Toxicity Characteristic Leaching Procedure Research Articles

Stabilization and solidification of arsenic contaminated silty sand using alkaline activated slag

Contamination of soils by arsenic represents a significant environmental and public health risk, making effective remediation strategies a pressing concern. One commonly employed technique is stabilization and solidification, which involves the addition of stabilizing binders such as cement to immobilize arsenic. This study investigates the potential of alkaline activated slag for stabilization and solidification of arsenic-contaminated soil, employing the toxic characteristic leaching procedure (TCLP) and unconfined compressive strength (UCS) tests. To assess the strength and leachability behavior, the research examines the effect of several factors, including binder content, curing time, curing conditions, alkaline activator solution to slag ratio, sodium silicate to sodium hydroxide ratio, and sodium hydroxide concentration. Additionally, field emission scanning electron microscopes (FE-SEM) in combination with energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) tests are employed to analyze the stabilization and solidification mechanism. The study shows that increasing the slag content to 20% by weight after 28 days of curing at ambient temperature leads to a decrease of almost 92% in the concentration of leached arsenic and an increase in UCS from 80 kPa to approximately 19 MPa. The formation of albite and anorthite crystals, along with gels such as (N, C)-A-S-H and C–S–H, contributes to enhanced strength and reduced leachability. As a result, the use of alkaline activated slag is identified as an effective and environmentally friendly approach for the stabilization/solidification of arsenic-contaminated soils. This study highlights the use of alkaline activated slag as an effective solution for remediating arsenic-contaminated soils while simultaneously reducing waste, greenhouse gas emissions, and energy consumption. Slag, a byproduct of metal production, is often wasted due to a lack of value and landfill space. However, alkaline activated slag demonstrates the potential to stabilize soil, immobilize heavy metals, and provide efficient and sustainable soil remediation.

Physicochemical Properties and Leaching Toxicity Assessment of Jarosite Residue

The safe disposal of hazardous waste from zinc hydrometallurgy, such as jarosite residue, is crucial for the sustainable development of the industry. The chemical, structural and morphological properties of jarosite residue from zinc smelting were studied by a combination of various characterizations, and environmental stability was evaluated using the toxicity characteristic leaching procedure (TCLP), Chinese standard leaching tests (CSLT) and long-term leaching experiments (LTLE). Phase composition analysis revealed that zinc ferrite and sodium jarosite were the main phases present in the jarosite residue. TCLP and CSLT analyses indicated that the Zn and Pb contents exceeded their respective toxicity identification standards by more than 30 times and 8 times, respectively, exceeding the threshold values of the standard. The LTLE results demonstrated that Pb concentrations continued to exceed the standard limits, even after long contact times. This study has paramount significance in the prediction of jarosite residue stability and the evaluation of its potential for secondary environmental pollution.

Characterization, risk assessment, and potential utilization of fly ash from PLTSa Merah Putih at Bantargebang, Bekasi, Indonesia

Municipal solid waste incineration (MSWI) has been adopted in Indonesia to reduce the volume and mass of municipal solid waste. However, as a by-product of waste combustion, fly ash generation cannot be avoided. It can cause an environmental problem if not managed properly. Hence, this study aims to investigate the characteristics of fly ash generated from Indonesia’s first waste-to-energy incineration pilot plant and its potential utilization. Fly ash was characterized using X-Ray fluorescence (XRF), BET surface area measurement, pore size distribution (PSD), scanning electron microscopy with energy-dispersive spectrometry (SEM-EDS), and X-ray diffraction (XRD). The characterization results showed that calcium was predominant, around 28%, followed by titanium, silica, iron, and aluminum, with values of 7.10%, 5.70%, 4.40%, and 4.30%, respectively. These results demonstrated that MSW incineration fly ash was classified as a very active high-calcium fly ash. The surface structure of fly ash was flaky and complex, with some irregular material. The particle size distribution of fly ash shows that the diameters of most particles were nearly 12 μm. The specific surface area of fly ash was 3.06 m2/g, the total-pore volume was 0.01 cm3/g, and the average pore size was 11.82 nm. In addition, the environmental risk assessment was evaluated using the toxicity characteristic leaching procedure (TCLP). TCLP concentrations of heavy metals, including arsenic, barium, beryllium, cadmium, chromium hexavalent, copper, lead, mercury, nickel, selenium, and zinc, were lower than the regulatory level, indicating a low environmental risk. Therefore, it has great potential for utilization as a paving block mixture material.

Converting sludge to slag through a high temperature slagging co-gasification process: An evaluation based on a demonstration trial and life cycle assessment

A variety of sludge-based waste materials with distinctive physical and chemical characteristics are co-treated with municipal solid waste (MSW) in a demonstration-scale high-temperature slagging gasifier. The waste materials tested include dewatered sewage sludge (originated from industrial and municipal wastewater treatment processes, respectively), dried sewage sludge, sludge incineration ashes, and waterworks sludge (originated from freshwater treatment process). Up to 49.5% by mass of dried sewage sludge is successfully co-gasified with MSW and up to 89.5% of the solid residues from the gasification process is recovered as reusable slag and recyclable metals. Despite feeding of various waste mixtures, stable operation of the slagging gasifier is achieved with the proper control of the temperature profiles inside the gasifier, the melting of inorganic residues, smooth discharge of the molten slag, and effective flue gas emissions control. Various types of waste-derived slag are characterized to evaluate their potentials to be applied as sustainable construction materials. Toxicity characteristic leaching procedure (TCLP) and compliance batch leaching test results indicate that the sludge-derived slag is environmentally safe with very low pollution potential. In addition, life cycle assessment (LCA) is performed to comprehensively evaluate the treatment of sludge-based waste materials via slagging co-gasification technology. These findings reveal the promising potentials of converting sludge to slag through the high temperature slagging gasification process based on both engineering and environmental aspects, as well as the possible areas for further advancement in the future.

The Synthesis of Lead-Bearing Jarosite and Its Occurrence Characteristic and Leaching Toxicity Evaluation

Lead is the main toxic factor in jarosite residue. It is important to study the release behavior of lead from simulated lead-bearing jarosite (SLBJ) for predicting the stability of jarosite residue and its secondary pollution to the environment. To identify the technical issues and limitations associated with its safe disposal, a comprehensive analysis of the chemical, structural, and morphological characteristics of SLBJ was conducted using various detection techniques including XRF, XRD, SEM-EDS, FTIR, XPS, etc. The environmental stability of SLBJ was assessed through the toxicity characteristic leaching procedure (TCLP), Chinese standard leaching tests (CSLT), and a long-term leaching experiment (LTLE). Phase composition analysis revealed that the primary components of SLBJ are sodium jarosite and lead sulfate. TCLP and CSLT results indicated that lead content surpassed the toxicity identification standard limit by more than 47 times. Furthermore, LTLE indicated that the lead concentration surpassed the standard limit about 15 times after prolonged contact time. This study is of great significance for predicting the stability of jarosite residue and its secondary pollution to the environment.

The reuse potential of spent ZIF-8@GO-COOH nano adsorption materials: Study on their impact on the hydration mechanism and mechanical properties of cement-based composites

Spent adsorbents often contain harmful substances, which may have an impact on the ecological environment if these substances enter the environment. This study mainly introduces a safe treatment method for solid waste formed after heavy metal ions are adsorbed by an efficient adsorbent, ZIF-8@GO-COOH. This work mainly discusses the impact of solidifying the spent adsorbent in the cement system on the cement mechanical properties and hydration process. The experimental results showed that when the dosage of the spent adsorbent was 0.01%, 0.02%, 0.03%, and 0.04%, respectively, the compressive strength of 28 d was increased by 4.24%, 6.16%, 7.19%, and 8.87%, respectively, compared with the cement without spent adsorbent. Introducing spent ZIF-8@GO-COOH in the cement production process does not have a negative impact on the cement mechanical properties, and can refine the cement pore structure and dense microstructure, thereby improving the compressive strength. The Toxicity Characteristic Leaching Procedure (TCLP) results showed that solidifying the spent adsorbent with cement can effectively reduce the leaching of heavy metal ions in the spent adsorbent. This study solidifies the spent ZIF-8@GO-COOH nanoadsorbent material into the cement system as a sustainable alternative to landfilling. This treatment method has certain economic benefits and safety, and has some reference significance for promoting the development and improvement of spent adsorbents and improving their safe treatment and reuse.

Leachability of lead, cadmium, and antimony in cement solidified waste in a silo-type radioactive waste disposal facility environment

The waste acceptance criteria for heavy metals in mixed waste should be developed by reflecting the leaching behaviors that could highly depend on the repository design and environment surrounding the waste. The current standards widely used to evaluate the leaching characteristics of heavy metals would not be appropriate for the silo-type repository since they are developed for landfills, which are more common than a silo-type repository. This research aimed to explore the leaching behaviors of cementitious waste with Pb, Cd, and Sb metallic and oxide powders in an environment simulating a silo-type radioactive waste repository. The Toxicity Characteristic Leaching Procedure (TCLP) and the ANS 16.1 standard were employed with standard and two modified solutions: concrete-saturated deionized and underground water. The compositions and elemental distribution of leachates and specimens were analyzed using an inductively coupled plasma optical emission spectrometer (ICP-OES) and energy-dispersive X-ray spectroscopy combined with scanning electron microscopy (SEM-EDS). Lead and antimony demonstrated high leaching levels in the modified leaching solutions, while cadmium exhibited minimal leaching behavior and remained mainly within the cement matrix. The results emphasize the significance of understanding heavy metals' leaching behavior in the repository's geochemical environment, which could accelerate or mitigate the reaction.

Moisture sensitivity and heavy metal leaching potentials of asphalt mixtures incorporating rubber modifiers and wax-based admixture

Moisture damage is one of the most frequent damages that occurs on asphalt pavements. The application of rubberized materials for asphalt is widely acceptable due to cost saving and environmentally friendly. The purpose of this study is to assess the performance of rubberized asphalt mixtures containing wax-based admixture, namely Tough Fix Hyper (TFH), in terms of moisture sensitivity and hazardous compound leaching potential. The moisture absorption, chemical immersion, and boiling water tests were used to assess the resistance to moisture damage on aggregate-binder bonding characteristics. Following that, the trend of moisture sensitivity was evaluated through an image analysis procedure. To simulate the environmental impact of leaching, the toxicity characterization leaching procedure (TCLP) test was adopted. The results showed that interaction between the crumb rubber or latex with TFH decreases the stripping value of the mixes significantly based on the chemical immersion test. Furthermore, the image analysis of the samples subjected to boiling water conditioning showed that pixels of asphalt samples without TFH, ranging from 524,949 to 472,346, decrease from 520,870 to 289,070, indicating that the asphalt remained coated area improved by 67.70–91.39%. The analysis of the leachate test revealed that the heavy metals, including arsenic (As), nickel (Ni), copper (Cu), chromium (Cr), vanadium (V), and cadmium (Cd), were not detected. However, zinc (Zn) was detected in the modified mixture, ranging from 0.05 to 0.114 mg/L, which lies within the allowable norm. In conclusion, the incorporation of crumb rubber and TFH in asphalt mixtures poses no environmental risk.

Utilization of fly ash and bottom ash for tailings stabilization

To anticipate the problem of acid mine drainage generation from the tailings, the experiments on the use of fly ash (FA) and bottom ash (BA) were carried out by considering the neutralizing potential contained in these materials. The basic characteristics of the tailings, FA, and BA materials were tested through mineralogy and geochemical test. Geochemical static tests were carried out on various compositions and rates of FA and BA mixtures added to the tailings. Based on the results of these tests and the results of the Toxicity Characteristic Leaching Procedure (TCLP) test, a kinetic geochemical test was carried out using a column to determine the neutralization performance of FA and BA. The results of the quality test of leachate water coming out from the column for 6 weeks test period showed that a mixture of FA and BA with a ratio of 9:1, which was added to the tailings at the rate of 15 kg/ton of tailings was the optimum mixture that could neutralize the acidity formed, and also reduced the Pb and Cu content in the leachate.

Studying the Properties of Chromium-Contaminated Soil Solidified by Polyurethane.

The solidification of chromium-contaminated soil using polyurethane (PU) was systematically investigated. The unconfined compression test was conducted to investigate the effects of the curing time, PU dosage and the content of chromium ions on the unconfined compressive strength (UCS) of chromium-contaminated soil. The effect of the PU dosage on the pore structure was investigated using nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM), and the mechanism of strength change was revealed by combining the strength law with the pore structure development law. In addition, the ability of the PU to solidify the chromium-contaminated soil was studied by the toxicity characteristic leaching procedure (TCLP). According to the above test results, the UCS and the ability of the PU to solidify the chromium ions increased with the increase in curing time. The NMR tests showed that with the increase in PU dosage, the porosity decreased and the soil became more compact, hence increasing the strength. When the chromium ion content was 2000 mg/kg and the PU dosage was 8%, the strength of the sample was 0.37 MPa after curing for 24 h, which met the requirement of 0.35 MPa set by the U.S. Environmental Protection Agency. Consequently, PU is a solidification agent with high-early strength.

Effective purification of high concentration chromium-containing wastewater and preparation of chromium ferrite

The formation of valuable ferrite in one step by adding ferrous sulfate to chromium-containing wastewater without adjusting to acidic conditions was studied under simple mechanical equipment. The process can not only solve the problem of heavy metals removal from wastewater, but also achieve the purpose of chromium resource recovery. Increasing temperature and pH can further promote the formation of chromium ferrite while prolonging the reaction time has no significant effect on the product’s performance. The response surface method (RSM) was utilized to optimize the process of ferrite formation: the Fe<sup>2+</sup>/Cr<sup>6+</sup> mole ratio of 7, pH value of 10.5, and temperature of 100℃, on which formed ferrite has a high saturation magnetization of 41.28 emu/g with Cr(VI) removal ratio around 100%. TCLP (Toxicity Characteristic Leaching Procedure) tests showed that the chromium concentrations leached from the solid were below the limit of 5 mg/L by USEPA. The molecular formula of chromium ferrite can be expressed as Fe<sub>3-x</sub>Cr<sub>x</sub>O<sub>4</sub> (where x is about 0.45), which has the potential to be recycled as a favorable material. The ferrite process has a low activation energy of 20.8 kJ/mol, which is an economical and environmentally friendly method for synthesizing ferrite from wastewater.

Leaching Test Protocols to Evaluate Contaminant Response of Nano-Calcium Silicate–Treated Tropical Soils

The current study deals with the interaction of toxic metals with soils by conducting a series of leaching tests on four kaolinitic soils procured from locations with varying pollution levels, such as a pristine location, vicinity of steel mill works, battery works unit, and an abandoned municipal waste dump site. The procured soils were spiked with 3,000 mg/kg of toxic metals, lead (Pb), and nickel (Ni) to understand the contaminant transport. An innovative amendment, such as synthesized nano-calcium silicate (NCS), was also mixed with the soils for toxic metal immobilization. Leaching tests. such as the toxicity characteristic leaching procedure (TCLP), extended TCLP, and caged TCLP, were conducted on soil samples. These tests were successful in predicting the interactive behavior of the composite mix containing soil, NCS, and toxic metals, comprehensively. The vulnerability of soils based on TCLP can be expressed in the order of Soil C > Soil D > Soil B > Soil A. These results were concurrent with extended TCLP, wherein the response of these composites at various pH levels was presented. Caged TCLP quantified the leaching rate for different mixtures, and it was found that the leaching rate for virgin soils reduced from 1.3 to 0.075 cm/yr after the NCS amendment. The addition of NCS resulted in the agglomeration and aggregation of soil particles. This increased the density and reduced the void spaces of the soil–nano mix, thereby encapsulating the toxic metals permanently within its interstices. The highest attenuation was observed for Pb2+ compared with Ni2+ for all the mixtures considered. Thus, leaching tests can be considered a prospective short-term technique for monitoring polluted soil media compared with tedious and time-dependent column studies.

Remediation of Cr(VI) contaminated soil by chitosan stabilized FeS composite and the changes in microorganism community

In-suit immobilization is one of the major strategies to remediate heavy metals contaminated soil with the effectiveness largely depends on the characteristics of the added chemical reagents/materials. In this study, chitosan stabilized FeS composite (CS–FeS) was prepared to evaluate the performance of remediating the high and toxic hexavalent chromium contaminated soil from the effectiveness and microbial response aspects. The characterization analysis confirmed the successful preparation of composite, and the introduction of chitosan successfully stabilized FeS to protect it from rapid oxidation as compared to bare FeS particles. With the addition dosage at 0.1%, about 85.6% and 81.3% of Cr(VI) was reduced in 3 d based on toxicity characteristic leaching procedure (TCLP) and CaCl2 extraction, and the reduction efficiency increased to 96.6% and 94.8% in 7 d, respectively. The Cr(VI) was non-detected in the TCLP leachates with increase the CS-FeS composites to 0.5%. The percentages of HOAc-extractable Cr decreased from 25.17% to 6.12% accompanied with the increase in the residual Cr from 4.26% to 13.77% and improvement of soil enzyme activity under CS-FeS composites addition. Cr(VI) contamination reduced the diversity of microbial community in soil. Three dominate prokaryotic microorganisms, namely Proteobacteria, Actinobacteria and Firmicutes, were observed in Cr-contaminated soil. The addition of CS-FeS composites increased the microbial diversity especially for that in relative lower abundance. The relative abundance of Proteobacteria and Firmicute related to Cr-tolerance and reduction increased in CS-FeS composites added soils. Taking together, these results demonstrated the potential and promising of using the CS-FeS composites for Cr(VI) polluted soil remediation.

Co-treatment of chromite ore processing residue and pyrolusite for simultaneous detoxification and resource utilization

Efficient and low-cost recovery of metals from pyrolusite and Chromite Ore Processing Residue (COPR) is of great significance for environmental protection and sustainable development of metal resources. This study proposes a new method for the simultaneous recovery of Cr and Mn from COPR and pyrolusite, to realize COPR and pyrolusite resource utilization and detoxification. Under the optimal conditions (i.e., the temperature of 150 ℃, the holding time of 1.5 h, the H2SO4 concentration of 3 M, the mass ratio of COPR to pyrolusite of 1:1, and the solid-liquid ratio of 1:20), the leaching efficiency of Cr and Mn reached 99.56% and 85.34% respectively. The Toxicity Characteristic Leaching Procedure (TCLP) total Cr concentration of the detoxified slag was 0.26 mg/L meeting the EPA standard (＜5 mg/L). The reaction mechanism was elucidated based on thermodynamics, kinetics, and mineral characterization. In addition, a high-quality utilization strategy was proposed, leaching solution to prepare manganese carbonate and sodium chromate, and the detoxified slag can be used for steel smelting. In brief, this method has good economic benefits and provides a new reference for the resource utilization of hazardous waste.

Insight into Chinese medicine residue biochar combined with ultrasound for persulfate activation in atrazine degradation: Acanthopanax senticosus precursors, synergistic effects and toxicity assessment

The synergistic activation of persulfate by multiple factors could degrade pollutants more efficiently. However, the co-activation method based on metal ions has the risk of leakage. The non-metallic coupling method could achieve the same efficiency as the metal activation and meanwhile release environmental stress. In this study, the original biochar (BC) was prepared through using Chinese medicinal residue of Acanthopanax senticosus as the precursor. Compared with other biochar, the pore size structure was higher and toxicity risk was lower. The ultrasonic (US)/Acanthopanax senticosus biochar (ASBC)/persulfate oxidation system was established for Atrazine (ATZ). Results showed that 45KHz in middle and low frequency band cooperated with ASBC600 to degrade nearly 70 % of ATZ within 50 min, and US promoted the formation of SO4− and OH. Meanwhile, the synergy index of US and ASBC was calculated to be 1.18, which showed positive synergistic effect. Finally, the potential toxicity was examined by using Toxicity Characteristic Leaching Procedure (TCLP) and luminescent bacteria. This study provides a promising way for the activation of persulfate, which is expected to bring a new idea for the win-win situation of pollutant degradation and solid waste resource utilization.

Toxic metals and the risks of sludge from the treatment of wastewater from beryllium smelting

The release of highly toxic beryllium in sludge (BCS) produced by physico-chemical treatment of beryllium-containing wastewater from Be smelting production has become a growing concern with the widespread use of Be in the defense industry. This work investigated the potential mobility of Be in BCS. The toxicity characteristic leaching procedure (TCLP) of BCS showed that the amount of leached Be was up to 202 mg L−1, which exceeded the regulated limit by nearly 10,000 times. The chemical fractionation analysis further revealed that the excessive amount of Be leached from BCS was contributed to the high content of acid-soluble fraction and reducible fraction of Be, which accounted for over 70% of the Be content. The results obtained from mineralogical automatic analyzer (MLA) showed that gypsum (23.23%) and epidote (19.55%) were the two major mineralogical phases of BCS. Both were small and loosely structured agglomerated particles with a D50 of 6.61 μm and 3.31 μm. ToF-SIMS results revealed that the Be distribution on the surface of BCS particles was relatively dispersed, with no aggregation or encapsulation. Be co-precipitated with gypsum and chlorite in the form of unstable Be(OH)2, which attached to the surface of these small particles. The unstable state of Be and the small size, loose structure and high liberation of the host material phases are the main reasons for the high leaching mobility of Be. The results of the risk assessment indicated that BCS posed an extremely high potential ecological risk, with Be being the most significant contributor.

Effect of mechanochemical activation on hydration properties of lead-zinc sulfide tailings for concrete construction

Proper management of mine tailings containing high amount of sulfide is a crucial issue in order to reduce their environmental hazards including generation of acid mine drainage and release of heavy metals. This study was conducted targeting to understand the effects of using lead-zinc sulfide tailings (LZSTs) as cement substitution on the hydration properties of concrete samples. Mechanochemical activation of the LZSTs containing 19.9% sulfide content in mixtures of 20%, 40%, 60%, 80% and 100% was performed to cement-replacement in concrete production. Analyzes were performed on the produced concrete samples. X-ray diffraction, compressive strength, toxicity characteristic leaching procedure (TCLP), mercury intrusion porosimetry, X-ray spectroscopy, scanning electron microscopy, energy-dispersive fourier-transform infrared spectroscopy, thermogravimetric analysis and differential thermal analysis were performed on the constructed concrete samples. The results of XRD analysis confirmed that introduction of activated tailings in the concrete construction led to form hatrurite (Ca3SiO5) phase. According to the TCLP, with increseasing curing days, leached amount of studied toxic metals decreased and was lower than the allowable limit. The results of the mercury intrusion porosimetry (MIP) indicated that as the curing time prolonged, the critical pore size was reduced from 150 to 50 nm in the constructed sample containing 20% activated tailings. Therefore, by activating the tailings, the specific surface area was enlarged and more cohesion took place in the concrete samples. The compressive strength results indicated that the compressive strength after 90 curing days in the constructed sample containing 20% activated tailings was 40.85 MPa, while it was 38.15 MPa in the control sample. In general, the results indicated that the mechanochemical activated of LZSTs with calcium carbonate can be a suitable alternative for cement (up to 40%) in the production of concrete samples.

Thermal treatment of petroleum-contaminated marine sediment according to oxygen availability and temperature: Product quality as a potential plant-growth medium

The sustainable management of dredged sediment from contaminated sites needs to consider the end-use of the treated sediment. In this regard, modifying conventional sediment treatment techniques to generate a product that is suitable for a range of terrestrial uses is necessary. In the present study, we evaluated the product quality of treated sediment as a potential plant-growth medium following the thermal treatment of marine sediment contaminated by petroleum. The contaminated sediment was subject to thermal treatment at temperatures of 300, 400, or 500 °C, and no, low, or moderate oxygen availability, and the resulting treated sediment was analyzed in terms of its bulk properties, spectroscopic properties, organic contaminants, water-soluble salts and organic matter, and the leachability and extractability of heavy metals. All operational combinations for the treatment process reduced the total petroleum hydrocarbon content of the sediment from 4922 mg kg−1 to lower than 50 mg kg−1. The thermal treatment process stabilized the heavy metals in the sediment, reducing the zinc and copper concentration by up to 58.9% and 89.6%, respectively, in the leachate from the toxicity characteristic leaching procedure. The hydrophilic organic and/or sulfate salt byproducts of the treatment were phytotoxic, but these can easily be removed by washing the sediment with water. By combining the sediment analysis results with experimental data from barley germination and early-growth tests, the end product was found to be of higher quality when higher temperatures and lower oxygen availability were employed in the treatment process. These findings demonstrate that it is possible to retain the natural organic resources of the original sediment by optimizing the thermal treatment, thus ensuring a suitably high product quality for use as a plant-growth medium.

Reutilization of ferro-arsenic waste sludge for the development of concrete blocks through solidification: conservation of natural aggregates with policy suggestion

ABSTRACT In the present study, arsenic sludge and iron sludge extracted from a laboratory scale water treatment plant were aimed to reutilize for the development of concrete blocks. Three different grades (M15, M20 and M25) of concrete blocks were made by blending of arsenic sludge and improved iron sludge (50% sand and 40% iron sludge) in the range of density of 425 to 535 kg/m3 at an optimum ratio of 10:90 (arsenic: iron sludge) followed by mixing of designed quantity cement, coarse aggregates, water and additives. Concrete blocks developed based on this such combination exhibited 26 MPa, 32 MPa and 41 MPa compressive strengths, and 4.68 MPa, 5.92 MPa and 7.78 MPa tensile strengths for M15, M20 and M25, respectively. In comparison with the developed concrete blocks and the blocks made with 10% arsenic sludge and 90% fresh sand, the developed ones (employing 50% sand, 40% iron sludge and 10% arsenic sludge) showed more than 200% higher strength perseverance on average. Successful Toxicity Characteristic Leaching Procedure (TCLP) and compressive strength of the sludge-fixed concrete cubes classified it as a non-hazardous and completely safe to use value-added material. This process involves stabilization of arsenic-rich sludge generated from high-volume long-run laboratory-based arsenic-iron abatement set-up from contaminated water with successful fixation in solid matrix of concrete through complete substitution of natural fine aggregates (river sand) in cement mixture. Techno-economic assessment reveals such concrete block preparation at $0.09 each which is lesser than 1/2 of the present market price of same quality concrete block in India

Surface modification of arsenic sulfide particles for their stabilization

Arsenic sulfide slag (ASS), the product of sulfide precipitation from arsenic wastewater, needs further processing to protect the environment. Oxidation of trivalent to pentavalent arsenic with H2O2 and stabilization of arsenic with CaO in the form of water-insoluble calcium arsenate, Ca3(AsO4)2, are now relatively mature methods. Their disadvantages, however, are the large amounts of sludge produced and of the oxidant H2O2 used. In this paper, to reduce the use of lime and the oxidizing agent, surface modification of arsenic sulfide particles (ASP) was studied. ASS was mixed with calcium oxide in different proportions to obtain mixed slag which was oxidized with hydrogen peroxide. High-resolution spectroscopic examination showed that calcium arsenate completely covered the ASP surface when the stoichiometric amounts of H2O2 and CaO were 50 and 100%, respectively. The stability of the arsenic in the surface modified ASP was evaluated using USEPA toxicity characteristics leaching procedure (TCLP) in which the ASS was cured with cement. The arsenic leaching concentration from the solidified body in which the ASS to cement ratio is 1:2 could be reduced to 0.63 mg/L for dosages of 30% CaO and >30% H2O2, which is lower than the USEPA and Chinese regulatory thresholds of 5 mg/L and 1.2 mg/L, respectively. The method developed successfully saves a large quantity of the hydrogen peroxide oxidant and has the potential to reduce industrial toxic arsenic slag discharge.