Dechelation of Cd-EDTA complex and recovery of EDTA from simulated soil-washing solution with sodium sulfide

A closed-loop process for spent washing solution from multi-metal contaminated soil: EDTA reclamation and recycling

The proper disposal of spent soil washing solution is a great challenge to ethylenediamine tetraacetate (EDTA)-base soil washing technologies, particularly when the solution contains multi-metals. In this paper, we proposed an environmentally friendly disposal of multi-metal spent washing solution, in which the multi-metals were concentrated as hazardous precipitates for further safe disposal, and EDTA was reclaimed and recycled to further wash contaminated soil together with the cleansed process water. The results showed that Cr3+ was poorly removed by sole heavy-metal-capturing agent (HMCA) chelation because of the high solubility of HMCA-Cr, which also yielded a low percentage of EDTA reclamation in the multi-metal spent washing solution. We established a closed-loop process for the disposal of multi-metal spent washing solution by combining coagulation-flocculation-sedimentation and HMCA chelation. The novel recycling process was able to remove 99.67% Cu, 99.62% Pb, 92.48% Cd, 88.19% Sb, 84.38% As, and 82.39% Cr as precipitates from the real spent washing solution, and up to 95.64% of EDTA was reclaimed in the cleansed process water. On the average, the overall efficiency of the reclaimed EDTA solution could reach 65% of the fresh EDTA solution in extracting various HMs from contaminated soil. The recycling method provides an efficient and promising alternative for spent soil washing solution with both EDTA and process water reusage in a closed-loop process.

Migration and morphological transformation patterns of heavy metals on sludge cells and extracellular polymeric substances (EPS) under the influence of different treatments

The impediment of sludge resource utilization stems from the presence of heavy metals within the sludge matrix. To optimize heavy metal removal techniques from undried sludge, it is essential to study the distribution of heavy metals in the sludge flocs structure and the changes in morphology in the sludge cells after different treatments. In this study, the sludge was subjected to chemical treatments using citric acid (CA), EDTA, and saponin, as well as electrokinetic treatment at 2 V/cm. The distribution and migration of Cu, Ni, and Zn in sludge flocs after various treatment methods were analyzed. The heavy metals were found to migrate from intracellular to extracellular polymeric substances (EPS) without causing extensive sludge cell lysis. They gradually diffused outward with the dispersion of the EPS layer. The migration efficiency of the three heavy metals in the sludge flocs was Zn, Ni, and Cu. This was mainly related to the initial distribution and morphology of the heavy metals. Under the influence of chemicals and an electric field, the acid-soluble and reducible heavy metals in the cells partially migrated to the EPS, while the stable heavy metals transformed into an unstable state. Furthermore, the order of chemical reagents in terms of their effect on the migration efficiency of heavy metals was CA > EDTA > Saponin, owing to the varying binding strengths of heavy metals and their impact on the degree of loosening of the EPS. Especially after CA treatment a greater proportion of Cu, Ni, and Zn were transferred from the cells to the EPS. The acidification effect near the anode during electrokinetic treatment intensifies the migration of heavy metals. This study provides basic research for subsequent engineering optimization aimed at removing heavy metals from sludge.

A review of organophosphonates, their natural and anthropogenic sources, environmental fate and impact on microbial greenhouse gases emissions - Identifying knowledge gaps

Organophosphonates (OPs) are a unique group of natural and synthetic compounds, characterised by the presence of a stable, hard-to-cleave bond between the carbon and phosphorus atoms. OPs exhibit high resistance to abiotic degradation, excellent chelating properties and high biological activity. Despite the huge and increasing scale of OP production and use worldwide, little is known about their transportation and fate in the environment. Available data are dominated by information concerning the most recognised organophosphonate - the herbicide glyphosate - while other OPs have received little attention. In this paper, a comprehensive review of the current state of knowledge about natural and artificial OPs is presented (including glyphosate). Based on the available literature, a number of knowledge gaps have been identified that need to be filled in order to understand the environmental effects of these abundant compounds. Special attention has been given to GHG-related processes, with a particular focus on CH4. This stems from the recent discovery of OP-dependent CH4 production in aqueous environments under aerobic conditions. The process has changed the perception of the biogeochemical cycle of CH4, since it was previously thought that biological methane formation was only possible under anaerobic conditions. However, there is a lack of knowledge on whether OP-associated methane is also formed in soils. Moreover, it remains unclear whether anthropogenic OPs affect the CH4 cycle, a concern of significant importance in the context of the increasing rate of global warming. The literature examined in this review also calls for additional research into the date of OPs in waste and sewage and in their impact on environmental microbiomes.

Enhanced decomplexation of Cu-EDTA and simultaneous removal of Cu(II) by electron beam irradiation accompanied with autocatalytic fenton-like reaction: Synergistic performance and mechanism

Widely existing heavy metal complexes with high stability and poor biodegradability are intractable to be eliminated by conventional methods. In this study, electron beam (EB) irradiation characterized by rapidly producing strong oxidizing radicals was employed to effectively decompose Cu-ethylenediaminetetraacetic acid (Cu-EDTA) with almost complete elimination at 5 kGy. In terms of heavy metal removal, EB irradiation at relatively low doses was insufficient to remove copper ions, which was only 17.2% under 15 kGy. However, with the extra addition of 8 mM H₂O₂, such an irradiation dose could result in 99.0% copper ions removal. Mechanism analysis indicated that EB irradiation combined with spontaneously induced Fenton-like reactions were responsible for its excellent performance. The prime function of EB irradiation was to destroy the structure of Cu-EDTA with in-situ produced ·OH, and the subsequent released Cu-based intermediates could activate H₂O₂ to initiate autocatalytic chain reactions, correspondingly accelerating the degradation of complexes and the liberation of metal ions. Highly oxidative ·OH and O₂·^- were demonstrated as main active species acted on different positions of Cu-EDTA to realize gradual decarboxylation, synchronously generating low molecular weight compounds. XRD and XPS analysis showed that the released copper ions were mainly precipitated in the form of CuO, Cu(OH)₂ and Cu₂(OH)₂CO₃. In general, EB/H₂O₂ was an adoptable strategy for the disposal of such refractory heavy metal complexes.

Feasibility of a combined solubilization and eluent drainage system to remove Cd and Cu from agricultural soil

Washing using low-molecular-weight organic acid is an effective and eco-friendly technique to permanently remove heavy metals from soil. There is still lack of evaluation of the application modes and the recovery methods, as well as the variations of heavy metal and nutrients fractions on a pilot-scale during washing. Here, we developed a simple combined solubilization and eluent drainage system in a pilot-scale washing box using industrial grade citric acid with feasible waste eluent treatment method to remove cadmium (Cd) and copper (Cu) from agricultural soil. The removal rates of Cd and Cu after sequential two-stage citric acid-water washing reach up 68.9% and Cu 41.4%, which was 7.5% and 10.0% higher than single citric acid. The removed the heavy metals were mainly in exchangeable and reducible fractions. The heavy metals at different soil depth were dissolved more effectively by citric acid-water washing with wheat straw as underdrain filling material than that of crushed stone. The potential risks of Cd in soils all decreased by approximately 75% from considerable to low risk. The two-stage citric acid-water washing significantly mitigated the effect of soil acidification. The average contents of soil organic matter, available ammonium N-NH₄ and available phosphorus increased by 40.9%, 57.3%, 32.0% after citric acid-water washing under wheat straw filling. The waste eluent can be efficiently recovered by clam powder, which removed 78.7% of Cd and 57.5% of Cu. The regenerated citric acid showed similar removal efficiencies for Cd and Cu compared to the fresh citric acid. These results indicate that the combination of washing, drainage and flocculation can effectively remove Cd and Cu from contaminated agricultural soil and realize the recycling of waste eluent.

One-step extraction and simultaneous quantitative fluorescence immunochromatography strip for AFB1 and Cd detection in grain

AFB₁ and heavy metal Cd are two common pollutants during grain storage. The rapid detection of grains before they enter the granary is particularly important. Hence, rapidly, accurately, and sensitively screening contaminated grains, simplifying the detection process, and reducing detection costs are necessary. In this study, linear ranges of time-resolved fluorescence microsphere - immunochromatographic test strip (TRFM-ICTS) detection were 0.01-30 ng/mL (AFB₁) and 0.01-60 ng/mL (Cd), and the IC₅₀ values were 0.536 ng/mL (AFB₁) and 3.331 ng/mL (Cd). In the TRFM-ICTS sample addition experiment, the recovery rates were all between 90% and 110%. The coefficient of variation was less than 8% in the actual sample detection process of grain. We have established a one-step extraction method for AFB₁ and Cd in grains to achieve simultaneous detection in one extraction. In addition, TRFM-ICTS could be stored for at least 12 months, providing technical support for the realization of commercial production.

Removal of heavy metals from contaminated paddy soils using chemical reductants coupled with dissolved organic carbon solutions

General acid washing is commonly used to treat heavy metal-contaminated soils, but it is sometimes difficult to achieve remediation aims in severely polluted soils. If we expose the surfaces of Fe oxide minerals to reductive dissolution during washing treatment, more of the metals initially adsorbed to these surfaces will be liberated, which may encourage the removal of heavy metals. Initially, the metal extraction capabilities of nine chemical reductants were compared in ten soil samples polluted by Cr, Cu, Zn, and Ni. Sodium dithionite (Na₂S₂O₄) and ferrous sulfate (FeSO₄) were screened for subsequent intensive research. In summary, the Na₂S₂O₄ solutions had higher Cr, Cu, and Zn removal rates than either the FeSO₄ or acid solution. Application of dissolved organic carbon (DOC) further increased the removal of heavy metals by complexation. About 15%, 86%, 32%, and 52% of the Cr, Cu, Zn, and Ni, respectively, were removed from the representative soil (M-2) by two-stage washing using 0.2 M Na₂S₂O₄ coupled with 1,500 mg L^-1 DOC solution at pH 2.0. Meanwhile, most soil fertility was preserved: ammonium nitrogen was increased 3.9 times; the increase in exchangeable potassium was 33%; and the reduction in available P was only 10%.

Selective leaching of lead from lead smelter residues using EDTA

Ethylenediaminetetraacetic acid (EDTA) has been widely used as an effective reagent for removal of lead from soil because of its high lead extraction efficiency caused by the high thermodynamic stability of the Pb(ii)-EDTA complex. In this study, EDTA was used as a lixiviant for recovery of lead from residues (matte and slag) of secondary lead smelter plants. The residues were composed mainly of iron (34-66 wt%) and lead (7-11 wt%). Leaching parameters (EDTA concentration, pH, temperature, liquid-to-solid ratio and leaching time) were optimized. The optimum leaching efficiency was achieved when leached for 1 h at room temperature using 0.05 mol L-1 EDTA at a liquid-to-solid ratio of 5 mL g-1. At such conditions, 72 to 80% of lead and less than 1% of iron were leached from both matte and slag. The high selectivity towards lead with minimal co-dissolution of iron is a major advantage since it reduces the chemical consumption and simplifies the downstream processes. Although the stability constants of the complexes Fe(iii)-EDTA, Fe(ii)-EDTA and Pb-EDTA are all large (log K S 25.1, 14.33 and 18.04, respectively), the leaching of iron was most likely limited by its presence in insoluble phases such as iron oxides, sulfides and silicates in the residues. 100% leaching of lead was achieved by a multi-step leaching process where the leaching residues were contacted three times by a fresh EDTA solution. To recover EDTA, first iron was precipitated as iron hydroxide by raising the pH of pregnant leach solution (PLS) above 12.6 using sodium hydroxide, followed by precipitation of lead as lead sulfide by adding ammonium sulfide. The recovered EDTA was successfully reused two times for leaching without significant changes in leaching yields.

Removal of Cd, Pb, Zn, Cu in smelter soil by citric acid leaching

Soil washing has been verified as a feasible technology for source reduction for contaminated soil with heavy metals. We conducted batch and column leaching experiments to investigate the removal of Cd, Pb, Cu and Zn from smelter soil by citric acid. The removal efficiency of heavy metals by batch leaching reached a maximum (89.1% Cd, 26.8% Pb, 41.7% Zn, 14.2% Cu) at a concentration of 0.1 M and a pH of 5. Citric acid also removed 91.3%, 11.1%, 39.2% and 11.1% of Cd, Pb, Zn, and Cu respectively after column leaching. Citric acid mainly removed exchangeable, carbonate bound and oxide bound Cd, Pb, Cu and Zn fractions. Vertical distributions of Cd, Cu and Zn similarly increased with increasing soil depth. Chestnut shells were applied to the recovery of citric acid from the waste eluent, which removed 84.4%, 97.5%, 74.6%, 70.3% of Cd, Pb, Zn, and Cu, respectively, due to chestnut shell chelation. Fresh and regenerated citric acid was used in batch leaching of heavy metal-contaminated soil, and they showed similar ability to extract heavy metals.

Low-Fe(III) driven UV/Air process for enhanced recovery of heavy metals from EDTA complexed system

The efficient recovery of heavy metals from complexed wastewater is an essential but challenging task. In this study, a novel low-Fe(III) driven UV/Air process (LFUA) was developed to break the strong complexation between ethylenediamine tetracetic acid (EDTA) and heavy metal ions (HMIs) and enable the enhanced recovery of HMIs via chelating resin adsorption (CRA). The inside mechanism of the LFUA process includes: 1) displacement of HMIs from HMI-EDTA complexes by Fe(III); 2) direct photolysis of Fe(III)-EDTA through a ligand-to-metal charge transition reaction (LMCT) and indirect photolysis of EDTA by HO₂·/O₂·^-. The iron dosage was orders of magnitude lower than that previously reported, due to the Fe(II)/Fe(III) redox cycle in the LFUA process. Fe(II) formed during the LMCT reaction of Fe(III)-EDTA was oxidized back to Fe(III) by O₂ and HO₂·, and the reformed Fe(III) was then recombined with EDTA to sustains the LMCT reaction. EDTA was completely removed in 20 min at a molar ratio of Fe(III)/EDTA = 0.05. In addition, following the LFUA process, the adsorption amounts of various HMIs onto D463 resin were at least two orders of magnitude higher than those reported using the direct adsorption process. Employing the integrated technique of LFUA + CRA enabled the efficient removal of up to 64.5 mg/L of Cu(II) from inlet wastewater, and residual Cu(II) was below 0.5 mg/L. The results of desorption experiments showed that over 90% of Cu(II) was recovered, and the desorption solution had a Cu concentration of 2.1 g/L and purity of 99%. Furthermore, the economic and practical feasibility of using the combined process of LFUA + CRA was analyzed to substantiate that the technique is highly efficient and clean (produces no harmful sludge). Therefore, it is an appropriate and practical process in removing HMIs-EDTA complexes and recovering HMIs from wastewater.

Soil washing remediation of heavy metal from contaminated soil with EDTMP and PAA: Properties, optimization, and risk assessment

Soil washing is a rapid and efficient remediation technique for soil contaminated by heavy metals. In this study, Cd, Pb, and Zn were removed from contaminated soil by ethylenediamine tetra (methylene phosphonic acid) (EDTMP) and polyacrylic acid (PAA). We then investigated the effect of varying the concentration, pH and duration of the washing processes. Single-factor experiments suggest that the PAA washing process may be dominated by electrostatic adsorption, and is suitable for remediation under weak acid and neutral conditions. Meanwhile, EDTMP remediation might be dominated by chelation, which is favorable in strong acid and alkaline environments. In a quadratic saturation D-optimization design (QSDD), we optimized the washing parameters and further explored the washing mechanism including primary factor, principal effect, interaction effect, and the optimal washing conditions, with simultaneously changing multiple influencing factors. The optimum efficiencies of Cd, Pb, and Zn removal were 92.74%, 96.14%, and 50.76% respectively in EDTMP remediation, and 84.62, 79.24, and 41.66% respectively in PAA remediation. The washing processes effectively reduced the availability of Cd, Pb, and Zn in contaminated soil, without noticeably affecting soil chemical properties. Therefore, the washing incurred little ecological risk. EDTMP and PAA are suitable remediation agents of soil contaminated by heavy metals.

Recycling of Chemical Eluent and Soil Improvement After Leaching

Ethylenediaminetetraacetic acid disodium salt (EDTA) was selected among various eluents due to its highest removal efficiency for lead (Pb) (43.7%) and zinc (Zn) (57.1%) leaching from Pb-Zn contaminated soil by soil column experiment. Compared with newly prepared EDTA eluent, using recycled EDTA eluent can still leaching down 71.1% of Pb and 63.2% of Zn respectively, which showed the reusable benefits of recycled EDTA eluent. After soils were leached by EDTA, soil quality decline, such as reducing of urease, catalase, invertase activities and microorganism numbers. However, adding 5% nutrition soil or earthworm fertilizer can significantly improve the quality of EDTA leached soil, and promote growth of peas and ryegrass compared with EDTA treatments. Overall, the improvement of EDTA leached soil by adding nutrition soil or earthworm fertilizer is important, and recycled EDTA eluent can recycle and re-use for Pb-Zn contaminated soil remediation.

Recovery of EDTA from soil-washing wastewater with calcium-hydroxide-enhanced sulfide precipitation

It is cost effective and thermodynamically feasible to recover EDTA and remove potential toxic elements (PTEs) with sulfide precipitation from soil-washing wastewater produced from EDTA washing PTEs-contaminated soil. However, poor solid-liquid separation and EDTA recovery restrict its application due to a large number of fine particles formed during the precipitation process. This study investigated the effect of single factor on PTEs (Cu, Pb, Cd, and Zn) removal and solid-liquid separation from wastewater. The results showed that Zn was the most difficult to remove compared with Cu, Pb, and Cd; with the aid of Ca(OH)₂, Zn removal efficiency was improved from 22.16% to 92.45%, and over 70.98 min, its average rate was 4.2 times that obtained without Ca(OH)₂ dosage; undissolved Ca(OH)₂ adsorbed suspended particles, acted as condensation nucleus, and promoted similar flocculation effect (self-flocculation); dissolved Ca(OH)₂ modified the charge on the surface of suspended particles by changing the zeta potential from -36.77 ± 1.2 mV to -25.39 ± 3.06 mV and weakened the electrostatic repulsion between the suspended particles, and promoted their adsorption and flocculation precipitation, thereby improving the solid-liquid separation. The acid-recovered EDTA was analyzed in the protonated form (H₄EDTA) using Fourier transform infrared (FT-IR) spectroscopy, and it maintained the same ability to extract PTEs from the soil as that of fresh EDTA over several cycles. This indicates that Ca(OH)₂-enhanced sulfide precipitation can effectively treat soil-washing wastewater and recover EDTA and potentially reduce the cost of remediation techniques for PTEs-contaminated soil with EDTA-enhanced soil washing.