Heavy metal contamination of crop lands surrounding mines in North Vietnam is a major environmental issue for both farmers and the population as a whole. Technology for the production of biochar at a village and household level has been successfully introduced into Vietnamese villages. This study was undertaken to determine if rice straw biochar produced in simple drum ovens could remediate contaminated land. Tests were also carried out to determine if biochar and apatite mixed together could be more effective than biochar alone. Incubation trials were carried out over 90 days in pots to determine the total changes in exchangeable Cd, Pb and Zn. Detailed tests were carried out to determine the mechanisms that bound the heavy metals to the biochar. It was found that biochar at 5% (BC5) and the mixture of biochar and apatite at 3% (BCA3) resulted in the greatest reduction of exchangeable forms of Cd, Pb and Zn. The increase in soil pH caused by adding biochar and apatite created more negative charge on the soil surface that promoted Pb, Zn and Cd adsorption. Heavy metals were mainly bound in the organic, Fe/Mn and carbonate fractions of the biochar and the mixture of biochar and apatite by either ion exchange, adsorption, dissolution/precipitation and through substitution of cations in large organic molecules.

In the present study, experiments using zeolite and Mg/Al LDH-zeolite for immobilization of Cd and Pb ions in artificial soil were conducted.

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This work investigated the effects of soil pH and the content ratio of natural zeolite on Cr contaminated soil. The immobilization experiments of the exchangeable Cr in contaminated soils were conducted using the batch method. The incubation experiments were carried out over 30 days in plastic bottles to determine five fraction of Cr existence (exchangeable fraction (F1), Fe/Mn/Oxide (F2), carbonate bound (F3), organic matters (F4) and residual (F5)) in amended soils after incubation. Results showed that the content and proportion of the exchangeable Cr decreased with an increase in soil pH from 5 to 9. At soil pH 5, the exchangeable Cr in soil reduced from 44.80±0.772 mg/kg (initial soil) to 17.72±0.300 mg/kg after 30 days of incubation with natural Zeolite 3%. Meanwhile, the exchangeable Cr of soil also decreased with increasing the content ratio of natural zeolite from 1% to 5% in soil. The ratio of 3% was suitable for incubation of the exchangeable Cr in contaminated soil with natural zeolite. The exchangeable Cr in contaminated soil decreased from 80.34% at un-amended soil treatment to 25.06% after incubation of 30 days. The forms of carbonate bound (F3) and organic matters (F4) in amended soils increased to 36.54% and 28% compared with 4.26% and 6.90% in un-amended contaminated soil. Ion exchange, precipitation and adsorption on the surface of natural zeolite  might be the potential mechanisms of immobilization of the exchangeable Cr. The results indicated that natural zeolite can be used as the effective adsorbent for immobilizing the exchangeable Cr in contaminated soils and leading to a decrease in the environmental risk from Cr toxicity.

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