Insights into solid phase characteristics and release of heavy metals and arsenic from industrial sludge via combined chemical, mineralogical, and microanalysis

Environ Sci Pollut Res (2015) 22:2205–2218 DOI 10.1007/s11356-014-3438-y RESEARCH ARTICLE Insights into solid phase characteristics and release of heavy metals and arsenic from industrial sludge via combined chemical, mineralogical, and microanalysis Tran Thi Thu Dung & Asefeh Golreihan & Elvira Vassilieva & Nguyen Ky Phung & Valérie Cappuyns & Rudy Swennen Received: 16 May 2014 / Accepted: 10 August 2014 / Published online: 31 August 2014 # Springer-Verlag Berlin Heidelberg 2014 Abstract This study investigates the solid phase characteristics and release of heavy metals (i.e., Cd, Co, Cu, Cr, Mo, Ni, Pb, and Zn) and arsenic (As) from sludge samples derived from industrial wastewater treatment plants The emphasis is determining the influence of acidification on element mobilization based on a multidisciplinary approach that combines cascade and pHstat leaching tests with solid phase characterization through X-ray diffraction (XRD), field emission gun electron probe micro analysis (FEG-EPMA), and thermodynamic modeling (Visual MinteQ 3.0) Solid phase characterization and thermodynamic modeling results allow prediction of Ni and Zn leachabilities FEG-EPMA is useful for direct solid phase characterization because it provides information on additional phases including specific element associations that cannot be detected by XRD analysis Cascade and pHstat leaching test results indicate that disposal of improperly treated sludges at landfills may lead to extreme environmental risks due to high leachable concentrations of Zn, Ni, Cu, Cr, Responsible editor: Philippe Garrigues T T T Dung : A Golreihan : E Vassilieva : V Cappuyns : R Swennen Department of Earth and Environmental Sciences, KU Leuven, Celestijnenlaan 200E, 3001 Leuven, Belgium V Cappuyns Faculty of Business and Economics, KU Leuven, Warmoesberg 26, 1000 Brussels, Belgium T T T Dung (*) University of Science, Faculty of Environment, Vietnam National University Ho Chi Minh City, 227 Nguyen Van Cu St., W4, D5, Ho Chi Minh City, Vietnam e-mail: tttdung@hcmus.edu.vn N K Phung Department of Science and Technology, 244 Dien Bien Phu St., W7, D3, Ho Chi Minh City, Vietnam and Pb However, high leachabilities under acid conditions of Ni and Zn as observed from pHstat leaching test results may provide a potential opportunity for acid extraction recovery of Ni and Zn from such sludges Keywords Heavy metals and arsenic Industrial sludge Cascade leaching test pHstat leaching test XRD FEG-EPMA Introduction All over the world, urbanization and industrialization have increased the production of sludge (Kazi et al 2005), especially in developing countries such as Vietnam Sludge generated from wastewater treatment plants is usually treated as solid waste and classified as hazardous or non-hazardous based on its constituents and leachability (VMONRE 2013; US EPA 2011) In Vietnam, management options for sludge include landfill disposal, stabilization/solidification, and incineration (VMOST 2004) The option of choice depends on the origin of the sludge and leachable concentrations of contaminants (VMONRE 2013) However, these management options apply only to sludge generated at factories registered at the National Environmental Protection Agency, the source of 65 % by mass of total sludge production (LBCD & Experco International 2010) The remaining sludge is disposed of at illegal dump sites Depending on its origin, sludge composition can vary from highly organic (domestic sludge) to mineral-enriched (industrial sludge) (Perez-Cid et al 2002) Industrial sludge that contains high quantities of heavy metals (HMs) and As can contaminate soil and groundwater, even when located in landfills (Salado et al 2008) In recent years, due to the increasing volume of industrially derived sludge 2206 Environ Sci Pollut Res (2015) 22:2205–2218 and associated environmental concerns, HMs and arsenic in sludge have become a popular research topic (Perez-Cid et al 2002; Kazi et al 2005; Hsieh et al 2008; Jamali et al 2009a; Jamali et al 2009b; Wu et al 2009; Gao et al 2012; Ozdemir and Piskin 2012; van der Sloot and Kosson 2012; Huang et al 2013; Milinovic et al 2014) Studies mostly deal with sludge bulk composition and leaching properties (Orescanin et al 2009; van der Sloot and Kosson 2012; Milinovic et al 2014) or possible treatment options—recovery of valuable metals via vitrification (Huang et al 2013; Chou et al 2012), acid leaching (Li et al 2010), solidification to immobilize HMs and As in Portland cement, or bio-stabilization via use of iron reducing microorganisms (Papassiopi et al 2009) Understanding chemical and mineralogical properties of industrial sludge and the factors that may influence release of HMs and As provides essential information for defining management options The present study investigates leaching of HMs (Cd, Co, Cu, Cr, Mo, Ni, Pb, and Zn) and As from industrial sludge using the cascade and pHstat leaching tests The focus is determining the effect of acidification on element mobilization Given that the leaching behavior of solid materials is largely dependent on the mineralogical characteristics of the solid phase (Ettler et al 2003), the second objective involves identification of possible mineralogical phases in sludge via X-ray diffraction (XRD) and field emission gun electron probe micro analysis (FEG-EPMA) Mineralogical composition can be related to leaching of particular elements Furthermore, the environmental risk of HMs and As is strongly dependent on chemical speciation We present predictions of leachate HM and As species distributions based on Visual MinteQ 3.0 thermodynamic models In this study, the term “heavy metals” denotes the elements Cd, Cr, Co, Cu, Ni, Pb, and Zn Arsenic (As), which is actually a metalloid, is mentioned separately Material and methods Sampling and sample pretreatment Three industrial sludge samples with different chemical composition originating from three wastewater treatment plants were collected in Binh Duong and Dong Nai provinces (southern Vietnam) in February 2013 These provinces host chemical, garment, shoe and leather, metal plating, iron and steel processing, and export industries (Dore et al 2008) Two of the industrial sludge samples were collected from centralized wastewater treatment plants in the industrial parks of Dong Nai (sample SI1) and Binh Duong (sample SI2) that accept the effluent from a variety of nearby industrial factories The third industrial sludge (sample SE) originated from an electroplating wastewater treatment plant in Binh Duong Electroplating sludge is regarded as a hazardous waste in Vietnam (VMONRE 2013) The dried bulk sample SI2 collected for use in experiments weighed ∼1 kg, while the wet samples SI1 and SE weighed ∼3 kg After collection, the samples were placed in sealed plastic bags and transported to KU Leuven, Belgium, for further analysis The moisture contents of samples SI1 and SE were, respectively, determined from the weight difference between the wet and the dry samples (105 °C) The moisture content was 90 % for SI1 and 79 % for SE Each sample was divided into two portions One portion was then dried in an oven at 60 °C until constant weight The other portion was air-dried The oven-dried samples were used for bulk chemical and mineralogical sample characterization, while the air-dried samples were used for leaching tests Once dry, the samples were ground and homogenized in a porcelain mortar and sieved through a 2-mm sieve Chemical characterization In this study, the term “total concentrations” applies to geochemical data collected using a three-acid digestion method, where samples were digested by HNO3conc, HClO4conc, and HFconc in a Teflon beaker on a hot plate All glassware was first rinsed with HNO3 0.5 mol/L and all reagents were of analytical grade Total concentrations of the elements Al, Ca, Fe, K, Mg, P, S, As, Cd, Co, Cr, Cu, Mn, Mo, Ni, Pb, and Zn were then determined via ICP-OES (Varian 720ES) In order to evaluate the quality of the analytical method, a certified reference material (NIST2782, industrial sludge) and sample duplicates were analyzed The comparison between the measured concentrations with the certified data in the certified reference material is given in Table The pH of each sample was measured (pH Hamilton single-pore electrode, calibrated at pH and 7) in a suspension solution of 5.0 g of sludge in 25.0 mL water, following shaking for h The content of Table Comparison between the measured concentrations with the certified data in the certified reference material (NIST2782, industrial sludge) (average±standard deviation of two replicates) Measured value Certified value As (mg/kg) Cd (mg/kg) Co (mg/kg) Cr (mg/kg) Cu (mg/kg) Mo (mg/kg) Ni (mg/kg) Pb (mg/kg) Zn (mg/kg) 123±2 166 6±0 4.17 57±1 66.3 102±12 109 2,778±15 2,594 11±4 10.07 136±22 154.1 509±69 574 1,158±136 1,254 Environ Sci Pollut Res (2015) 22:2205–2218 organic matter was determined by the Walkley and Black manual titration method (Nelson and Sommers 1982) Solid phase characterization We performed XRD analysis on the original samples and on residues from the pHstat test in order to identify any changes in the mineralogical composition of the solid phase during the leaching test (Fig 1) A Philips PW1830 diffractometer with Bragg/Brentano θ–2θ setup, CuK radiation, 45 kVand 30 mA, and graphite monochromator was used Data from FEGEPMA analysis were used to complement the XRD characterization For FEG-EPMA analysis, sludge samples were embedded in a resin and prepared as polished thin sections coated with a ∼14-nm-thick carbon layer The polished thin sections were examined with a Jeol JXA8530F machine, with energy dispersive spectrometer (EDS) mode (in spot analysis) or wavelength dispersive spectrometer mode (WDS) (in mapping mode) Leaching tests Cascade leaching test We used a cascade leaching test (CLT, NEN 7349 1995) to assess the extent of leaching as a function of liquid/solid (L/S) ratio This is a serial batch test in which material is successively extracted five times, resulting in L/S ratios of 20, 40, 60, 80, and 100 (L/kg) This test was replaced by compliance test NEN-EN 12457-3:2002 in 1999 consisting of two successive extractions at liquid-to-solid ratio and L/kg dry matter 2207 However, because our interest is leaching characteristics rather than compliance with environmental standards, we chose to use CLT in the present study Extractions were carried out in duplicate in acid-rinsed 50 mL polyethylene centrifuge tubes with screw caps Thirty milliliters of Milli-Q water, acidified to pH with HNO3 (ultrapure), was added to 1.5 g of dry sample, shaken (for 24 h), centrifuged at 3,000 rpm for 10 min, and filtered (0.45 μm, Chromafil® PET-45/25, Macherey) No pH adjustment was performed during the test pHstat leaching test We performed a pHstat leaching test (CEN/TS 14429, 2004) at a pH of to assess the influence of acidic conditions on the release of HMs and As from sludge The test was based on an automatic multititration system (Titro-Wico Multititrator, Wittenfield and Cornelius, Bochum, Germany) Eighty grams of dried sample was put in an erlenmeyer flask together with 800 mL of Milli-Q water (L/S ratio=10 L/kg) We continually monitored the pH and adjusted it by automatic addition of HNO3 solution Preliminary experiments showed that when a HNO3 solution with a concentration of mol/L was used, a very large volume of HNO3 mol/L solution was needed to maintain the predefined pH (pH 4) for sample SE Therefore, we adapted the concentration of the HNO3 solution to the different samples (SI1 and SI2 samples, mol/L; SE sample, 2.5 mol/L) At regular time intervals (0, 1, 3, 6, 12, 24, 48, 72, 96 h), a sample of the suspension (10 mL) was taken over a filter (0.45 μm, Chromafil® PET-45/25, Macherey-Nagel GmbH & Co KG, Germany) by means of a syringe attached to a flexible tube Fig XRD patterns of original sample and sample after the pHstat test (pH=4, sample SI2) 2208 Analysis of leachates Part of each leachate sample (CLT and pHstat test) was acidified with a drop of concentrated HNO3, for measurement of Al, Ca, Fe, K, Mg, P, S, As, Cd, Co, Cr, Cu, Mn, Mo, Ni, Pb, and Zn by ICP-OES whereas the other part was not acidified, and stored in cool and dark conditions until anion (F−, Cl−, SO42−, S2O32−, and PO43−) analysis by ion chromatography (IC-Dionex ICS-2000) We used the colorimetric diphenylcarbazide (DCB) method (USEPA 1995) to determine the Cr(VI) content in the leachates Cr measured by ICP-OES constitutes total Cr Cr(III) was obtained by subtraction of Cr(VI) from total Cr Environ Sci Pollut Res (2015) 22:2205–2218 composition of industrial sludge may vary widely, depending on industry type As mentioned, sample SI1 and SI2 originate from centralized industrial wastewater treatment plants of industrials parks that actually treat mixed wastewaters from factories within the industrials parks, making comparison to published data difficult Regarding the electroplating sludge sample (SE), concentrations of major elements (Al, Ca, and Fe) and Zn are higher and Mg, Mn, As, Cd, and Cu are lower compared to those reported by other workers Zn concentration is 75 times higher than the value reported by Wu et al (2012) Concentrations of Cr, Ni, and Pb lie within the same range Solid phase characterization Aqueous speciation based on modeling by Visual MinteQ, version 3.0 We performed calculations based on Visual MinteQ version 3.0 to determine speciation of HMs in the leachate Input data include concentrations of elements (Al, K, Ca, Mg, Fe, Co, Cr, Mn, Mo, Ni, Pb, Zn, S, P) and anions (F−, Cl−, SO42−, S2O32−, and PO43−) measured in the leachate during the pHstat leaching test (96 h) Concentrations of NO3− were derived from acid dosage in the pHstat test Measured Eh and pH values were used in the calculation The specified redox couples were Fe2+/Fe3+, Co2+/Co3+, Cu+/Cu2+, Cr(OH)2+/CrO42−, HS−/ SO42−, and Mn2+/Mn3+ Results General sludge characteristics Bulk chemical composition The three sludge samples have a nearly neutral to slightly alkaline pH (Table 2) Elevated concentrations of Al, Ca, Fe, P, S, and organic matter (percentage level) were found in all samples These elements might derive from the use of chemical precipitants and coagulants (e.g., lime, ferrous sulfate, ferrous phosphate, and poly aluminum chloride (PAC)) in the chemical remediation step in wastewater treatment (Hsieh et al 2008) Sample SI2 has a notably high concentration of Pb (8,130 mg/kg), likely associated with the battery and steel processing factory in the industrial park The electroplating sludge (sample SE) has the highest concentrations of As and HMs (Co, Cu, Mo, and Zn) and extremely high concentrations of Cr and Ni (13,208 and 55,732 mg/kg, respectively) (Table 2) Chemical data related to samples SI1, SI2, and SE and a comparison with published data from electroplating sludges (Sophia and Swaminathan 2005; Le 2007; Wu et al 2012; Huang et al 2013) appear in Table The elemental Understanding the correlation between solid phase properties and leaching behavior of materials requires a detailed knowledge of the minerals present together with their chemical composition (Bayuseno and Schmahl 2010) XRD analysis reveals the presence of silicates, including quartz (SiO2 in all three samples) and narcrite (Al2Si2O5(OH)4 in sample SI1), and carbonates such as calcite (CaCO3 in SE) The occurrence of quartz and gypsum in sample SE is in accordance with results reported by Ozdemir and Piskin (2012), who also reported the presence of quartz and gypsum as common phases in metal plating sludge Calcite is also a common byproduct of the neutralization step by lime in wastewater treatment systems due to its precipitation from high calcium content and carbonate fraction in industrial sludge (Zinck 2005) Some phosphate and sulfate minerals are also present: phosphosiderite (FePO4·2H2O in sample SI1), aluminum phosphate (Al16P16O64 in SI2), gypsum (CaSO4·2H2O in all three samples), and lanarkite (Pb2SO4O in SI2) Mineral phases detected by XRD analysis are consistent with high contents of Al, Ca, Fe, P, and S in all samples in ICP-OES data Apart from Pb, no HMs as discrete mineral phases was detected by XRD We applied FEG-EPMA as a check on the consistency of XRD results and to identify the phases that could not be detected by XRD Table provides the chemical composition from selected EDS spot analysis by FEG-EPMA In sample SI1, 62 spots were analyzed by FEG-EPMA to examine elemental compositions of the phases Results reveal the presence of quartz (spots and 2, Table and Fig 2), phosphorus (spots 3, 4, 5, 6, 7, 12, and 13), and sulfur species Although some HMs (Cr, Ni, and Zn) were detected with FEG-EPMA, no Cd or Co was detected As was only detected in one spot in a matrix of glass (Si-Ca-Mg-Na) (spot 8, Fig 2) A different distribution of Cr, Ni, and Zn was observed (Table 3) Cr-rich spots were observed in Fe-rich spots (spots 9, 10, and 11) or in a matrix with Zn, Ni, P and small amount of Ca, Al, and Fe (spot 12) Ni was found in Zn-rich spots containing P, Al, Fe (spots 12 and 13), and Si (spot 14) Most Cd (mg/kg) 1.4±0.5 3.0±0.5 1.2±0.2 NA NA NA 30 As (mg/kg) 11±2 10±2 35±2 NA NA NA 97.6 SI1 SI2 SE Sophia and Swaminathan (2005) Le (2007) Wu et al (2012) Huang et al (2013) SI1 SI2 SE Le (2007) Wu et al (2012) Sophia and Swaminathan (2005) Huang et al (2013) NA not available, DL below detection limit Ca (%) 8.01±0.04 7.06±0.04 9.82±0.16 NA NA NA 3.19 Al (%) 4.36±0.05 7.93±0.06 3.36±0.06 NA NA NA 0.16 Co (mg/kg) 260±6 11±1 230±1 NA NA NA NA Fe (%) 3.63±0.09 2.91±0.03 5.36±0.04 NA NA NA 1.5 Cr (mg/kg) 5,039±51 501±6 13,208±148 4,656–7,274 28,828–133 85,000±1,400 220 P (%) 1.97±0.09 2.72±0.04 6.17±0.02 NA NA NA NA Cu (mg/kg) 10,851±29 542±146 9,790±95 NA NA NA 19,000 S (%) 7.98±0.02 2.19±0.02 1.30±0.01 NA NA NA NA Mo (mg/kg) DL 5±1 8±1 NA NA NA NA K (mg/kg) 1,223±14 820±3 193±22 NA NA NA 220 Ni (mg/kg) 14,118±159 174±6 55,732±388 50,229 308–4,976 590±76 267,000 Mg (mg/kg) 2,702±83 1,727±6 5,153±113 5,600±360 NA NA 2,6800 Pb (mg/kg) 122±1 8,130±100 268±1 NA 424 100±14 5,700 Mn (mg/kg) 427±20 168±3 157±2 NA NA NA 250 Zn (mg/kg) 72,347±2,223 1,124±9 77,352±1,061 NA 1,036 20,100±720 17,300 OC (%) 7.8±0.2 7.2±0.7 3.0±0.5 NA NA NA NA IR (%) 8.5±0.9 9.2±0.7 14.0±1.3 NA NA NA NA pH (H2O) 7.7±0.3 7.16±0.3 7.82±0.3 NA NA NA NA Table Chemical characteristics of samples SI1, SI2, and SE (mean±standard deviation of replicates) and data from electroplating sludge reported by Sophia and Swaminathan (2005), Le (2007), Wu et al (2012), and Huang et al (2013) Environ Sci Pollut Res (2015) 22:2205–2218 2209 2210 Environ Sci Pollut Res (2015) 22:2205–2218 Table Chemical composition (wt%) of selected spots by FEG-EPMA, the spot numbers refer to the numbers given in Fig Sample Spot P SI1 SI2 SE SE Fe O 10 11 12 13 14 15 16 17 55.56 55.57 16.45 43.17 14.63 1.72 33.15 16.21 10.95 43.59 16.48 10.69 43.1 16.96 9.57 43.21 44.72 85.6 1.13 87.84 85.92 1.06 8.43 5.82 37.4 8.41 5.14 40.16 3.99 4.93 38.71 58.41 43.85 38.47 12.84 12.14 43.76 18 19 20 21 22 23 24 25 26 27 28 29 30 13.4 13.21 44.37 24.07 24.06 2.13 1.33 48.77 11.47 6.74 26.64 51.76 50.04 3.7 26.55 10.51 37.44 39.79 25.72 22.09 10.05 34.22 14.59 35.9 Pb N Na Mg Al Si S Ca Ti Mn Cr Ni Zn As Zr Sn 44.44 44.43 10.27 0.69 5.18 3.73 2.66 38.2 6.95 7.95 5.5 21.64 10.62 12.09 12.02 61.84 59.15 1.13 0.66 7.53 12.25 19.6 41.59 7.37 3.47 4.15 4.66 5.49 1.07 1.14 3.42 12.29 7.22 Zn-rich spots occurred within P-Fe-rich spots (spots 12, 13 and 14) or in a complex matrix of Na, Al, Si, and Ca with small amounts of S and Cl (data not shown) WDS mapping was also used to deduce the micro-scale elemental distributions in a selected area The map (Fig 2) shows a variety of phases, namely phases with high Zn-Ni-P-Fe and phases with high Al-Si content The coexistence of Zn, Ni, P, and Fe, as observed in the map, suggests that Fe and PO43−-rich phases are important host phases for Zn and Ni The coexistence of Al-Si suggests the presence of nacrite (Al2Si2O5) which was also identified by XRD In sample SI2, 42 spots were analyzed in combination with WDS mapping of selected areas Representative EDS point analyses are given in Fig and Table In general, SI2 consists of a large variety of elements that are mostly P, Si, Al, and S, most likely coinciding with quartz, aluminum phosphate and Si, Al-rich phases (spots 15, 16, 17, and 18) Pb was the only HM that was detected with FEG-EPMA, most 1.47 30.11 38.75 29.24 29.73 30.26 7.01 4.08 11.47 23.04 8.21 1.34 4.54 3.68 13.28 12.16 13.01 4.46 11.77 21.09 8.01 30.33 5.83 21.44 6.22 1.06 0.7 32.64 9.91 7.96 45.23 49.96 0.95 1.15 2.3 36.52 5.14 0.97 1.94 0.88 5.81 55.12 5.61 17.77 26.16 11.4 9.09 11.59 19.15 8.4 14.35 47.33 35.17 likely because of its elevated concentration (8,130 mg/kg Pb in SI2) and the heterogeneous distribution of other HMs in this sludge sample In spot analyses, traces of Pb were observed in Ti-rich spots with trace amount of P, Fe, Al, and Si (spot 21) Pb-rich spots were also found in Al-Si-rich spots (spots 19 and 20) and in Al-, P-, S-rich spots containing small amounts of Fe, Si, and Ca (spot 22) Mapping of selected areas revealed the presence of quartz and lead sulfide by the coexistence of Si-O and Pb-S in the selected area With regard to sample SE, 61 spot analyses were performed Major phases in SE comprise calcite (spots 23 and 24) and metal-rich spots with Fe, Ni, Sn, and Zn (spots 29 and 30) Results reveal an elevated content of some HMs, including Ni, Zn, Cr, and Sn Cd, Co, and Mo were not detected Znrich spots were identified in 30 of the 61 analyzed spot which is in accordance with the fact that Zn is the most abundant HM in sample SE (see also Table 2) Ni- and Zn-rich spots with varying contents of Al, Fe and Sn and, in some cases, traces of Environ Sci Pollut Res (2015) 22:2205–2218 2211 Fig Graph of selected spots for EDS analysis and area for WDS mapping analyzed by FEG-EPMA From left to right: a SI1; b WDS mapping of selected area revealed the coexistence of Ni, P, and Fe in SI1; c SI2; d SE The spot numbers refer to the numbers given in Table Cr (spots 25, 26, and 28), S (spots 25 and 26), and As and Pb (data not shown) were found Arsenic was also observed in a Ca-Mg-O matrix (spots 23 and 27) Leaching tests Although many different elements were measured, the following discussion mainly focuses on As, Cd, Co, Cr, Cu, Ni, Mo, Pb, and Zn because of their potential toxicity Major elements (Al, Ca, K, Mg, P, and S) and anions (SO42− and PO43−) are mentioned because of their relevance for interpretation of release mechanisms of elements of interest Cascade leaching tests Figure shows pH and cumulative leached concentrations of HMs and SO42− from the CLT Concentrations of As in the leachates are below detection limit in all three samples In the CLT, the final pH of the leachates varied from step to step (Fig 3) According to Cappuyns and Swennen (2008a), the extent of pH change mainly depends on the acid neutralization capacity (ANC) of the samples From this result, we deduce that sample SE has the highest ANC because its pH change was smallest during the CLT Below, “leachability” refers to the cumulative leachability (sum of five extractions steps), expressed in percent of an element leached relative to its total content in a sample In sample SI1, the leachability is in the following order: S (43 %)>Ca (32 %)>K (15 %)>Mg (9 %)>Co≈Mn (3 %)>Ni (2 %)>Zn (1 %)>Cu≈P (0.1–0.2 %) The leachability of Al, Fe, and Cr is negligible (Co (9 %)>Zn (4 %) Speciation analysis of Cr indicates that the leachates of sample SI2 not contain Cr(VI) In general, sample SE displays a lower leachability of elements compared to SI1 and SI2 Similar to SI2, Mo showed 2212 Environ Sci Pollut Res (2015) 22:2205–2218 Fig Cumulative amount leached of HMs including Cr(VI) (sample SE), and SO42−, and pH change during the cascade leaching test the highest release among the HMs (23 % of the total content was released) The leachability of Zn, Ni, and Cu is negligible ( SE>SI1 for SO42− and SI1>SI2>SE for PO43− pHstat leaching test Heavy metals and As Figures and display release patterns of major elements, ANC, anions, HMs, and As from the pHstat leaching test In the following section, leachability refers to leachable concentration after 96 h of pHstat leaching, expressed in percent relative to total concentration Acid neutralization capacity, major elements, and anions We calculated ANC based on the quantity of acid added to maintain a particular pH value During the pHstat leaching test, the highest ANC (at 96 h) was observed in sample SE During pHstat leaching at pH 4, Cd, Co, Ni, and Zn leachability is high (9–31 % of the total content) This is consistent with Kazi et al (2005), who report identifying Cd and Ni mainly in the acid-soluble fraction of industrial sludge samples, suggesting release at low pH Cu exhibits a medium leachability (1–5 %), while Cr and Pb show a low leachability (