Remediation Of Heavy Metal Polluted Soils Research Articles

Sorption Stability and Mechanism Exploration of Palygorskite as Immobilization Agent for Cd in Polluted Soil

Clay minerals have been utilized for the remediation of heavy metal-polluted soil. However, information on the remediation stability of various clay minerals with different performances is limited. In this study, a kind of palygorskite (PAL) with a sorption amount for Cd2+ about 40 mg/g, which is much larger than common minerals, was selected as amendment for in situ immobilization field demonstration. Besides, sorption stability which is essential for remediation was investigated in an ideal solid solution system by sorption and desorption behaviors of Cd2+ on PAL, including isotherms, kinetics, and various stimulated environmental factors such as pH, temperature, and background electrolytes. The calculated thermodynamic parameters confirmed the sorption process was endothermic and driven by entropy changes. Only minimal desorption was caused by stimulated irrigation or runoff and acid rain. The temperature, pH, and background electrolyte dependence confirmed that the sorption of Cd2+ on PAL was stable. Various characterization results including X-ray photoelectron spectroscopy (XPS), scanning electron microscopy-energy dispersive spectrometry (SEM-EDS) mapping, X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FT-IR) confirmed the sorption mechanisms were surface precipitation of CdCO3 and surface complexation with hydroxyl groups.

In situ field-scale remediation of Cd polluted paddy soil using sepiolite and palygorskite

At present, the remediation of heavy-metal polluted soil is a considerable problem. In this study, in situ immobilization field experiments were conducted by planting rice (Oryza sativa L.) in Cd contaminated paddy soil to determine the optimal remediation mode that would reduce the accumulation of Cd in brown grain. Sepiolite and palygorskite were utilized as amendments, while ordinary and low Cd-accumulating rice cultivars were selected as model plants. The remediation effects of the amendments on the soil and rice were investigated and the potential immobilization mechanisms were analyzed using X-ray diffraction and X-ray photoelectron spectroscopy. The application of sepiolite and palygorskite was found to increase the pH value of the paddy soil, by varying amounts, and significantly reduce the HCl, TCLP, CaCl2, and NH4OAc-extractable Cd concentrations resulting in a notable decrease in the Cd concentration in brown rice. Both sepiolite and palygorskite were found to reduce exchangeable Cd concentrations and increase the carbonate-bound and residual fractions in paddy soil. Precipitation of Cd as carbonates or hydroxides and surface complexation were the main immobilization mechanisms for sepiolite and palygorskite. The concentration of Cd in brown rice of the low Cd-accumulating cultivar from the control plot was 0.72mg·kg−1. Sepiolite treatment reduced the value to 0.18mg·kg−1 below the maximum levels reported by Codex Stan 193-1995 and the national standard of China. It was concluded that chemical immobilization combined with crop cultivar selection could be a suitable proposal for crop safety regulation and sustainable agriculture.

A simple synthesis of 2-keto-3-deoxy-D-erythro-hexonic acid isopropyl ester, a key sugar for the bacterial population living under metallic stress

2-Keto-3-deoxy-D-erythro-hexonic acid (KDG) is the key intermediate metabolite of the Entner Doudoroff (ED) pathway. A simple, efficient and stereoselective synthesis of KDG isopropyl ester is described in five steps from 2,3-O-isopropylidene-D-threitol with an overall yield of 47%. KDG isopropyl ester is studied as an attractive marker of a functional Entner Doudoroff pathway. KDG isopropyl ester is used to promote growth of ammonium producing bacterial strains, showing interesting features in the remediation of heavy-metal polluted soils.

Electrodialytic Remediation of Different Heavy Metal-Polluted Soils in Suspension

Electrokinetic remediation of heavy metal-polluted soil faces different challenges in relation to implementation. One challenge is to cope with the nonlinear and transient geochemical changes in the soil and another is to increase the remediation rate. Both these challenges are met when treating the soil in a suspension in an electrodialytic cell. The soil suspension is stirred and uniform during treatment. Previously, it has been shown that a faster remediation can be obtained when remediating a stirred soil suspension compared to a stationary water saturated soil (all other parameters the same). The present work shows that the method for treating stirred suspensions was robust in the sense that in 1–3 weeks, three of four soils were decontaminated from heavy metal concentrations, where the soils must be deposited to concentrations where the soil can be allowed used for some purposes in Denmark. From the fourth soil of the investigation, 92 % Pb was removed during 14 days, but as the initial concentration was very high (33.6 g Pb/kg), the final concentration was still high and the soil maintained classified where there are no reuse options in Denmark, so optimization of the treatment for this soil is necessary to meet the goal. The good results were obtained even without optimization of processing parameters, but the investigation underlined that the optimal parameters are highly soil and pollution specific.

Characterization of endophytic Rahnella sp. JN6 from Polygonum pubescens and its potential in promoting growth and Cd, Pb, Zn uptake by Brassica napus

Microbe-assisted phytoremediation has been considered as a promising measure for the remediation of heavy metal-polluted soils. In this study, a metal-tolerance and plant growth-promoting endophytic bacterium JN6 was firstly isolated from roots of Mn-hyperaccumulator Polygonum pubescens grown in metal-contaminated soil and identified as Rahnella sp. based on 16S rDNA gene sequence analysis. Strain JN6 showed very high Cd, Pb and Zn tolerance and effectively solubilized CdCO3, PbCO3 and Zn3(PO4)2 in culture solution. The isolate produced plant growth-promoting substances such as indole-3-acetic acid, siderophore, 1-aminocyclopropane-1-carboxylic deaminase, and also solubilized inorganic phosphate. Based upon its ability in metal tolerance and solubilization, the isolate JN6 was further studied for its effects on the growth and accumulation of Cd, Pb and Zn in Brassica napus (rape) by pot experiments. Rape plants inoculated with the isolate JN6 had significantly higher dry weights, concentrations and uptake of Cd, Pb and Zn in both above-ground and root tissues than those without inoculation grown in soils amended with Cd (25mgkg−1), Pb (200mgkg−1) or Zn (200mgkg−1). The isolate also showed a high level of colonization in tissue interior of rapes. The present results demonstrated that Rahnella sp. JN6 is a valuable microorganism, which can cost-effectively improve the efficiency of phytoremediation in soils contaminated by Cd, Pb and Zn.

Chelate-assisted phytoextraction using canola (Brassica napus L.) in outdoors pot and lysimeter experiments

Phytoextraction is an emerging technology for non-destructive remediation of heavy metal-polluted soils. This study was conducted to test chelate-assisted phytoextraction of Cu, Pb and Zn using EDTA and canola (Brassica napus L. cv. Petranova) on a moderately polluted industrial soil (loamy sand) in the sub-continental climate of Eastern Austria. The effects of the rate (up to 2.1 g kg−1 soil) and mode (single versus split) of EDTA application on the biomass, water contents and metal concentrations in shoots and roots were investigated along with changes of metal lability in soil and leaching from the root zone in parallel outdoors pot and lysimeter experiments. Labile (1 M NH4NO3-extractable) metal concentrations in soil increased considerably upon application of EDTA, indicating enhanced phytoavailability. However, this was also associated with enormously increased metal concentrations in the leachates collected below the root zone. Enhanced metal labilities and leachate concentrations persisted for more than 1 year after harvest. Metal lability was more enhanced by EDTA in rhizosphere relative to bulk soil, indicating interactions of EDTA with root activities. Shoot biomass and water contents of canola were virtually unaffected by EDTA, revealing that canola can tolerate excessive metal concentrations in soil pore water. Metal concentrations in shoots were increased considerably, but were insufficient to obtain reasonable extraction rates. Split applications were generally more effective than the same amounts of EDTA added at once. Metal concentrations in roots decreased after each application of EDTA, possibly indicating metal removal from roots by free protonated EDTA, but increased again within several days. As the application of chelate-assisted phytoextraction is limited by the risk of groundwater pollution, further work should focus on natural, continuous phytoextraction technologies.