Arsenite removal from water by electro-coagulation on zinc–zinc and copper–copper electrodes

Removal of arsenite from aqueous solution was carried out using electro-coagulation method. The experiments were conducted using copper–copper and zinc–zinc electrodes. The optimized experimental parameters were 2.0 mg/L initial concentration, 16.0-min processing time, 6.0 pH, 3.0-V applied voltage and 30 °C temperature for zinc–zinc electrodes while these values for copper–copper electrodes were 2.0 mg/L initial concentration, 20.0-min processing time, 7.0 pH, 5.0-V applied voltage and 30 °C temperature. The results demonstrated that zinc–zinc and copper–copper electrodes removed arsenite up to 99.89 and 99.56 %, respectively. The treated water was clear, colorless and odorless without any secondary contamination. There was no change in water quality after the removal of arsenite. The reported method is capable to remove arsenite from water at 6–7 pH range, which is a pH range of natural water. Therefore, this method may be the choice of arsenite removal from natural ground water.     

Lab-scale performance of selected expandable clays under HLW repository conditions

Smectite clay has been proposed for embedding canisters with highly radioactive waste in deep repositories because of its isolating capacity. Montmorillonite-rich bentonite is a premier buffer candidate for many national organizations that are responsible for disposal of such waste. Experience from the use of drilling mud at large depths indicates that other smectite clay minerals are more stable chemically and saponite is one of them. The physical properties of smectitic mixed-layer minerals like Friedland clay are known to be less sensitive to high salt contents and such clay may also be a buffer candidate. Montmorillonite-rich MX-80 clay, Greek saponite with a minor amount of palygorskite, and Friedland clay were investigated in hydrothermal tests with dense samples confined in oedometers with 95 °C temperature at one end, which was made of copper, and 35 °C at the other, for 8 weeks. A 1 % CaCl₂ solution was circulated through a filter at the cold end. At the end of the tests, the samples were sliced into three parts, which were tested with respect to expandability, hydraulic conductivity, and chemical composition. The tests showed that while the saponite was hardly changed at all and did not take up any copper, MX-80 underwent substantial changes in physical performance and adsorbed significant amounts of copper. The Friedland clay sample was intermediate in both respects.     

Empirical and mechanistic evaluation of sodium exchange isotherms on natural mineral and organic adsorbents and organically functionalized nanoparticles

This study was conducted to evaluate the efficiency of low-cost adsorbents including bentonite, kaolinite and zeolite saturated with calcium and potassium, potato and wheat residues, and three metal oxide nanoparticles functionalized with an acidic extract of potato residues in improving the quality of sodic waters. The optimization of factors such as pH, contact time, and adsorbent dosage was investigated using a solution containing sodium, calcium, magnesium, and potassium. The optimal pH and contact time were 7.0 and 24 h, respectively. The optimal dosage for using functionalized nanoparticles was 0.1 g and for using other adsorbents was 1.0 g. The sodium exchange isotherms were conducted in binary sodium–calcium and sodium–potassium and quaternary sodium–calcium–magnesium–potassium systems. Zeolite saturated with potassium was the most effective adsorbent in removing sodium from aqueous solutions with an average removal efficiency of 69.2 and 66.5 % in binary and quaternary systems, respectively. Freundlich and Langmuir equations fitted well to experimental data in both binary and quaternary systems. Cation selectivity coefficients calculated based on the Gaines–Thomas convention varied with changing pH and adsorbent dosage. Graphical and statistical evaluations confirmed that the mechanistic cation exchange model using average Gaines–Thomas selectivity coefficients in geochemical PHREEQC program was able to successfully simulate the sodium exchange on different adsorbents in both systems. The Gaines–Thomas selectivity coefficient values greater than unity and as a consequence, the negative values of the Gibbs free energy change of adsorption indicated that sodium exchange reactions in the presence of different adsorbents used is this study were exergonic and spontaneous.     

Study of mango biomass (Mangifera indica L) as a cationic biosorbent

Unfertilizable fruiting buds of mango plant Mangifera Indica L, an agrowaste, is used as a biomass in this study. The efficacy of the biosorbent was tested for the removal of lead, copper, zinc and nickel metal ions using batch experiments in single and binary metal solution under controlled experimental conditions. It is found that metal sorption increases when the equilibrium metal concentration rises. At highest experimental solution concentration used (150 mg/L), the removal of metal ions were 82.76 % for lead, 76.60 % for copper, 63.35 % for zinc and 59.35 % for nickel while at lowest experimental solution concentration (25 mg/L), the removal of metal ions were 92.00% for lead, 86.84 % for copper, 83.96 % for zinc and 82.29 % for nickel. Biosorption equilibrium isotherms were plotted for metal uptake capacity (q) against residual metal concentrations (C_f) in solution. The q versus C_f sorption isotherm relationship was mathematically expressed by Langmuir and Freundlich models. The values of separation factor were between zero and one indicating favourable sorption for four tested metals on the biosorbent. The surface coverage values were approaching unity with increasing solution concentration indicating effectiveness of biosorbent under investigation. The non-living biomass of Mangifera indica L present comparable biosorption capacity for lead, copper, zinc and nickel metal ions with other types of biosorbent materials found in literature and is effective to remove metal ions from single metal solutions as well as in the presence of other co-ions with the main metal of solution.     

Biosorption of Cu(II) and Pb(II) from aqueous solutions by chemically modified spent coffee grains

In this research, spent coffee grains were modified with citric acid solutions (0.1 and 0.6 M) to increase the quantity of carboxylic groups improving its metal adsorption capacity. Added functional groups on modified and non-modified spent coffee grains were identified and quantified by attenuated total reflection Fourier transform infrared analyses and potentiometric titrations, respectively. These adsorbents were used for the removal of lead (II) and copper (II) from aqueous solutions at 30 °C and different pH in batch systems. In addition, adsorption–desorption experiments were conducted to evaluate the possibility of re-using the modified adsorbent. Potentiometric titrations data reveal that the quantity of carboxylic groups was increased from 0.47 to 2.2 mmol/g when spent coffee grains were modified with 0.1 and 0.6 M citric acid. Spent coffee grains treated with 0.6 M citric acid, achieved a maximum adsorption capacity of 0.77 and 1.53 mmol/g for lead (II) and copper (II), respectively, whereas non-modified spent coffee grains only reached 0.24 and 0.19 mmol/g for lead (II) and copper (II), respectively. Desorption of lead (II) and copper (II) achieved around 70 % using 0.1 N HCl for non-modified and modified spent coffee grains with 0.6 M citric acid. It is suggested that lead (II) and copper (II) species were adsorbed mainly on the carboxylic groups of modified spent coffee grains and these metals may be exchanged for hydrogen and calcium (II) ions during adsorption on non-modified spent coffee grains. Finally, the adsorption equilibrium was reached after 400 min for modified spent coffee grains with 0.6 M citric acid. Modified spent coffee grains are a promising option for removing metal cations from aqueous solutions due to its low cost and high adsorption capacity (about 10 times higher than the activated carbons).     

Adsorption kinetic models for the removal of Cu(II) from aqueous solution by clay liners in landfills

Liners are commonly used in engineered waste disposal landfill to minimize the potential contamination of the aquatic environment. The adsorption behavior of Cu(II) from aqueous solution onto clay admixed with various mix ratios of quarry fines was investigated. The amount of Cu(II) adsorption increases with increase in contact time. The copper removal efficiencies of the composite mixture gradually decrease from 94.53 % (raw clay) to 85.59 % (20 % of quarry fines with clay), and appreciable decrease in percent removal 75.61 % was found with 25 % of quarry fines with clay. The kinetic adsorption data were analyzed by pseudo-first-order, pseudo-second-order, Bhattacharya–Venkobachar and Natarajan–Khalaf kinetic models to classify adsorption process mechanisms. Kinetic experimental data were good agreement with pseudo-second-order kinetic model with the degree of fitness of the data (R ²) 0.9999 for the adsorption of Cu(II). The results revealed that quarry fines can be used with optimum of 20 % replacement of natural clay for removal of Cu(II) as a liner material in landfills.     

Fluoride contamination of groundwater in parts of eastern India and a preliminary experimental study of fluoride adsorption by natural haematite iron ore and synthetic magnetite

Incidence of high fluoride (F^?) in groundwater (>1.5 mg/L) in two tribal belts of eastern India, one around Chukru in the Palamau district of Jharkhand and the other around Karlakot in the Nuapada district of Odisha, has been studied. The maximum concentration of F^? in groundwater from dug wells and tube wells is 10.30 mg/L in Chukru and 4.62 mg/L in Karlakot. The groundwaters are mildly alkaline with pH ranges of 7.52–8.22 and 7.33–8.20 in Chukru and Karlakot, respectively. The F^? concentration is positively correlated with pH, electrical conductivity and SO₄ ^2? in both areas. The high F^? in groundwater resulted mainly from dissolution of biotite and fluorapatite in quartzofeldspathic gneiss. The ionic dominance pattern (in meq/L) is mostly in the order Ca²⁺ > Na⁺ > Mg²⁺ > K⁺ among cations and HCO₃ ^? > SO₄ ^2? > < Cl^? > F^? among anions in the Karlakot groundwater. Preliminary adsorption experiments were conducted on natural haematite iron ore and synthetic magnetite to evaluate their potential for F^? removal from water. Effects of different parameters such as contact time, pH, adsorbent dose and initial F^? concentration on the adsorption capacity of these materials were investigated. Strong dependence of F^? removal on pH was observed for both the adsorbents. With natural haematite iron ore, the maximum F^? removal of 66 % occurred at an initial pH of 3.2 for a solution with F^? concentration of 3 mg/L, adsorbent dose of 7 g/L and overnight contact time. The haematite iron ore was observed to increase the pH of the F^? solution. Adsorption equilibrium was not achieved with this adsorbent even after a contact time of 45.2 h. In the case of synthetic magnetite, 84 % F^? removal was achieved after 2 min of contact time for a solution with F^? concentration of 6 mg/L, adsorbent dose of 10 g/L and initial pH of 7. The results indicate high potential of both natural haematite iron ore and synthetic magnetite as adsorbents of F^? in water.     

Adsorption efficiency,thermodynamics and kinetics of Schiff base-modified nanoparticles for removal of heavy metals

Nanosilica particles modified by Schiff base ligands 3-methoxy salicylaldimine propyl triethoxysilane (MNS₁), 5-bromo salicylaldimine propyl triethoxysilane (MNS₂) and 3-hydroxy salicylaldimine propyl triethoxysilane (MNS₃) were prepared, and their potential for separation of copper, lead, zinc, cadmium, cobalt and nickel ions from aqueous solutions was examined. The effect of parameters influencing adsorption efficiency including aqueous-phase pH, amount of adsorbent, stirring time and initial concentration of the metal ions was assessed and discussed. Although MNS₁ and MNS₃ removed lead ions efficiently, all adsorbents showed strong selectivity toward copper ions. It was shown that, under some circumstances, MNS₃ decreased the amount of other ions, particularly cobalt, in the aqueous phase. The adsorbents were also applied for removal of copper and lead ions from real samples. Possible quantitative desorption of the metal ions loaded onto the adsorbents suggests their multiple uses in adsorption–desorption process. Investigation of temperature dependency of the process led to determination of the ΔH°, ΔS° and ΔG° values. This investigation indicates that the adsorption of copper ions onto the all studied adsorbents and lead ions onto MNS₁ and MNS₃ is endothermic. The Langmuir, Freundlich, Temkin and Dubinin–Radushkevich isotherms were tested to describe the equilibrium data. Pseudo-first-order, pseudo-second-order, Elovich and intra-particle diffusion equations were applied to study the kinetics of copper and lead adsorption onto the modified nanoparticles. This investigation indicates that the process for all adsorbents follows pseudo-second-order kinetics and suggests a chemisorption mechanism for the adsorption processes by the studied adsorbents.     

Simultaneous electrokinetic removal of polycyclic aromatic hydrocarbons and metals from a sediment using mixed enhancing agents

The maintenance of harbor waterways generates large amounts of dredged sediments which are often rich in coexisting organic and inorganic contaminants. Electrokinetic remediation treatments have recently been developed for the simultaneous removal of heavy metals and polycyclic aromatic hydrocarbons (PAHs), using various enhancing agents generally tested sequentially. In this study, different processing fluids were tested, alone or mixed, to improve the decontamination of aged model sediment contaminated with cadmium, lead, chromium, copper, zinc, and five PAHs. Nitric acid (NA) and citric acid (CA) were tested to avoid the formation of an alkaline front into the sediment and favor the metals removal, while an anionic surfactant [sodium dodecyl sulfate (SDS)] and a nonionic surfactant (Tween 20) were tested to solubilize and mobilize PAHs. Processing fluids were circulated under a constant voltage gradient of 1 V cm^?1 for 10–14 days. NA showed an excellent potential to remove metals (76.8–99.9 % removal) and PAHs (70.3–89.7 % removal) in a single run. Besides, the mixture of Tween 20 and CA, more environmental friendly, could be considered as a relatively good processing fluid for the simultaneous removal of metals (10.3–90.8 % removal) and PAHs (53.6–61.6 % removal) from the fine-grained sediment, while SDS mixed to CA was not a good candidate for this purpose (0.1–65 % removal for metals and 34.1–41.0 % removal for PAHs).     

A study on soil physico-chemical, microbial and metal content in Sukinda chromite mine of Odisha, India

Soil samples from chromite mining site and its adjacent overburden dumps and fallow land of Sukinda, Odisha, were analysed for their physico-chemical, microbial and metal contents. Chromite mine soils were heterogenous mixture of clay, mud, minerals and rocks. The pH of the soils ranges between 5.87 and 7.36. The nutrient contents of the mine soils (N, P, K and organic C) were found to be extremely low. Analysis of chromite mine soils revealed accumulation of a number of metals in high concentrations (Fe > Cr > Mn > Ni > Zn > Pb > Sr) which exceeded ecotoxicological limits in soil. Correlation and cluster analysis of metals revealed a strong relation between Cr, Ni, Fe, Mn among the different attributes studied. Assessment of different microbial groups such as fungi, actinomycetes and bacteria (heterotrophic, spore forming, free-living nitrogen fixing, phosphate solubilising and cellulose degrading) from mine soils were found to be either extremely low or absent in some soil samples. Further chromium tolerant bacteria (CTB) were isolated using 100 mg/L Cr(VI) enriched nutrient agar medium and were screened for their tolerance towards increasing concentrations of hexavalent chromium and other toxic metals. Out of 23 CTB isolates, three bacteria tolerated up to 900 mg/L, 6 up to 500 mg/L, 20 up to 200 mg/L of Cr(VI). These bacteria were also found to be sensitive towards Cu > Co > Cd and very few CTB strains could show multiple metal tolerance. These strains have great scope for their application in bioremediation of toxic chromium ions in presence of other metals ions, which needs to be explored for their biotechnological applications.     

Effectiveness of Mg–Al-layered double hydroxide for heavy metal removal from mine wastewater and sludge volume reduction

Health hazards from heavy metal pollution in water systems are a global environmental problem. Of similar concern is sludge that results from wastewater treatment due to unsatisfactory sludge management technology. Therefore, the effectiveness of using Mg–Al-layered double hydroxide in the removal of heavy metals from mine wastewater was tested and compared with that of calcium hydroxide [Ca(OH)₂], which is a common treatment method for heavy metal removal. Initially, the mine wastewater contained cations of the heavy metals iron (Fe), zinc (Zn), copper (Cu), and lead (Pb). The Mg–Al-layered double hydroxides were able to remove 371, 7.2, 121, and 0.4 mg/L of these pollutants, respectively, using the co-precipitation method. The removal of these metals is most effective using 0.5 g Mg–Al-layered double hydroxide (Mg/Al molar ratio 4) and 20 min of shaking. Zn was removed by the formation of Zn(NO₃)(OH)·H₂O and Zn₅(NO₃)₂(OH)₈ when LDH, Mg/Al molar ratios of 4 and 2, respectively, were used. Similarly, Fe, Cu, and Pb were removed by the formation of Fe–Al-layered double hydroxide, Cu₂(OH)₃·NO₃ and Pb₄(OH)₄(NO₃)₄, respectively. While Ca(OH)₂ is also capable of reducing the heavy metal concentrations below the Japanese recommended values, this analysis shows that using 0.5 g Mg–Al-layered double hydroxide is a better treatment condition for mine wastewater, because it generates lower sludge volumes than 0.1 g of Ca(OH)₂. The measured sludge volume was 1.5 mL for Mg–Al-layered double hydroxide and 2.5 mL for Ca(OH)₂, a nearly twofold further reduction.     

Mineralogical attenuation for metallic remediation in a passive system for mine water treatment

Passive systems with constructed wetlands have been consistently used to treat mine water from abandoned mines. Long-term and cost-effective remediation is a crucial expectation for these water treatment facilities. To achieve that, a complex chain of physical, chemical, biological, and mineralogical mechanisms for pollutants removal must be designed to simulate natural attenuation processes. This paper aims to present geochemical and mineralogical data obtained in a recently constructed passive system (from an abandoned mine, Jales, Northern Portugal). It shows the role of different solid materials in the retention of metals and arsenic, observed during the start-up period of the treatment plant. The mineralogical study focused on two types of materials: (1) the ochre-precipitates, formed as waste products from the neutralization process, and (2) the fine-grained minerals contained in the soil of the wetlands. The ochre-precipitates demonstrated to be poorly ordered iron-rich material, which gave rise to hematite upon artificial heating. The heating experiments also provided mineralogical evidence for the presence of an associated amorphous arsenic-rich compound. Chemical analysis on the freshly ochre-precipitates revealed high concentrations of arsenic (51,867 ppm) and metals, such as zinc (1,213 ppm) and manganese (821 ppm), indicating strong enrichment factors relative to the water from which they precipitate. Mineralogical data obtained in the soil of the wetlands indicate that chlorite, illite, chlorite–vermiculite and mica–vermiculite mixed-layers, vermiculite, kaolinite and goethite are concentrated in the fine-grained fractions (<20 and <2 μm). The chemical analyses show that high levels of arsenic (up to 3%) and metals are also retained in these fractions, which may be enhanced by the low degree of order of the clay minerals as suggested by an XRD study. The obtained results suggest that, although the treatment plant has been receiving water only since 2006, future performance will be strongly dependent on these identified mineralogical pollutant hosts.     

Dynamic fuzzy neural network for simulating the fixed-bed adsorption of cadmium,nickel, and zinc on bone char

This study introduces the application of a dynamic fuzzy neural network for fitting and simulating the adsorption of nickel, cadmium, and zinc ions in mono- and bi-metallic solutions (nickel–cadmium, nickel–zinc, and cadmium–zinc) using packed-bed columns with bone char. This neural network model has shown a flexible and self-adaptive architecture with a faster learning speed than that of traditional artificial neural approaches. Results showed that this neural network model was reliable for representing the high asymmetry behavior of concentration profiles in both mono- and bi-metallic breakthrough curves where its accuracy was quite reasonable. Breakthrough parameters for mono-component and binary systems of tested heavy metals were calculated and compared. This analysis showed that the removal of these heavy metal ions in binary systems was a strong competitive adsorption process where the presence of co-ions reduced the removal performance of bone char at fixed-bed adsorbers. Results of surface characterization of adsorbent samples with X-ray photoelectron and infrared spectroscopy supported a removal mechanism based on an ion exchange between calcium from hydroxyapatite of bone char and heavy metal ions in the solution forming new metal–phosphate interactions in the adsorbent surface.     

Adsorptive removal of Cu(II) and Pb(II) onto mixed-waste activated carbon: kinetic,thermodynamic, and competitive studies and application to real wastewater samples

This work aimed to investigate the adsorption characteristics, both kinetically and thermodynamically, of Cu(II) and Pb(II) removal from aqueous solutions onto mixed-waste activated carbon, as well as to study the competitive behavior found in mixed heavy metal solution systems. This study shows that activated carbon prepared from mixed waste is an effective adsorbent for the removal of Cu(II) and Pb(II) from aqueous solutions, with the aim of detoxifying industrial effluents before their safe disposal onto water surfaces. The adsorption process was characterized in terms of kinetic and thermodynamic studies. In addition, the influence of presence of Cu(II) and Pb(II) in a competitive system was investigated. The results showed that the maximum adsorption capacities were gained at a pH of 6 with a contact time of 180 min, a metal solution concentration of 300 ppm, and an adsorbent dose of 0.3 g/L. The adsorption process was found to follow a pseudo-first-order kinetic model. Thermodynamic parameters such as ΔG ^o, ΔH ^o, and ΔS ^o showed that the sorption process was spontaneous and endothermic in nature. A competitive study demonstrated the applicability of mixed-waste activated carbon to adsorb Cu(II) and Pb(II) from a solution of mixed metals. In addition, the adsorption capacity was found to be as effective as other adsorbents reported in the literature. The developed adsorptive removal procedure was applied for treatment of real wastewater samples and showed high removal efficiency.     

Isolation and characterization of metal-resistant bacterial strain from wastewater and evaluation of its capacity in metal-ions removal using living and dry bacterial cells

A novel strain of the genus Micrococcus isolated from wastewater was studied for resistance to seven heavy metals and forty antibiotics. Its capacity to accumulate metal ions was also realized at different pH. The strain exhibited high minimal inhibitory concentration values for metal ions tested and resist to 15 antibiotics. The living cells of the bacterial strain show a largest uptake capacity at pH 6–8.5 for copper, nickel, and zinc with values ranging from 51.45 to 83.90 %, 52.59 to 78.81 %, and 59.55 to 78.90 %, respectively. It was also able to absorbed 59.81–80.08 % of chromium and 58.09–79.41 % of cobalt at pH 7.3–8.5. The maximum lead uptake was obtained at pH 5.5–8.5 with an amount of 55.28–91.06 %. The significant absorption of cadmium was shown at pH 6.5 with 38 %. In 25 µg mL-1 zinc, chromium, and nickel solutions, dead cells of the isolate were able to biosorbed 20.46, 22.5, and 23.98 µg mL^?1, respectively, after 30 min of contact. In other solutions with higher concentrations 50 and 100 µg mL^?1, the amount of each metal immobilized was, respectively, as follows: 38.02 and 90.21 µg mL^?1 for zinc, 39.78 and 89.23 µg mL^?1 for chromium, and 47.19 and 86.83 µg mL^?1 for nickel. Due to its high-metal accumulation capacity in aerobic conditions, these Gram-positive bacteria may be potentially applicable in situ bioremediation of heavy metals contaminating aqueous systems.     

Adsorption of copper and zinc by oil shale

 Oil shale is able to remove appreciable amounts of copper and zinc ions from aqueous solutions. It was noted that an increase in the adsorbent concentration with constant copper or zinc concentration resulted in greater metal removal from solution. An increase in the copper or zinc concentration with a constant sorbent concentration resulted in higher metal loading per unit weight of sorbent. For both metals, copper and zinc, equilibrium was attained after 24-h contact time. Increase in the initial pH or temperature of the metal solution resulted in an increase in the metal uptake per unit weight of the sorbent. Freundlich isotherm model was found to be applicable for the experimental data of Cu²⁺ and Zn²⁺. The results showed that oil shale could be used for the adsorption of the Cu²⁺ and Zn²⁺ with higher affinity toward Zn²⁺ ions. Addition of sodium salt to the metal solution influenced copper removal positively, but inhibited zinc removal. Received: 3 January 2000 · Accepted: 27 June 2000     

Competitive sorption and desorption of cadmium and lead in paddy soils of eastern China

To assess the competitive sorption and desorption of cadmium (Cd) and lead (Pb), batch equilibrium experiments were performed using single- and binary-metal solutions in surface samples of three paddy soils from eastern China. Sorption isotherms were well fitted with one-metal and competitive Langmuir equation for single- and binary-metal system, respectively. The distribution coefficient (K _d) values were K _{d single} (Pb) > K _{d binary} (Pb) > K _{d single} (Cd) > K _{d binary} (Cd), indicating that Pb was stronger sorbed by these soils than Cd in binary metal system. Soils with high pH and clay content had the greatest sorption capacity as estimated by the maximum sorption parameter (Q). The co-existence of both metals reduces their tendency of sorption, whereas Cd sorption was affected to a greater extent than that of Pb. The Langmuir binding strength parameter (b) in binary sorption system was greater than that in single sorption system for all soils (b < b ₁), indicating that competition for sorption sites promote the retention of both metals into more specific sorption sites. Sorption of Cd and Pb decreased soil pH by 1.61 U for YRS, 1.39 U for PCS, and 0.91 U for SLS. The decreases of pH in binary metal system were greater than in single-metal system for three soils. Cadmium and Pb desorption increased with increasing Cd and Pb sorption saturation for all soils; however, Cd desorption ratio in binary metal system (d _Cd*) was much greater than Pb (d _Pb*), indicating that under the competitive sorption conditions, the sorbed Cd was more readily desorbed from the soils than the sorbed Pb.     

Effect of electrode configuration on pH distribution and heavy metal ions migration during soil electrokinetic remediation

Generation, migration, and distribution of H⁺ and OH^? have remarkable influence on heavy metal removal from soil during electrokinetic remediation. A series of experiments were carried out to investigate the effects of electrode configuration and voltage on pH distribution and heavy metal migration, based on the preliminary exploratory experiments. In the first phase, three soil samples were used to observe the effect of electrode configuration on pH distribution. Then, three more soil samples were used to explore the effect of voltage on ion migration. Finally, three other soil samples were used to explore the effect of electrode configuration (i.e. angle CAC) on pH distribution and heavy metal migration. The results showed that the soil was divided into acid, base, and pH-jump zones and that heavy metals migrated under low pH conditions and were deposited at the pH-jump zone. Heavy metal distribution was fairly consistent with pH distribution. Under the optimal condition of $ \angle $ CAC 60º and voltage 2.33 V/cm, 4.22 mg copper and 0.51 mg zinc migrated from acid zone and deposited in pH-jump zone with 6.44 kJ energy consumption.     

Adsorptive removal of azo dyes from aqueous solutions by natural bentonite under static and dynamic flow conditions

This work reports new findings on the physisorption of Foron Blue 291 on natural bentonite under both static and dynamic flow conditions of the aqueous solution that may prove important when using the adsorption technique for wastewater treatment. The quantity of dye retained by the solid matrix under both ideal exchange conditions between dyes and adsorbents given in a batch reactor and a pulse injection of dissolved azo dye in a soil column composed of a mixture of mean grain-sized sand and bentonite at different flow rates and at various mass fractions of bentonite was studied. Column experiments involving the use of a non-reactive tracer (Fluorescein) were performed to examine hydrodynamic behaviours of the clay/sand mixture studied. It is shown that advective–dispersive transport across a clay/sand mixture may be characterized by a double porosity medium. The results obtained on the reactive transport of Foron Blue 291 underscore that Foron Blue 291 adsorption depends heavily on the mass fraction of clay in a clay/sand mixture. With a clay mass fraction of 30 %, 77 % of the dissolved Foron Blue 291 mass was irreversibly removed by the adsorbent. At low flow rates, removal ratios obtained from the dynamic reactive system were similar to those obtained through previous static reactor experiments.     

Transformation of metal speciation in purple soil as affected by waterlogging

This study was conducted to investigate the effect of waterlogging on copper, lead and cadmium fractionation in Chinese purple soil. Heavy metals were added to purple soil at 80 % field capacity and waterlogging regimes as nitrate salts of 500 mg kg^?1 of copper and lead, and 5 mg kg^?1 of cadmium. Metals in the incubated soil samples were fractionated termly from 1 to 35 days by the sequential extraction procedure. Under both treatments, the heavy metals spiked in the soil were transformed slowly from the exchangeable fractions into more stable fractions, whereas their residual fractions barely changed. The transformation process of exchangeable fraction in soil was estimated by Elovich kinetic equation for the above incubation periods, and the constant B in Elovich equation was applied to reflect the transformation rates of metal speciation. It was found that waterlogging incubation could immobilize heavy metals, resulting in decreased lability and availability of the metals in purple soil. The effect of waterlogging on the redistribution of heavy metals in purple soil might be mainly related to the changes of pH, potential redox and hydrous oxides in varying soil-water systems.