Removal of metals in leachate from sewage sludge using electrochemical technology

Nitrate removal from groundwater using a batch and continuous flow hybrid Fe-electrocoagulation and electrooxidation system

During the last two decades nitrate contaminated groundwater has become an extensive worldwide problem with wide-reaching negative effects on human health and the environment. In this study, a combination of electrocoagulation (EC) and electrooxidation (EO) was studied as a denitrification process to efficiently remove nitrates and ammonium (a by-product produced during EC) from real polluted groundwater. Initially, EC experiments under batch operating mode were performed using iron electrodes at different applied current density values (20-40 mA cm^-2). Nitrate percentage removal of 100 % was recorded, however high ammonium concentrations were performed (4.5-6.5 mg NH₄⁺-Ν L^-1). Therefore, a continuous flow system was examined for the complete removal of both nitrates and EC-generated ammonium cations. The system comprised an EC reactor, a settling tank and an EO reactor. The applied current densities to the EC process were the same as those in the batch experiments, while the volumetric flow rates were 4, 6 and 8 mL min^-1. Regarding the current density of the EO process was kept constant at the value of 75 mA cm^-2. The percentage nitrate removal recorded during the EC process ranged between 52.0 and 100 %, while the NH₄⁺-N concentration at the outlet of the EO reduced significantly (53-100 %) depending on the applied current density and the volumetric flow rate. Also, the dissolved iron concentration in the treated water was always below the legislated limit of 0.2 mg L^-1 (up to 0.027 mg L^-1). These results indicate that the proposed hybrid system is capable of denitrifying real nitrate contaminated groundwater without generating toxic by-products, therefore making the water suitable for human consumption.

Feasibility of bioleaching of heavy metals from sediment with indigenous bacteria using agricultural sulfur soil conditioners

Sediment bioleaching using a sulfur substrate is a promising approach to the removal of heavy metals. Compared with commercial sulfur powder used as the sulfur substrate, agricultural sulfur soil conditioners may reduce secondary pollution and facilitate the reuse of sediment. This study explored the bioleaching effect of three agricultural sulfur soil conditioners, including sulfur-coated urea, bentonite sulfur, and bio-sulfur, and the bioleaching potential of the indigenous sediment bacteria. The results showed that the sulfur-coated urea had a comparable bioleaching effect with sulfur powder (Ni 35.35%, Cu 74.27%, Zn 69.92%) and the highest maximum bioleaching rate because of the additional nitrogen. The bentonite sulfur leached the least but increased the proportion of the residual state due to its adsorption of heavy metal. Similar changes to the microbial flora structure and bioleaching mechanism were found with the use of sulfur powder, sulfur-coated urea, and bentonite sulfur as the bioleaching substrate. There was no significant difference between the indigenous bacteria and the sludge-enriched bacteria in the bioleaching effect except for bio-sulfur, which only performed well with the sludge-enriched bacteria. In the absence of inoculum, the bio-sulfur hindered the bioleaching process due to high levels of organic matter. This study provides insights into the practical application of bioleaching heavy metal removal technology from the perspective of sulfur substrate selection.

Preparation of polyelectrolyte-modified membranes for heavy metal ions removal

Polyethersulfone membranes were modified by polyelectrolyte (PE) multilayers, made of poly(allylamine hydrochloride) with poly(styrene sulfonate), to remove Cu²⁺, Zn²⁺ and Ni²⁺ heavy metal cations from aqueous solutions in a wide range of metal concentration (50-1200 ppm). After characterization of the modified membranes, the efficiency of the process was estimated for single heavy metal ions solution leading to high rejection rates (>90% for 50 ppm) and good adsorption capacities (7.0-8.5 mg cm^-2) whatever the metal ion tested. The stability in time of the modified membranes was proved by repeating successive filtrations with the same membrane. The filtration process was also used with mixed solutions composed of Cu²⁺, Zn²⁺ and Ni²⁺ ions. The rejection rates obtained for these ternary systems were very similar to the ones obtained for the single metal solutions, showing that the filtration process is still efficient for mixed solutions and can be applied for the decontamination of complex solutions. The long-term stability of the modified membranes was also demonstrated for mixed solutions. The high efficiency of the filtration process and the good adsorption capacities of the modified membranes are due to the ability of the PEs used to complex all the metallic dications tested in this study.

Heavy-metal extraction from sewage sludge using phosphorous-based salts: optimization process with Na2H2P2O7

Land application is one of the important disposal alternatives for sewage sludge, but availability of potential toxic metals often restricts its uses. Three phosphorous-based salts (Na₂H₂P₂O₇, K₄P₂O₇, KH₂PO₄) were studied as potential metal extractants. The conclusions of the research were that greater extractive efficiency is achieved through a 30-min process of vertical shaking with disodium diacid pyrophosphate - Na₂H₂P₂O₇ - at a concentration of 0.2 M at pH 2. Alternatively, the optimized process with oscillating shaking equipment would require 60 min. In both cases the average of set of extracted metals is around 50%. A second extraction process with potassium pyrophosphate - K₄P₂O₇ at pH 6 achieved the reduction of further total amounts of metal, upper 65% with respect to the initial content. In this way the sludge could be used in land applications, with restrictions on each soil, according to the limit values specified in the future regulations.

Lead Removal from Contaminated Shooting Range Soil using Acetic Acid Potassium Chloride Washing Solutions and Electrochemical Reduction

BACKGROUND

Cleanup of soils contaminated with toxic metals is a difficult task due to the method inefficiency and the destructive nature of clean-up techniques on soil ecosystems.

OBJECTIVES

This study was performed to improve the removal efficiency of an acetic acid washing solution for the removal of lead (Pb) from soil. Acetic acid was used in combination with different concentrations of potassium chloride. In order to maximize the removal of Pb from the leachate, different electrode combinations were applied to the washing solutions.

METHODS

Acetic acid/potassium chloride washing solutions and electrochemical reduction were applied to lead-contaminated soil obtained from an impact berm of a major military shooting range in Ibadan, southwestern Nigeria. The soil was subjected to 5% acetic acid/5% potassium chloride (KCL) and 5% acetic acid/10% KCL solutions in an ex-situ batch experiment. The leachate was electrochemically reduced using 12 volt direct current with a current of 7 amps and 2.5 amps, with aluminum (Al)-Al, iron (Fe)-Fe, Al-Fe and Fe-Al electrodes.

RESULTS

The 5% acetic acid/5% KCL proved more efficient for Pb removal in soil with values ranging from 74.9% to 86.9% for 3% soil pulp densities with one washing time of 6 hours. Removal efficiency of Pb from the contaminated soil significantly decreased as the soil pulp density increased. The Al-Al and Al-Fe bipolar electrode combinations showed better removal efficiency of Pb from the leachates with values of 93.7% and 95.6% for 7 amps and 94.5% and 97.3% for 2.5 amps, respectively.

CONCLUSIONS

The combined 5% acetic acid and 5% potassium chloride washing solution enhances the removal efficiency of Pb in soil and poses less risk to the soil ecosystem and the environment in general.

Characterization, Recovery Opportunities, and Valuation of Metals in Municipal Sludges from U.S. Wastewater Treatment Plants Nationwide

U.S. sewage sludges were analyzed for 58 regulated and nonregulated elements by ICP-MS and electron microscopy to explore opportunities for removal and recovery. Sludge/water distribution coefficients (KD, L/kg dry weight) spanned 5 orders of magnitude, indicating significant metal accumulation in biosolids. Rare-earth elements and minor metals (Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) detected in sludges showed enrichment factors (EFs) near unity, suggesting dust or soils as likely dominant sources. In contrast, most platinum group elements (i.e., Ru, Rh, Pd, Pt) showed high EF and KD values, indicating anthropogenic sources. Numerous metallic and metal oxide colloids (<100-500 nm diameter) were detected; the morphology of abundant aggregates of primary particles measuring <100 nm provided clues to their origin. For a community of 1 million people, metals in biosolids were valued at up to US$13 million annually. A model incorporating a parameter (KD × EF × $Value) to capture the relative potential for economic value from biosolids revealed the identity of the 13 most lucrative elements (Ag, Cu, Au, P, Fe, Pd, Mn, Zn, Ir, Al, Cd, Ti, Ga, and Cr) with a combined value of US $280/ton of sludge.

Removal of copper in leachate from mining residues using electrochemical technology

This research is related to a laboratory study on the performance of a successive mining residues leaching and electrochemical copper recovery process. To clearly define the experimental region for response surface methodology (RSM), a preliminary study was performed by applying a current intensity varying from 0.5 A to 4.0 A for 60 min. By decreasing the current intensity from 4.0 A to 0.5 A, a good adhesion and a very smooth and continuous interface of copper was formed and deposited on the cathode electrode. However, the removal rate of Cu decreased from 83.7% to 37.9% when the current intensity passed from 4.0 A to 0.5 A, respectively. Subsequently, the factorial design and central composite design methodologies were successively employed to define the optimal operating conditions for copper removal in the mining residues leachate. Using a 2(3) factorial matrix, the best performance for copper removal (97.7%) was obtained at a current intensity of 2.0 A during 100 min. The current intensity and electrolysis time were found to be the most influent parameters. The contribution of current intensity and electrolysis time was around 65.8% and 33.9%, respectively. The treatment using copper electrode and current intensity of 1.3 A during 80 min was found to be the optimal conditions in terms of cost/effectiveness. Under these conditions, 86% of copper can be recovered for a total cost of 0.56 $ per cubic meter of treated mining residues leachate.

Immobilization of polyethylenimine nanoclusters onto a cation exchange resin through self-crosslinking for selective Cu(II) removal

Donnan membrane principle provides great opportunities for development of highly efficient adsorbents for toxic metals abatement. Based on the principle we prepared a new composite adsorbent by immobilizing polyethylenimine (PEI) nanoclusters within a macroporous cation exchanger D001 through self-crosslinking by glutaraldehyde upon Cu(II)-template process. Negligible PEI loss was observed from the resultant composite adsorbent D001-PEI-GA to solution of pHs 1-12. Increasing solution pH from 1 to 6 results in more favorable Cu(II) retention by D001-PEI-GA, and Cu(II) adsorption onto D001-PEI-GA follows the pseudo-second-order kinetic model well. Compared to D001, D001-PEI-GA displays more preferable Cu(II) sequestration in the presence of co-ions Mg(2+), Ca(2+), Sr(2+) at higher levels. Fixed-bed adsorption of a synthetic solution containing Cu(II) and other co-ions showed that Cu(II) sequestration on D001-PEI-GA could result in its conspicuous decrease from 5mg/L to below 0.01 mg/L with the treatment volume as high as 630 BV per run, while that for D001 was only ∼ 85 BV. Also, the spent composite adsorbent can be readily regenerated by HCl (0.3M)-NaCl (0.5M) binary solution for repeated use with negligible capacity loss.

Treatment of cadmium and nickel electroplating rinse water by electrocoagulation

Treatments of cadmium-cyanide and nickel-cyanide electroplating rinse water were investigated in an electrochemical reactor equipped with iron plate electrodes in a batch mode by electrocoagulation (EC). Effects of the process variables such as pH, current density, and operating time were explored with respect to removal efficiencies of cadmium, nickel and cyanide in electroplating rinse water and operating costs as well. Removal efficiencies and operating costs under the optimum conditions (30 A/m2, 30 min and pH 8-10 for cadmium; 60A/m2, 80 min and pH 8-10 for nickel) for the EC process in electroplating rinse water were determined as 99.4% and 1.05/m3 for cadmium, 99.1% and 2.45/m3 for nickel and > 99.7% for cyanide, respectively. The results indicated that EC was very effective treatment for the removals of cadmium, nickel, and cyanide ions from the electroplating rinse water.

Selective removal of Cu(II) ions by using cation-exchange resin-supported polyethyleneimine (PEI) nanoclusters

A novel hybrid adsorbent D001-PEI was fabricated for selective Cu(II) removal by immobilizing soluble polyethyleneimine (PEI) nanoclusters within a macroporous cation exchange resin D001. Negligible release of PEI nanoclusters unexpectedly observed during operation may result from the porous cross-linking nature of D-001 as well as the electrostatic attraction between PEI and D001. Increasing solution pH from 1 to 6 results in more favorable Cu(II) retention by D001-PEI, and Cu(II) adsorption onto D001-PEI follows the Langmuir model and the pseudosecond-order kinetic model well. Compared to the host cation exchanger D001, D001-PEI displays more preferable adsorption toward Cu(II) in the presence of competing Mg(2+), Ca(2+), Sr(2+) at greater levels in solution. Fixed-bed adsorption runs showed that Cu(II) sequestration on D001-PEI could result in its conspicuous decrease from 5 mg/L to below 0.01 mg/L. Also, the spent hybrid adsorbent can be readily regenerated by 6-8 BV HCl (0.2 mol/L)-NaCl (0.5 mol/L) binary solution for repeated use with negligible capacity loss. The results reported herein validate that D001-PEI is a promising adsorbent for enhanced removal of Cu(II) and other heavy metals from waste effluents.

Bioleaching of heavy metals from sewage sludge: a review

During the treatment of sewage, a huge volume of sludge is generated, which is disposed of on land as soil fertilizer/conditioner due to the presence of nitrogen, phosphorus, potassium and other nutrients. However, the presence of toxic heavy metals and other toxic compounds in the sludge restricts its use as a fertilizer. Over the years, bioleaching has been developed as an environmentally friendly and cost-effective technology for the removal of heavy metals from the sludge. The present paper gives an overview of the various bioleaching studies carried out in different modes of operation. The various important aspects such as pathogen destruction, odor reduction and metal recovery from acidic leachate also have been discussed. Further, a detailed discussion was made on the various technical problems associated with the bioleaching process, which need to be addressed while developing the process on a larger scale.

Decolourization of dye-containing effluent using mineral coagulants produced by electrocoagulation

The colour and colour causing-compounds has always been undesirable in water for any use, be it industrial or domestic wastewaters. The discharge of such effluents causes excessive oxygen demand in the receiving water and then a treatment is required before discharge into ecosystems. This study examined the possibility to remove colour causing-compounds from effluent by chemical coagulation, in comparison with direct electrocoagulation. The inorganic coagulants (C1, C2 and C3) in the form of dry powder tested, were respectively produced from electrolysis of S1=[NaOH (7.5 x 10(-3)M)], S2=[NaCl (10(-2)M)], and S3=[NaOH (7.5 x 10(-3)M)+NaCl (10(-2)M)] solutions, using sacrificial aluminium electrodes operated at an electrical potential of 12 V. Reactive textile dye (CI Reactive Red 141) was used as model of colour-causing compound prepared at a concentration of 50 mgl(-1). The best performances of dye removal were obtained with C(2) having a chemical structure comprised of a mixture of polymeric specie (Al45O45(OH)45Cl) and monomeric species (AlCl(OH)2.2H2O and Al(OH)3). The removal efficiency (R(A)) evaluated by measuring the yields of 540 nm-absorbance removal varied from 41 to 96% through 60 min of treatment by imposing a concentration of C2 ranging from 100 to 400 mg l(-1). The effectiveness of the treatment increased and the effluent became more and more transparent while increasing C(2) concentration. The comparison of chemical treatment using C2 coagulant and direct electrocoagulation of CI Reactive Red 141 containing synthetic solution demonstrated the advantage of chemical treatment during the first few minutes of treatment. A yield of 88% of absorbance removal was recorded using C2 coagulant (400 mg l(-1)) over the first 10 min of treatment, compared to 60% measured using direct electrocoagulation while imposing either 10 or 15 V of electrical potential close to the value (12 V) required during C2 production. However, at the end of the treatment (after 60 min of treatment), CI Reactive Red 141 pollutant was completely removed from solution (540 nm-absorbance removal of 100%) using direct electrochemical treatment, compared to 96.4% of absorbance removed while treating dye-containing synthetic solution by means of C2 coagulant.

Effectiveness of soil washing, nanofiltration and electrochemical treatment for the recovery of metal ions coming from a contaminated soil

This research was conducted to integrate soil washing, nanofiltration (NF) membranes and electrochemical treatment as feasible methods for the remediation of contaminated soils. For this investigation, two acidic leachates (pH=2) were prepared using HCl and H(2)SO(4)-NaCl as soil-washing agents. The results of the soil washing indicated that HCl and the combined H(2)SO(4)-NaCl were effective for the extraction of ions resulting from a contaminated soil. It was observed that both leachates presented similar chemical compositions. Following this procedure, the leachate solutions were pre-filtered by microfiltration, followed by NF using the Desal-5 (DK) membrane. The experiment results showed that NF membranes presented a high ion-retention rate for the two leachates. In general, better retentions were observed with the leachate prepared with HCl than H(2)SO(4)-NaCl. In order to treat the concentrate resulting from the NF treatment, it underwent an electrochemical procedure (electrochemical deposition) as an alternative method for safe disposal. The results showed a high reduction of toxic ions, such as Pb and Cu, from the solution. These three processes applied in conjunction not only indicated that the treatment of solutions heavily contaminated with inorganic pollutants resulting from contaminated soils were feasible but also suggested the possibility of treating different types of heavy industrial effluents.

Highly effective removal of heavy metals by polymer-based zirconium phosphate: A case study of lead ion

Zirconium phosphate (ZrP) has recently been demonstrated as an excellent sorbent for heavy metals due to its high selectivity, high thermal stability, and absolute insolubility in water. However, it cannot be readily adopted in fixed beds or any other flowthrough system due to the excessive pressure drop and poor mechanical strength resulting from its fine submicrometer particle sizes. In the present study a hybrid sorbent, i.e., polymer-supported ZrP, was prepared by dispersing ZrP within a strongly acidic cation exchanger D-001 and used for enhanced lead removal from contaminated waters. D-001 was selected as a host material for sorbent preparation mainly because of the Donnan membrane effect resulting from the nondiffusible negatively charged sulfonic acid group on the exchanger surface, which would enhance permeation of the targeted metal ions. The hybrid sorbent (hereafter denoted ZrP-001) was characterized using a nitrogen adsorption technique, scanning electron microscope (SEM), and X-ray diffraction (XRD). Lead sorption onto ZrP-001 was found to be pH dependent due to the ion-exchange mechanism, and its sorption kinetics onto ZrP-001 followed the pseudo-first-order model. Compared to D-001, ZrP-001 exhibited more favorable lead sorption particularly in terms of high selectivity, as indicated by its substantially larger distribution coefficients when other competing cations Na(+), Ca(2+), and Mg(2+) coexisted at a high level in solution. Fixed-bed column runs showed that lead sorption on ZrP-001 resulted in a conspicuous decrease of this toxic metal from 40 mg/L to below 0.05 mg/L. By comparison with D-001 and ZrP-CP (ZrP dispersion within a neutrally charged polymer CP), enhanced removal efficiency of ZrP-001 resulted from the Donnan membrane effect of the host material D-001. Moreover, its feasible regeneration by diluted acid solution and negligible ZrP loss during operation also helps ZrP-001 to be a potential candidate for lead removal from water. Thus, all the results suggested that ZrP-001 offers excellent potential for lead removal from contaminated water.

Comparison between electrocoagulation and chemical precipitation for metals removal from acidic soil leachate

This paper provides a quantitative comparison between electrocoagulation and chemical precipitation based on heavy metals (Cd, Cr, Cu, Ni, Pb and Zn) removal from acidic soil leachate (ASL) at the laboratory pilot scale. Chemical precipitation was evaluated using either calcium hydroxide or sodium hydroxide, whereas electrocoagulation was evaluated via an electrolytic cell using mild steel electrodes. Chemical precipitation was as effective as electrocoagulation in removing metals from ASL having low contamination levels (30 mg Pbl(-1) and 18 mg Znl(-1)). For ASL enriched with different metals (each concentration of metals was initially adjusted to 100 mg l(-1)), the residual Cr, Cu, Pb and Zn concentrations at the end of the experiments were below the acceptable level recommended for discharge in sewage urban works (more than 99.8% of metal was removed) using either electrocoagulation or chemical precipitation. Cd was more effectively removed by electrochemical treatment, whereas Ni was easily removed by chemical treatment. The cost for energy, chemicals and disposal of metallic residue of electrocoagulation process ranged from USD 8.83 to 13.95 tds(-1), which was up to five times lower than that recorded using chemical precipitation. Highly effective electrocoagulation was observed as the ASL was specifically enriched with high concentration of Pb (250-2000 mg Pbl(-1)). More than 99.5% of Pb was removed regardless of the initial Pb concentration imposed in ASL and, in all cases, the residual Pb concentrations (0.0-1.44 mg l(-1)) were below the limiting value (2.0 mg l(-1)) for effluent discharge in sewage works.

Pilot-plant study of wastewater sludge decontamination using a ferrous sulfate bioleaching process

The objective of this research was to investigate the performance of the ferrous sulfate bioleaching (FSBL) process in a pilot plant for decontamination and stabilization of wastewater sludge. Batch and continuous experiments, conducted with two 4-m3 bioreactors using indigenous iron-oxidizing bacteria (20% v/v of inoculum) with addition of 4.0 g ferrous sulfate heptahydrate per liter of sludge initially acidified to pH 4.0, were sufficient for effective heavy metal (cadmium, copper, manganese, zinc, and lead) removal yields. The average metal removal yields during the FSBL process were as follows: cadmium (69 to 75%), copper (68 to 70%), manganese (72 to 73%), zinc (65 to 66%), and lead (16%). The FSBL process was also found to be effective in removing both fecal and total coliforms (abatement > 5 to 6 log units). The nutrients content (nitrogen, phosphorus, and magnesium) were also preserved in decontaminated sludge.

Electrolytic recovery of lead in used lime leachate from municipal waste incinerator

The purification of municipal incinerator gases generates huge quantities of both fly ashes and used lime, containing high amount of heavy metals (Pb, Cd, Zn and Hg). The used lime is mainly contaminated by Pb and often considered as hazardous waste. This paper is related to a laboratory study on the performance of electrochemical techniques used to recover Pb from leachate produced during used lime decontamination. Two types of electrolytic units (monopolar and bipolar electrode cells) using different electrode materials were studied. Effectiveness was measured in terms of energy consumed, weight of residue sludge produced and reduction in Pb concentration. Results showed that the best performances for Pb removal (in terms of effectiveness and cost) were obtained by initially adjusting the pH of UL leachate to pH 7.5-8.5 with sulphuric acid followed by electrochemical treatment using bipolar electrode (mild steel) system operated at current intensity of 1.5A through only 5 min. The yield of Pb removal was 98.8+/-1.3% and an amount of 4.9+/-1.3 kg trt-1 of metallic sludge was produced. The optimal conditions determined for Pb recovery involved a total cost of CAN$ 1.66 t-1 of dry residue treated, including acid consumption, energy consumption and metallic sludge disposal.