Toxicity characterization and long-term stability studies on copper slag from the ISASMELT process

Detailed studies on the toxicity and long-term stability of copper slag of varying heavy metal concentration generated over a 14-week period in an Indian copper plant through the ISASMELT process was carried out using USEPA's toxicity characteristic leaching procedure (TCLP), multiple extraction procedure and sulfuric acid leaching of granulated and mechanically activated slag as a function of pH at two different temperatures. The TCLP, acid leaching and multiple extraction test results carried out on a large number of slag samples of varying compositions derived from the use of several copper concentrates indicate poor leachability of the heavy metals and assures long-term stability even in extreme atmospheres. Leaching tests on mechanically activated samples gives an idea of the resistance to leaching of the heavy metals even upon weathering. The multiple extraction leaching tests indicate that the heavy metals present in the slag are stable and are not likely to dissolve significantly even through repetitive leaching under acid rain in a natural environment. The highest concentration of all the elements is far below the prescribed limits in USEPA 40CFR Part 261

Studies on the electrochemical decontamination of wastewater containing arsenic

The arsenic removal from aqueous solutions by electrocoagulation (EC) using mild steel electrodes was studied. Effect of electrolytes such as NaCl, NaNO3 and Na2SO4 on anodic dissolution of iron and in turn the arsenic removal was deliberated. The arsenic removal was observed to be 98% in the presence of NaCl whereas it is around 75% in the presence of Na2SO4 and NaNO3. The removal of arsenic by EC process was found to be almost similar irrespective of its oxidation state. Almost 95% of the total arsenic was removed within 5 min from its initial concentration of 10 mg L−1. The precipitates formed during EC were characterized using FT-IR, SEM-EDAX, XRD, XPS and magnetometer. The iron oxy-hydroxide precipitate formed during EC was identified as maghemite and lepidocrocite. The magnetic property, particle size and surface properties of the iron oxy-hydroxide precipitate were found to be influenced by arsenic adsorption. The oxidation state of arsenic and iron in EC products was ascertained by XPS. It was observed that some amount of As3+ was converted to As5+ during EC. The electrokinetic and FT-IR measurements revealed the co-precipitation of arsenic by specific chemical interaction between arsenic species and iron oxide precipitate. The effect of initial As3+concentration and current density on arsenic removal was also discussed. The TCLP test (USEPA method-1311) was conducted on EC products to assess their toxicity. It was confirmed that the solid waste generated during EC process is non-hazardous and can be safely disposed

Stability of copper smelter slag in sea water

The long-term stability of the slag in sea water is an essential requirement for its use in marine environment. Because no specific standards exist for studying the leachability of slag in sea water, its long-term stability was assessed through four types of indigenously designed extraction tests: (i) extraction tests on the slag similar to that specified in the multiple extraction procedure USEPA 1320, however, using sea water as the leachant; (ii) mechanical activation of the slag followed by extraction test using sea water as leachant; (iii) pressure leaching of the slag (at 1000 psi) in N2 and O2 for 4 h at different temperatures up to 120 degrees C; and (iv) mechanical activation of slag followed by pressure leaching of the slag in the temperature range of 40120 degrees C. In all these four tests, the leachability of the copper smelter slag was found to be higher in sea water compared to that in the Toxicity Characteristic Leaching Procedure medium and in acids. However, the highest concentration of all the relevant elements in the leachate was below the prescribed limits in USEPA method 1311. (C) 2010 American Institute of Chemical Engineers Environ Prog, 201

Adsorption of arsenate on synthetic goethite from aqueous solutions

Goethite was synthesized from the oxidation of ferrous carbonate precipitated from the double decomposition of ferrous sulfate doped with sodium lauryl sulfate (an anionic surfactant) and sodium carbonate in aqueous medium. The specific surface area and pore volume of goethite were 103 m(2) and 0.50 cm(3) g(-1) . Batch experiments were conducted to study the efficacy of removal of arsenic(V) using this goethite as adsorbent for solutions with 5-25 mg l(-1) of arsenic(V). The nature of adsorption was studied by zeta-potential measurements. The adsorption process followed by Langmuir isotherm and diffusion coefficient of arsenate was determined to be 3.84 x 10(11) cm(2) s(-1). The optimum pH of adsorption was found to be 5.0. The kinetics of adsorption was evaluated with 10 mg 1(-1) and 20 mg 1(-1) of As(V) solutions and activation energy of adsorption, as calculated from isoconversional method was in the range of 20 U mol(-1) to 43 U mol(-1). This suggests that the adsorption process is by diffusion at the initial phase and later through chemical control. FT-IR characterization of arsenic treated goethite indicated the presence of both As-O-Fe and As-O groups and supported the concept of surface complex formation. (c) 2005 Elsevier B.V. All rights reserved

Adsorption studies of arsenic on Mn-substituted iron oxyhydroxide

: Mn-substituted iron oxyhydroxide (Mn0.13Fe0.87OOH) was prepared by the oxidation of ferrous carbonate precipitated from ferrous sulfate and sodium carbonate solutions. X-ray diffraction analysis led to the conclusion that the sample was basically iron manganese hydroxide with bixbyite structure. The sample exhibited a surface area of 101 m(2) g(-1) and a pore volume of 0.35 cm(3) g(-1). Batch experiments were conducted to study the adsorption of arsenite and arsenate species onto Mn-substituted iron oxyhydroxide (MIOH) and adsorption equilibrium time was evaluated. The temperature of adsorption was varied from 30 to 60 degrees C. The maximum uptake of arsenite and arsenate was found to be 4.58 and 5.72 mg g(-1), respectively. Zeta potential measurements and FT-IR spectral studies were also conducted to study the nature of adsorption. In both cases. adsorption was best described by Langmuir isotherm and activation energies as calculated from a model-free isoconversional method were found to be on the order of 15-24 and 45-67 kJ mol(-1) for arsenate and arsenite, respectively. (c) 2006 Elsevier Inc. All rights reserved

Removal of Cr (VI) by electrochemical reduction

Removal of Cr (VI) from aqueous solutions by electrochemical reduction was studied using steel rods connected in unipolar mode. An aqueous solution of potassium dichromate was used as the model contaminant. Three different supporting electrolytes, viz. NaCl, NaNO3 and Na2SO4 were used during electrolysis. Only NaCl was found to be effective to arrest the passivation of electrodes by Cr (VI). The effect of initial concentration of Cr (VI) and current density on chromium removal was investigated. The rate of conversion of Cr (VI) to chromite is fast at higher current densities and the sludge is contaminated with more iron oxide. X-ray diffraction studies have revealed the formation of chromite (FeCr2O4) as main product in the electrochemically precipitated sludge. Though chromite is a predominant species other products such as Cr2O3, Fe2O3 are also formed during the process. At low current densities the electrochemical reduction of Cr (VI) was found to be initially under current control and turns to diffusion control after certain time. (c) 2007 Elsevier B.V. All rights reserved

Studies on adsorption of Cr(VI) onto Strychnos potatorum seed from aqueous solution

The removal of Cr(VI) from aqueous solution using the seeds of the plant species Strychnos potatorum was studied. The plant is popularly known as Nirmali tree, grown all over India. The dried Nirmali seeds were powdered and used as adsorbent. The SEM-EDX and FT-IR spectrum of the adsorbent revealed that the powder is fibrous in nature and contains functional groups such as OH and COOH. The zeta potential measurements indicated negative surface charge beyond the pH of 1.50. Batch type experiments were conducted to study the influence of pH and initial Cr(VI) concentration on the removal of Cr(VI) from aqueous solutions. Maximum removal of Cr(VI) was observed at pH 1.0. The adsorption equilibrium was attained within a sort contact time of 4 min and the maximum uptake of Cr(VI) was found to be 59 mg g-1. The influence of anions such as Cl-, NO3-, and SO42- on Cr(VI) removal was investigated and found that the removal was slightly affected in the presence of NO3- and SO42-. The rate of removal of Cr(VI) was found to obey pseudo second order rate equation. The high free energy value of adsorption (ΔG = -137 kJ mol-1) indicated that the adsorption of Cr(VI) on Nirmali seed powder was mainly attributed due to chemical and electrostatic interaction. © 2011 American Institute of Chemical Engineers (AIChE)

Studies on electrochemical detoxification of trichloroethene (TCE) on Ti/IrO2–Ta2O5 electrode from aqueous solution

Anodic oxidation of trichloroethene (TCE), a toxic chlorinated water contaminant, is carried out using Ti/IrO2–Ta2O5 electrode. The redox behaviour of TCE molecule, fouling of electrodes and generation of oxygen was studied by cyclic voltammetry. The studies on cyclic voltammetry reveals that the TCE oxidation takes place directly on electrode surface at the potential region above 0.3 V vs. SCE by direct electron transformation and oxygen evolution from anode. The effect of the electrolyte, pH of the aqueous medium and applied current density on the mineralization behaviour of TCE is also investigated. The degradation of TCE is monitored in terms of TOC and TCE concentration with electrolysis time and corresponding chloride evolution. The complete mineralization of TCE is achieved using Na2SO4 as supporting electrolyte. Conversely, the mineralization of the TCE molecule was observed to be poor because of the electrode fouling in the presence of NaNO3. The dechlorination of TCE is found to increase with electrolysis time and the complete dechlorination is observed at all Iappl studied. The kinetics of TCE degradation was found to follow pseudo-first order reaction whose rate constant was calculated to be 0.0081 cm s1 at Iappl value of 13.39 mA cm2. The performance of Ti/IrO2–Ta2O5 anode was proved to be better compared to Pt under similar experimental conditions

Enhanced photocatalytic performance at a Au/N–TiO2 hollow nanowire array by a combination of light scattering and reduced recombination

We demonstrate one-step gold nanoparticle (AuNP) coating and the surface nitridation of TiO2 nanowires (TiO2-NWs) to amplify visible-light photon reflection. The surface nitridation of TiO2-NW arrays maximizes the anchoring of AuNPs, and the subsequent reduction of the band gap energy from 3.26 eV to 2.69 eV affords visible-light activity. The finite-difference time-domain (FDTD) simulation method clearly exhibits the enhancement in the strengths of localized electric fields between AuNPs and the nanowires, which significantly improves the photocatalytic (PC) performance. Both nitridation and AuNP decoration of TiO2-NWs result in beneficial effects of high (e−/h+) pair separation through healing of the oxygen vacancies. The combined effect of harvesting visible-light photons and reducing recombination in Au/N-doped TiO2-NWs promotes the photocatalytic activity towards degradation of methyl orange to an unprecedented level, ∼4 fold (1.1 × 10−2 min) more than does TiO2-NWs (2.9 × 10−3 min−1). The proposed AuNP decoration of nitridated TiO2-NW surfaces can be applied to a wide range of n-type metal oxides for photoanodes in photocatalytic applications

Advanced Oxidation and Electrooxidation As Tertiary Treatment Techniques to Improve the Purity of Tannery Wastewater

The option of electro-oxidation and advanced oxidation as tertiary treatment technique for the purification of tannery wastewater was explored. The TOC removal of 85% was achieved by UV/O3/H2O2 process, whereas it is hardly 50% by electr-ooxidation. However the power consumption to remove unit mass of TOC by electro-oxidation process was estimated to be 738 kW h/kg, which is ten times less than that of 7600 kW h/kg, required for advanced oxidation process. The kinetic data indicated that the degradation of organics by electro-oxidation is a current control process. To minimize the power consumption, we attempted a two-stage process involving electro-oxidation in the first stage and advanced oxidation in the second stage. The results indicated that the TOC removal by advanced oxidation became sluggish, when the wastewater was processed initially by electro-oxidation. However, the effluents processed by EO were found to be completely disinfected