the arsenic and other heavy metals in acid soluble and reducible fraction is classified as direct effect phases for environmental availability and ecological risk because they are presented as a loosely bound phase or thermodynamically unstable phase, respectively, which are likely to release into the environment. Meanwhile, arsenic associated with the oxidizable fraction is identified as a potential effect fraction because it can be liberated or transformed into an acid soluble and reducible fraction under oxidizing conditions. Only the residual fraction is believed to be a stable fraction because it contains mainly primary and secondary minerals, which may retain metal elements within their crystal or glass structure [29]. The relative percentage of As and other heavy metals extracted in the different steps of the BCR test is presented in Table7. After the hydrothermal treatment, the acid soluble, reducible and oxidizable soluble fractions of arsenic dramatically decrease from 1.16%, 0.08% and 90.00% to 0.01%, 0.01% and 36.23%, respectively, indicating that direct toxicity effect fractions are reduced [30]. The acid soluble, reducible and oxidizable soluble states of other heavy metals (Cd, Cr, Cu and Zn) also significantly decreased. In terms of residual fractions, the arsenic and other heavy metals (Cd, Cr, Cu, Pb and Zn) of them increased 54.99%, 36.82%, 15.48%, 25.65%, 10.37%, and 43.63%, respectively. Thus, an important conclusion is that the chemical species of arsenic and other heavy metals in sludge are significantly transformed to residual fractions by the hydrothermal treatment, resulting in a restrained environmental availability.

Table 7. Chemical speciation of arsenic and heavy metals by BCR procedure (wt.%) in the raw and treated HB-ASR.

Classiﬁcation As Cd Cr Cu Pb Zn Acid soluble 1.16 17.59 12.61 0.49 1.12 22.96 Reducible 0.08 1.96 2.64 0.24 1.12 1.71 Before treatment Oxidizable 90.00 70.37 18.3 70.49 63.43 51.25 Residual 8.76 10.08 66.45 28.78 34.33 24.08 Acid soluble 0.01 0.40 1.66 0 19.49 1.03 Reducible 0.01 0 0 0 7.21 0.07 After treatment Oxidizable 36.23 51.98 16.41 45.57 28.6 31.19 Residual 63.75 47.62 81.93 54.43 44.7 67.71 Int. J. Environ. Res. Public Health 2018, 15, 1863 8 of 15

3.6. Phase Transformation Figure3 shows the XRD patterns of ASRs before and after treatment. As it can be seen from Figure3, the ASRs before and after treatment were mainly amorphous with only a few diffraction peaks. Only the peaks of arsenic trioxide were found in the raw AH-ASR, probably due to the oxidation of theInt. waste J. Environ. residue Res. Public during Health the2018,long-term 15, x storage. The crystalline form of sulfur (S8) was8 observed of 15 in 2− 2− the raw HB-ASR, which might be attributed to the reaction of S and SO3 in the previous waste observed in the raw HB-ASR, which might be attributed to the reaction of S2− and SO32− in the acid treatment process. No diffraction peaks were found in SD-ASR. previous waste acid treatment process. No diffraction peaks were found in SD-ASR.

FigureFigure 3. 3.The The XRD XRD patternspatterns of of the the raw raw and and treated treated ASR ASRs.s.

Moreover, for AH-ASR, As2O3 species were apparently detected in the raw material and the Moreover, for AH-ASR, As2O3 species were apparently detected in the raw material and the intensity of the As2O3 peaks increased somewhat after treatment. This could explain the decline in intensitythe As of leaching the As2 concentrationO3 peaks increased as the better somewhat crystalline after structure treatment. meant a Thislower could contact explain area with the the decline in theleac Ashing leaching agent. concentrationOn the other hand, as the it bettercould be crystalline found that structure the crystalline meant sulfur a lower (S8) contactin HB-ASR area with the leachingcompletely agent. disappeared On the after other the hand, treatment. it could During be foundthe hydrothermal that the crystalline process, the sulfur sulfur (S(S88) could in HB-ASR completelydecompose disappeared into H2S by after disproportionation the treatment. reaction During [38 the]. T hydrothermalhe H2S gas could process, precipitate the heavy sulfur metals (S8) could and thus reduced their leaching concentrations. For SD-ASR, it was amorphous and no changes on decompose into H2S by disproportionation reaction [38]. The H2S gas could precipitate heavy metals and thusits XRD reduced pattern their could leaching be observed concentrations. after the treatment. For SD-ASR, Surprisingly, it was amorphous no new crystalline and no phase changes of on its arsenic sulfide compounds, such as realgar and orpiment, were found after hydrothermal treatment. XRD pattern could be observed after the treatment. Surprisingly, no new crystalline phase of arsenic This is probably because under these conditions it was difficult to generate crystalline As2S3 (c-As2S3) sulfide[39,40 compounds,]. Although such the XRD as realgar analysis and results orpiment, can explain were thefound decrease after in the hydrothermal leaching concentrations treatment. of This is probablysome because metals, it under is insufficient these conditions and the reason it was for difficult dehydration to generate and volume crystalline reduction As 2mayS3 (c-As be related2S3)[ 39,40]. Althoughto thethe morphology XRD analysis of the ASRs, results which can will explain be discussed the decrease in the following in the leaching section. concentrations of some metals, it is insufficient and the reason for dehydration and volume reduction may be related to the morphology3.7. Morphology of the ASRs,Change which will be discussed in the following section. The SEM images of treated HB-ASR were selected as a representative to illustrate the 3.7. Morphologymorphology Change changes during hydrothermal treatment. Figure 4 shows that there are obvious differences in the morphology of the HB-ASR with different treatment times. Before treatment, the The SEM images of treated HB-ASR were selected as a representative to illustrate the morphology ASR was flocculent particles that were composed of extremely fines of arsenic sulfide (Figure 4a,b). changesAfter during being treated hydrothermal for 120 min, treatment. the flocculent Figure particles4 shows bonded that together there areand obvious formed a differencesnetwork of a in the morphologyporous body of the (Figure HB-ASR 4c,d). with In our different experiments, treatment it was times. found Beforethat the treatment, product generated the ASR under was this flocculent particlescondition that were was very composed easily crushed. of extremely Finally, fines a large of bulk arsenic with sulfide a smooth (Figure surface4a,b). was Afterobtained being after treated 240 for 120 min,min the (Figure flocculent 4e,f). There particles were bondedsome spherular together pits and on the formed fracture a network surface of of the a porousbulk, which body might (Figure be 4c,d). In ourcause experiments,d byH2S gas it generated was found by sulfur that the decomposition. product generated Because the under amorphous this condition and flocculent was veryASR easily crushed.was Finally,converted a large into a bulk large with bulk, a smooththe water surface content was and obtained volume of after ASR 240 reduced min (Figure dramatically.4e,f). There This were phenomenon was also found in the process of coal treatment by a hydrothermal method [35]. In some spherular pits on the fracture surface of the bulk, which might be caused byH2S gas generated summary, the densification of ASRs in the hydrothermal process is the main reason for volume by sulfur decomposition. Because the amorphous and flocculent ASR was converted into a large reduction, dehydration and stabilization/solidification. bulk, the water content and volume of ASR reduced dramatically. This phenomenon was also found in the process of coal treatment by a hydrothermal method [35]. In summary, the densification

Int. J. Environ. Res. Public Health 2018, 15, 1863 9 of 15 of ASRs in the hydrothermal process is the main reason for volume reduction, dehydration and Int.stabilization/solidiﬁcation. J. Environ. Res. Public Health 2018, 15, x 9 of 15

Int. J. Environ. Res. Public Health 2018, 15, x 9 of 15

Figure 4. SEM images of raw HB-ASR (a,b) and treated HB-ASRs (c,d) treated for 120 min, (e,f) Figure 4. SEM images of raw HB-ASR (a,b) and treated HB-ASRs (c,d) treated for 120 min, (e,f) treated treated for 240 min). forFigure 240 min). 4. SEM images of raw HB-ASR (a,b) and treated HB-ASRs (c,d) treated for 120 min, (e,f) treated for 240 min). 3.8. The Analysis of Raman and XPS 3.8. The Analysis of Raman and XPS 3.8. The Analysis of Raman and XPS The Raman technique was further applied to investigate the structural variation in the treated The Raman technique was further applied to investigate the structural variation in the treated HB-ASRThe (Figure Raman 5). technique In the raw was ASR, further four applied peaks tolocated investigate at approximately the structural ~153, variation ~219 ,in ~340, the treated and ~474 HB-ASR (Figure5). In the raw ASR, four peaks located at approximately ~153, ~219, ~340, cmH−1B -wereASR (found.Figure 5).The In peak the raw at ~340ASR, cmfour−1 peaksindicated located the at existence approximately of arsenic(III) ~153, ~219 sulfide, ~340, [41,42and ~474]. The and ~474 cm−1 were found. The peak at ~340 cm−1 indicated the existence of arsenic(III) sulfide [41,42]. otherscm−1 (~153,were found. ~219, andThe ~474 peak cm at −1~340) suggested cm−1 indicated the presence the existence of sulfur of [ 43arsenic(III)]. After the sulfide sample [41,42 was]. treatedThe The others (~153, ~219, and ~474 cm−1) suggested the presence of sulfur [43]. After the sample was forothers 120 min,(~153, the ~219 peak, and at~474 362 cm cm−1) suggested−1 was found the presenceand the of intensity sulfur [43 became]. After thestronger sample after was treated240 min, treated for 120 min, the peak at 362 cm−1 was found and the intensity became stronger after 240 min, indicatingfor 120 min, that thearsenic(II) peak at sulfide362 cm −1became was found clearly and observable the intensity [44 –became46]. The s trongertrend of after the 240peak min, (~340 indicating that arsenic(II) sulfide became clearly observable [44–46]. The trend of the peak (~340 cm−1) indicating−1 that arsenic(II) sulfide−1 became clearly observable [44–46]. The trend of the peak (~340 cm ) shifting to peak (~362−1 cm ) was probably a reflection of the decomposition of arsenic(III) shifting−1 to peak (~362 cm ) was probably−1 a reflection of the decomposition of arsenic(III) sulfide into sulfidecm ) intoshifting arsenic(II) to peak sulfide. (~362 cm ) was probably a reflection of the decomposition of arsenic(III) arsenic(II)sulfide into sulfide. arsenic(II) sulfide.

FigureFigure 5. 5. TheThe Raman Raman spectra ofof the the raw raw and and treated treated HB HB HB-ASR.-ASR-ASR. .

XPS survey-spectra of the raw HB-ASR and sample treated for 240 min are shown in Figure 6. It XPS survey survey-spectra-spectra of of the the raw raw HB HB-ASR-ASR and and sample sample treated treated for for 240 240 min min are are shown shown in inFigure Figure 6. 6It. indicates the presence of S, As and O. It was obvious that the intensity of peak O 1s was lower after indicatesIt indicates the the presence presence of of S, S, As As and and O. O. It It was was obvious obvious that that th thee intensity intensity of of peak peak O O 1s 1s was was lower lower after the treatment. The As 3d and S 2p spectra for the raw and treated sample are presented in Figure 7. the treatment. The The As As 3d 3d and and S S 2p 2p spectra spectra for for the the raw raw and and treated treated sample sample are are presented presented in in Figure Figure 77.. The Gaussian-Lorentzian resolving was performed to analyse the components of the sample [47]. The Gaussian-Lorentzian resolving was performed to analyse the components of the sample [47]. Raw spectra were fitted using a least-squares procedure with peaks of convoluted Gaussian (80%) Raw spectra were fitted using a least-squares procedure with peaks of convoluted Gaussian (80%) and Lorentzian (20%) peak shape after subtraction of a Shirley baseline. The S 2p spectra were and Lorentzian (20%) peak shape after subtraction of a Shirley baseline. The S 2p spectra were modeled as doublets of 2p1/2 and 2p3/2, separated by 1.2 eV and the area of the S 2p1/2 peak was half modeled as doublets of 2p1/2 and 2p3/2, separated by 1.2 eV and the area of the S 2p1/2 peak was half the area of S 2p3/2 peak. The As 3d spectra were modeled as doublets of 3d3/2 and 3d5/2, separated by the area of S 2p3/2 peak. The As 3d spectra were modeled as doublets of 3d3/2 and 3d5/2, separated by

Int. J. Environ. Res. Public Health 2018, 15, 1863 10 of 15

The Gaussian-Lorentzian resolving was performed to analyse the components of the sample [47]. Raw spectra were ﬁtted using a least-squares procedure with peaks of convoluted Gaussian (80%) and Lorentzian (20%) peak shape after subtraction of a Shirley baseline. The S 2p spectra were modeled as doublets of 2p1/2 and 2p3/2, separated by 1.2 eV and the area of the S 2p1/2 peak was half the area of Int.Int. J.J. Environ.Environ. Res.Res. PublicPublic HealthHealth 2018,2018, 15,15, xx 1010 ofof 1515 S 2p3/2 peak. The As 3d spectra were modeled as doublets of 3d3/2 and 3d5/2, separated by 0.7 eV. The area of the As 3d peak3/2 was two-thirds the area of the As 3d peak [5/248]. A higher binding 0.70.7 eV.eV. TheThe areaarea ofof thethe3/2 AsAs 3d3d3/2 peakpeak waswas twotwo--thirdsthirds thethe areaarea ofof thethe5/2 AsAs 3d3d5/2 peakpeak [[4848]].. AA higherhigher bindingenergybinding is energyenergy indicative isis indicativeindicative of a higher ofof oxidation aa higherhigher state oxidationoxidation of arsenic statestate and ofof a lowerarsenicarsenic binding andand aa energylowerlower correspondsbindingbinding energyenergy to a correspondslowercorresponds oxidation toto aa state. lowerlower oxidationoxidation state.state.

FigureFigure 6.6. XPSXPSXPS spectraspectra spectra ofof of thethe the rawraw raw HBHB HB-ASR--ASRASR andand and thatthat that treatedtreated treated forfor for 240240 240 minmin min usingusing using aa highhigh a high passpass pass energyenergy energy ofof 100100 of eV100eV.. eV.

FigureFigure 7. 7. XPSXPS spectra spectra of of S S 2p 2p andand AsAs 3d3d peakspeaks forfor thethethe raw rawraw HB-ASR HBHB--ASRASR and andand that thatthat treated treatedtreated for forfor 240 240240 min minmin using usingusing a alowa lowlow pass passpass energy energyenergy of ofof 30 3030 eV. eVeV..

TheThe surfacesurface compositionscompositions ofof sulfursulfur areare shownshown inin FigureFigure 77 andand TableTable 88.. TheThe majormajor peakspeaks ofof thethe SS The surface compositions of sulfur are shown in Figure7 and Table8. The major peaks of the S 2p 3/2 2p3/2 spectrum of raw ASR were located at 162.80 and 164.02 eV, which were assigned to 2pspectrum3/2 spectrum of raw of ASR raw were ASR located were at located 162.80 andat 164.02162.80 eV,and which 164.02 were eV assigned, which towere orpiment-like assigned S2to− 2− orpiment-like S2− and S(0), respectively. The reported precipitates of arsenic sulfide formed in the orpimentand S(0), respectively.-like S and TheS(0), reported respectively. precipitates The reported of arsenic precipitates sulfide formed of arsenic in the As(III) sulfide removing formed processin the 3/2 As(III)As(III) removingremoving processprocess showedshowed thethe SS 2p2p3/2 bindingbinding energiesenergies ofof 162.6162.6 eVeV andand 163.1163.1 eV,eV, whichwhich werewere showed the S 2p3/2 binding energies of 162.6 eV and 163.1 eV, which were assigned to the orpiment-like assignedassigned toto thethe orpimentorpiment--likelike sulfidesulfide ionion ofof SS--As(III)As(III) andand realgarrealgar--likelike ssulfideulfide ionion ofof SS--As(II),As(II), respectivelyrespectively [49,50[49,50]].. AsAs shownshown inin TableTable 6,6, afterafter thethe treatment,treatment, thethe atomatom ratioratio ofof SS speciesspecies increasedincreased fromfrom 67%67% toto 71%.71%. InIn addition,addition, thethe S(0)S(0) speciesspecies accountedaccounted forfor 49%49% ofof thethe totaltotal sulfursulfur concontenttent inin thethe rawraw ASRASR andand itit increasedincreased toto 54%54% afterafter treatment.treatment. Finally,Finally, itit isis notednoted thatthat thethe realgarrealgar--likelike sulfidesulfide ionion ofof SS--As(II)As(II) alsoalso increasedincreased toto 11%.11%.

Int. J. Environ. Res. Public Health 2018, 15, 1863 11 of 15 sulfide ion of S-As(III) and realgar-like sulfide ion of S-As(II), respectively [49,50]. As shown in Table6, after the treatment, the atom ratio of S species increased from 67% to 71%. In addition, the S(0) species accounted for 49% of the total sulfur content in the raw ASR and it increased to 54% after treatment. Finally, it is noted that the realgar-like sulfide ion of S-As(II) also increased to 11%. Int. J. Environ. Res. Public Health 2018, 15, x 11 of 15 Table 8. Peak areas for component peaks used in ﬁtting the As (3d) and S (2p) peaks for the raw HB-ASRTable and8. Peak treated areas ASRs. for component peaks used in fitting the As (3d) and S (2p) peaks for the raw HB-ASR and treated ASRs. Bending Energy and Percent Peak Area of Each Component Bending Energy and Percent Peak Area of Each Component As 3d S 2p As 3d5/2 S 2p3/2 Sample As 3d S 2p As 3d5/2 S 2p3/2 Sample As AsS SAs(II)-S As(II)-S As(III) As(III)-S-S As(III) As(III)-O-O S-S S-S S-As(III) S-As(III) S-As(II) S-As(II) BE 43.83 43.83163.73 163.7343.10 43.10 43.48 43.48 44.8044.80 164.02164.02 162.80162.80 163.10163.10 Raw BE (FWHM) Raw ASR (FWHM) (2.03) (2.03)(2.86) (2.86)(1.00) (1.00)(1.00) (1.00) (1.00)(1.00) (1.37)(1.37) (1.37)(1.37) (1.37)(1.37) ASR PeakPeak areas areas 33% 33%67% 67%0 072% 72%28% 28% 49% 49% 51% 51% 0 0 BE 43.69 43.69163.58 163.5843.28 43.28 43.42 43.42 44.5044.50 163.66163.66 162.64162.64 163.10163.10 Treated BE (FWHM) Treated ASR (FWHM) (1.61) (1.61)(2.47) (2.47)(1.00) (1.00)(1.00) (1.00) (1.00)(1.00) (1.37)(1.37) (1.37)(1.37) (1.37)(1.37) ASR PeakPeak areas areas 29% 29%71% 71%17% 17%70% 70%13% 13% 54% 54% 35% 35% 11% 11% BE:BE: Bending Bending energy energy (FWHM); (FWHM); Narrow Narrow-scan-scan XPSXPS spectraspectra were were obtained obtained using using pass pass energies energies of 30 of eV. 30 eV.

The binding energy of the As 3d5/2 peak is fitted with three As components consisting of As(III) The binding energy of the As 3d5/2 peak is fitted with three As components consisting of As(III) and and As(II). The reported peaks of As 3d5/2 at 43.1, 43.4 and 44.8 eV were attributed to As(II)-S, As(II). The reported peaks of As 3d5/2 at 43.1, 43.4 and 44.8 eV were attributed to As(II)-S, As(III)-S and As(III)-O,As(III)-S respectively and As(III)- [O,4,27 respectively,48,51]. As shown[4,27,48,51 in Table]. As6 shown, before in the Table treatment, 6, before the the content treatment, of As(III)-S the (72%)content predominated of As(III)-S the (72%) As speciation,predominated followed the As by speciation, As(III)-O followed (28%). However, by As(III) the-O content(28%). However, of As(III)-S the content of As(III)-S decreased to 70% and new As(II)-S reached to 17% after the hydrothermal decreased to 70% and new As(II)-S reached to 17% after the hydrothermal treatment. This change treatment. This change was consistent with results of Raman analysis. On the other hand, Gallegos et was consistent with results of Raman analysis. On the other hand, Gallegos et al. [52] reported it is al. [52] reported it is thermodynamically favorable for As(III) sulfide to decompose into As4S4-like thermodynamically favorable for As(III) sulfide to decompose into As S -like phase and S under reducing phase and S under reducing conditions. Hence, it can be reasoned that4 4 the hydrophobic sulfur melts conditions. Hence, it can be reasoned that the hydrophobic sulfur melts to a liquid to encapsulate the to a liquid to encapsulate the arsenic sulfide compounds, making the particles bond together during arsenicthe hydrothermal sulfide compounds, process and making thus theresulting particles in the bond dehydration, together during volume the reduction hydrothermal and S/S process of heavy and thusmetals. resulting in the dehydration, volume reduction and S/S of heavy metals. 0 BasedBased on on the the SEM, SEM, Raman Raman and and XPS XPS results,results, itit isis believedbelieved that the the hy hydrophobicdrophobic sulfur sulfur (S (S0) and) and reactionreaction of of As(III)-S As(III)-S had had a a significant significant influence influence onon thethe densificationdensification of of ASR. ASR. The The schematic schematic diagram diagram forfor this this mechanism mechanism is is illustrated illustrated in in Figure Figure8 .8. First, First, As(III)As(III) sulfidesulfide generates hydrophobic hydrophobic sulfur sulfur and and As(II)As(II) sulfide sulfide under under reductive reductive conditions. conditions. Second,Second, the fine fine particles particles of of As(III) As(III) sulfide sulfide or or As(II) As(II) sulfide sulfide werewere bound bound together together by by the the melted melted sulfur.sulfur. Finally,Finally, the small formed formed pieces pieces grew grew into into a alarger larger ASR ASR bulkbulk under under high high pressure pressure due due to adhesionto adhesion by sulfur.by sulfur. The The hydrophobicity hydrophobicity of sulfur of sul mightfur might be the be reason the forreason the satisfactory for the satisfactory results of results dehydration of dehydration and volume and volume reduction. reduction.

FigureFigure 8. 8.Schematic Schematic illustrationillustration of the hydrothermal procedure procedure..

Int. J. Environ. Res. Public Health 2018, 15, 1863 12 of 15

4. Conclusions This study reported the hydrothermal treatment of ASRs for the purpose of dehydration, volume reduction and S/S. The results show that hydrothermal treatment had obvious effects on the dehydration and volume reduction of ASRs. The moisture contents of treated ASR were less than 7% and the dehydration ratios reached 89%~97%. The slurry residues were changed into a hard bulk solid and the volume of the ASRs was reduced by 60%~78%. The stabilization ratios of arsenic, copper and lead were close to 100%, while most of the chromium and zinc was dissolved in the hydrothermal fluid during the dehydration process. After the treatment, the leaching concentrations of As, Cd, Cr and Zn declined significantly and the available arsenic in sludge is significantly transformed to a residual fraction. Based on the further analysis, it was supposed that the As(III) sulfide generates hydrophobic sulfur and As(II) sulfide under reductive conditions. The densification by melting hydrophobic sulfur might play an important role in the dehydration, volume reduction and the decline of heavy metals leaching concentrations. The presented research offers a simple and efficient process for the treatment of ASRs. However, the evidence from XPS and Raman for the densification mechanism of ASRs is still insufficient. In particular, the action mechanism of action of sulfur requires further deeper analysis and demonstration. Overall, more work has to be done to determine the mechanism of arsenic sulfide and sulfur structural changes during the hydrothermal process and it is also very necessary to perform further tests about its impact on long term landfilling so that the approach can be more reliable and effective.

Author Contributions: Conceptualization, X.M. & Y.K.; Data curation, L.Y; Formal analysis, L.Y., Y.L. & K.Y.; Funding acquisition, X.M.; Investigation, L.Y., H.X. & K.Y.; Methodology, L.Y.; Project administration, X.M.; Resources, Y.K.; Software, Y.K.; Writing—original draft, L.Y.; Writing—review & editing, X.M. Funding: This research was funded by National Key R&D Program of China (2017YFC0210402), the key project of the National Natural Science Foundation of China (51634010), the Natural Science Foundation of China (51474247 and 51474102), and the China Postdoctoral Science Foundation (2017M612583). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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