At the onset of 21st century, the pollution of surface and groundwater by toxic metals continues to represent a challenge for the authorities responsible for environmental protection. The uncontrolled rejection of metals in aquatic ecosystems such as Ag, As, Be, Cd, Cr, Cu, Hg, Ni, Pb, Sb, Tl and Zn, constitute a serious threat to human and animal health. Several methods of treatment of waters polluted by metals have been proposed during the last several decades. However, the technique of precipitation of metals remains the most favorable option on an industrial scale due to reasons of cost-effectiveness, performance, and simplicity. The present review presents current knowledge on various technical alternatives for precipitation of metals. The discussion relates to the individual characteristics of the metal contaminants, as well as their behavior compared to various techniques of precipitation.

Sewage sludge decontamination requires the removal of toxic metals and it can be performed by chemical leaching. The commercialization of such a decontamination process needs to consider the preservation of the sludge fertilizing properties and the ease of dewatering of the acidic treated sludge. Moreover, operating costs must be acceptable under optimal operational parameters. Chemical leaching assays were performed at the laboratory bench scale with biological sludge. Afterwards, a multicriteria analysis has been conducted to determine the optimal operational parameters allowing the sludge decontamination while meeting all performance criteria. The analysis pointed out adequate working ranges in terms of sulfuric acid, ferric chloride, and hydrogen peroxide concentrations to be used for chemical decontamination of the sludge. Moreover, it allowed establishing a mathematical equation to help identify the optimal working range for further studies having different contamination scenarios.

Bioleaching of heavy metals from sewage sludge has been shown to be a promising technique for sludge decontamination in such a complex matrix. The effect of two types of substrates ( ${\mathrm{Fe}}^{2+}$ and ${\mathrm{S}}^0$ ) and their combination on metal removal efficiencies was studied in a batch system using Acidithiobacillus ferrooxidans. The results showed that the inoculation of A. ferrooxidans and coaddition of ${\mathrm{Fe}}^{2+}$ and ${\mathrm{S}}^0$ accelerated pH reduction and metal solubilization. After 4–10 days of bioleaching, the following removal efficiencies were obtained: Cr 80%; Cu 100%; and Zn 100%, which were the maximum that have been reported. The time required for Cr to reach the maximum was 10 days, whereas for Cu and Zn the time needed was only 4–6 days. Three different patterns of solubilization for Cr, Cu, and Zn were established as a function of pH and ${\mathrm{Fe}}^{3+}$ concentration. Cr required a threshold pH of around 2.0 to initiate its solubilization, whereas Cu solubilization was controlled simultaneously by pH and ${\mathrm{Fe}}^{3+}$ concentration. The pH appeared to be the sole factor responsible for the solubilization of Zn that was initiated at 4.0–4.5. Fundamental strategy for enhancing metal removal efficiencies is to lower the pH and to sustain high ${\mathrm{Fe}}^{3+}$ concentration, which can be achieved by increasing sulfate concentration, facilitating ${\mathrm{Fe}}^{2+}$ biooxidation, and decreasing the precipitation of jarosite and schwermannite.

The objective of the present study was to optimize the energy source addition required for bioleaching of heavy metals from undigested sewage sludge after secondary treatment. Bioleaching was conducted in a batch system with both inoculation of iron-oxidizing bacteria and the addition of ${\mathrm{FeSO}}_4\cdot 7{\mathrm{H}}_2\mathrm{O}$ in the range of $0–17.5\mathrm{g}{\mathrm{L}}^{-1}$ . The results showed that the pH of the sludge decreased with an increase in the ferrous iron concentrations and reached the maximum acidity of pH 2.1–3.0 for treatments receiving both bacterial inoculation and substrate addition. This led to a significant solubilization of metals from the solids fraction of sewage sludge. However, solubilization behavior differed for different metal species. Solubilization of Zn was not affected by the concentration of the substrate in the range of $7.5–17.5\mathrm{g}{\mathrm{L}}^{-1}$ , whereas solubilization of Cu and Cr was highly dependent on substrate concentration. After 10 days of bioleaching the following heavy metal solubilization efficiencies were obtained: Zn 100%, Cu 83–98%, and Cr 9–75%. In contrast, no obvious solubilization of Cu and Cr occurred except that less than 20% of Zn was leached out in the control without substrate addition. The result from the present study showed that $10\mathrm{g}{\mathrm{L}}^{-1}$ of substrate was sufficient to provide an optimum bioleaching condition. However, the biomass reduction rate was far below the criteria for sludge stabilization and thus, a further stabilization process is necessary prior to land application of the bioleached sludge.

The effectiveness of insoluble starch xanthate (ISX) for removal and recovery of heavy metals [ $\mathrm{Cd}\left(\mathrm{II}\right)$ , $\mathrm{Cu}\left(\mathrm{II}\right)$ , $\mathrm{Hg}\left(\mathrm{II}\right)$ , and $\mathrm{Ni}\left(\mathrm{II}\right)$ ] from simulated wastewater was evaluated. The metal–xanthate interaction is considered as sorption process involving two simultaneous reactions: (1) complexation of metal with xanthate groups; and (2) decomposition of xanthates. The kinetics and equilibrium studies indicate that the rate of metal uptake by ISX is quite rapid and the Freundlich constants indicate that the interaction is chemisorption. The metal uptake capacity of ISX remains almost the same in the pH range 5–7. The affinity, and also the selectivity of metals for xanthate, follow the reciprocal of metal–ethylxanthate solubility product. The aqueous phase conditions and ISX dosage can be controlled to selectively remove metals from the aqueous phase containing mixtures of metals [ $\mathrm{Cd}\left(\mathrm{II}\right)$ , $\mathrm{Cu}\left(\mathrm{II}\right)$ , $\mathrm{Hg}\left(\mathrm{II}\right)$ , and $\mathrm{Ni}\left(\mathrm{II}\right)$ ]. Also selective recovery of metals from ISX sludge containing the metals is feasible.

This research aimed at evaluating the adsorption of trivalent chromium present in high concentrations [about $100–170\mathrm{mg}\mathrm{Cr}\left(\mathrm{III}\right){\mathrm{L}}^{-1}$ ] in effluents on various types of wastewater sludges, namely, primary sludge (PWS), secondary sludge (SWS), mixed sludge $(\mathrm{primary}+\mathrm{secondary})$ (MWS), physicochemical treatment sludge (CWS), and agro-processing industry sludge (AWS). Adsorption tests of chromium were carried out at $25\pm 1°\mathrm{C}$ in $500\mathrm{mL}$ Erlenmeyer flasks at various adsorbent concentrations (2, 5, 10, 15, 20, and $30\mathrm{g}{\mathrm{L}}^1$ ) for all types of sludges studied (PS, SWS, MWS, CWS, and AWS). A synthetic chromium nitrate $\left[\mathrm{Cr}\right({\mathrm{NO}}_3{)}_3\cdot 9{\mathrm{H}}_2\mathrm{O}]$ solution adjusted to ${\mathrm{pH}}_{\mathrm{i}}=3.2$ at $112\mathrm{mg}{\mathrm{L}}^{-1}$ was used. The results revealed that CWS had the best adsorption capacity for chromium, followed by SWS, and adsorption capacity of the different wastewater sludges was in the following order: $\mathrm{CWS}>\mathrm{SWS}⩾\mathrm{MWS}>\mathrm{PWS}>\mathrm{AWS}$ . Kinetic adsorption studies also showed that almost complete removal of chromium in solution was reached in the first $2\mathrm{h}$ of reaction with all types of sludges. Finally, the adsorption tests of chromium on tannery effluent $\left(\right[\mathrm{Cr}{]}_i=143\mathrm{mg}{\mathrm{L}}^{-1})$ confirmed the effectiveness of SWS for the removal of chromium. A removal yield of 40.8% of chromium was observed following $2.0\mathrm{h}$ of adsorption on $5.0\mathrm{g}{\mathrm{L}}^{-1}$ of SWS at a ${\mathrm{pH}}_i=3.94$ . The quantity of chromium adsorbed on sludge during these tests corresponded to a load of $11.6\mathrm{mg}\mathrm{Cr}{\mathrm{g}}^{-1}$ dry weight.

Heavy metals have polluted many sites in industrial countries and pose risks for ecological systems. In the United States, metals are present at about three-quarters of Superfund and Department of Defense sites. Further, there are relatively few soil treatment projects that have treated metals compared to organic contaminants. The enormous quantity and the complexity of metal-contaminated soils (MCS) constitute an important challenge for their remediation. This paper provides: (1) a summary of the remedial options and treatment technologies for MCS; and (2) a review of the field applications of the treatment technologies performed in the United States over the past 25 years. The discussion focuses on the alternatives to ex situ solidification/stabilization (S/S) through a review of 128 case studies. Additionally, a comparison of the practices used in Europe and in the United States relating to the treatment of MCS is also presented. Soil washing, phytoremediation, electrokinetics and, in situ stabilization techniques provide an effective alternative to conventional S/S and disposal options. Further research is needed to improve understanding of the phytoremediation and in situ metal stabilization processes.

The phytoremediation of metal-contaminated soils in combination with chemical methods such as soil washing could be more effective than single technology. In this study, pot experiments were carried out to study the effects of chelators including a mixture of chelators (MC), monosodium glutamate waste liquid (MGWL), and ethylene diamine tetraacetic acid (EDTA) to enhance Zn, Cd, and Pb uptake by Sedum alfredii Hance. The metal washing efficiency and the environment risk caused by these chelators were also assessed. Furthermore, the rhizofiltration with Rumex crispus was established to investigate the potential of plant hairy roots to remove metals in leachates. Results showed that compared to MGWL and EDTA, MC significantly improved Cd phytoextraction efficiency and left lower environment risk to underground water. MC may be the appropriate choice to enhance phytoextraction by S. alfredii. Leaching with the MC or EDTA removed Pb, to which phytoextraction is not effective. However, heavy metals and organics in leachates were also increased with the addition of chelators and could lead to the pollution of underground water, particularly with EDTA and MGWL. The combination of MC enhanced phytoextraction with hyperaccumulator S. alfredii and in situ soil washing could be an interesting technology to remediate metals (Zn, Cd, and Pb)-contaminated soils if an appropriate leachate handling method is found.

Because of the potentially high metal concentrations found in soils immediately adjacent to chromated copper arsenate (CCA)-treated wood structures and utility poles, CCA-contaminated soil ingestion may be a significant exposure route to Cr, Cu, and As for children. Therefore, a strong need exists to provide data on oral bioavailability of these elements in field-collected CCA-contaminated soils. Recently, As bioaccessibility in CCA-contaminated soils was assessed but data for Cr and Cu are lacking. Therefore, the objectives of this study were (1) to assess the bioaccessibility of Cr and Cu in contaminated soils collected near in-service CCA-treated utility poles; and (2) to determine the influence of selected soil properties on their bioaccessibility. Cr and Cu bioaccessibilities (in vitro gastrointestinal method) were determined on surface soil samples collected immediately adjacent to 12 CCA-treated utility poles after 18 months of service. Bioaccessible Cr and Cu were also determined in three certified reference materials. Total soil Cu concentrations in soils varied from $95.6\pm 4.5$ to $\mathrm{3,580}\pm 0680\mathrm{mg}∕\mathrm{kg}$ , whereas total soil Cr concentrations were lower and ranged between $26.4\pm 2.1$ to $394\pm 53\mathrm{mg}∕\mathrm{kg}$ . Copper gastrointestinal bioaccessibility ranged between $19.4\pm 3.6\%$ and $89.4\pm 4.9\%$ (mean value: $54.1\pm 23.4\%$ ), whereas Cr bioaccessibility varied from below detection $(<0.3\%)$ to $32.9\pm 17.6\%$ (mean value: $8.5\pm 10.1\%$ ). Bioaccessible Cr and Cu were both negatively correlated with silt content ( $r^2=0.39$ , $p<0.05$ , and $r^2=0.65$ , $p<0.005$ , respectively). Copper bioaccessibility was also negatively correlated with total organic carbon content ( $r^2=0.64$ , $p<0.01$ ). Using conservative exposure parameters, the mean potential Cr intake from incidental ingestion of soil near CCA-treated utility poles was very low $(0.03\pm 0.03\mathrm{\mu }\mathrm{g}\mathrm{Cr}∕\mathrm{kg}\mathrm{body}\mathrm{weight}∕d)$ . The estimated mean Cu intake was also low $(2.5\pm 1.2\mathrm{\mu }\mathrm{g}\mathrm{Cu}∕\mathrm{kg}\mathrm{body}\mathrm{weight}∕d)$ in spite of its higher bioaccessibility. These values are much lower than the dietary reference intakes and oral minimal risk levels for Cu and Cr.

Oilseed rape, a Cd accumulator, is effective in the uptake of plant available Cd from the soil. We hypothesize that the efficient removal of bioavailable Cd from soil solution by the Cd-accumulator oilseed rape (Brassica napus var. Xikou Huazi) might reduce the Cd uptake of coplanted or rotated Cd sensitive Chinese cabbage (Brassica campestris var. Zhongyou 821). Xikou Huazi and Zhongyou 821 were cocultivated in uncompartmentalized pots, or pots that were divided by plastic film or mesh barriers to limit the extent of root intermingling at a Cd concentration of $60\mathrm{mg}{\mathrm{kg}}^{-1}$ . The shoot biomass of Xikou Huazi was significantly increased when its root were permitted to intermingle with Zhongyou 821. Cocropping of Xikou Huazi with Zhongyou 821 not only increased its biomass, but also increased the Cd uptake by Xikou Huazi when there was no barrier. The results were reversed for Cd sensitive oilseed rape (Zhongyou 821) in such a system. There was no significant difference in biomass, Cd concentration, and total Cd uptake for both the oilseed rape species under mixed-culture and monoculture, when grown in the pot with mesh and plastic film barrier. Successive cropping systems at Cd treatment concentrations of 0, 5, and $10\mathrm{mg}{\mathrm{kg}}^{-1}$ , showed that first cropping of Xikou Huazi had little influence on Cd uptake of successive cropping of Zhongyou 821. Cd-accumulator oilseed rape could reduce the Cd uptake of cocropped Chinese cabbage species.