The fate and lability of added soluble Ag in soils over time was examined by measurement of labile metal (E-value) by isotopic dilution using the 110mAg radioactive isotope and the solid-phase speciation of Ag by X-ray absorption near edge structure (XANES) spectroscopy. After two weeks of ageing the E-values for Ag decreased by 20–90% with a further decrease of 10–40% after six months. The overall decrease in labile Ag for all soils after the 6 month ageing period was 50–100%. The ageing was more rapid and pronounced in the alkaline soils. XANES results for Ag in soils indicated that for the majority of soils the added Ag+ was reduced to metallic Ag over time, and associations with Fe-oxohydroxides and reduced S groups in organic matter also decreased Ag lability. Strong positive correlations were found between metallic Ag and non-labile Ag and between organic carbon and Ag bonded with S species.

A cadmium/zinc hyperaccumulator extracted metals from four contaminated soils over three years in a glasshouse experiment. Changes in plant metal uptake and soil total (aqua regia-extractable) and available metals were investigated. Plant Cd concentrations in a high-Cd acid soil and plant Zn concentrations in two acid soils decreased during repeated phytoextraction and were predicted by soil available metal concentrations. However, on repeated phytoextraction, plant Cd concentrations remained constant in lightly Cd-polluted acid soils, as did plant Cd and Zn in alkaline soils, although soil available metal concentrations decreased markedly. After phytoextraction acid soils showed much higher total metal removal efficiencies, indicating possible suitability of phytoextraction for acid soils. However, DGT-testing, which takes soil metal re-supply into consideration, showed substantial removal of available metal and distinct decreases in metal supply capacity in alkaline soils after phytoextraction, suggesting that a strategy based on lowering the bioavailable contaminant might be feasible.

In this study, lead (Pb) and antimony (Sb) sorption experiments were conducted to elucidate the mechanisms of Pb and Sb sorption by combined applications using single or combined applications of hydroxyapatite (HAP) and ferrihydrite (FH), to evaluate the contribution of each material in Pb and Sb sorption, and to assess the chemical stability of the sorbed Pb and Sb. In the combined application, isotherms of Pb sorption and Sb sorption were well fitted to Langmuir and Freundlich isotherm models, respectively. The Pb and Sb amounts sorbed in the combined application were the same levels as the summed totals of those sorbed in the single applications, indicating that in the combined application, Pb sorption and Sb sorption were not suppressed. Pb was mainly sorbed on HAP in the combined application, at a 90 % level of the total adsorbed Pb. The HAP and FH contributions to Sb sorption were 32 and 68 % of the total adsorbed Sb, respectively, and Sb was sorbed on each material independently even in the combined application. The percentages of both Pb and Sb dissolved from the sorbed materials in the combined applications at pH 4 and 6 were the same levels as those in the single applications. However, the percentages of Sb dissolved in both combined and single applications were high at an alkaline pH. These results suggest that HAP and FH in a combined application would be useful for simultaneous Pb and Sb immobilization in soil with acidic to neutral pH, but not in soil with an alkaline pH.

Effect of biochar, derived from unfertilized dates, on the immobilization of Cd and Ni, in a sandy loam alkaline soil, was investigated. The biochar was applied to the soil columns at the rate of 0.5, 1, and 2 % (w/w) artificially polluted with 10 mg kg⁻¹Cd and 100 mg kg⁻¹Ni. After 1 month incubation of soil-biochar mixture under ambient conditions, the soil bulk density was reduced by 0.19 g cm⁻³as compared with no biochar addition with increase in soil pH. A reduction of 53 % in the NH₄NO₃-extractable soil Ni was recorded as compared with the corresponding control without biochar addition. After incubation, the water-soluble Ni and NH₄NO₃-extractable soil Cd and Ni contents were significantly lower in all the biochar treatments than the control. A reduction of 53 % in the NH₄NO₃-extractable soil Ni was recorded as compared with the corresponding control. The biochar content separated from the incubated soil showed low concentrations of NH₄NO₃-extractable Cd and Ni. The total Ni and Cd contents recovered from biochar samples after incubation were 35.2 and 3.7 mg kg⁻¹, respectively. Their contents in soil were substantially reduced by the incorporation of biochar amendment (114 to 57.2 mg kg⁻¹Ni, 9 to 5.6 kg⁻¹Cd) as compared with the no-biochar control. Therefore, addition of the biochar improved the soil physical properties and succeeded in immobilizing the studied metals.

Alkalisation of soil by dust pollution from a cement plant was assumed to be the principal cause of changes in heavy metal uptake and allocation between hybrid aspen (Populus tremula × Populus tremuloides Michx.) compartments. Emission of over 40 years of alkaline dust (pH 12.3–12.6) into the atmosphere had resulted in an increase of pH and an elevated concentration of total heavy metals in the upper layer of the soil (0–30 cm), which is considerable even 14 years after dust pollution has stopped. The accumulation and allocation of heavy metals in stem, shoot and leaves varied between themselves and between the trees from polluted and unpolluted plantations depending more on the mobility of elements and pH than element concentrations in the alkaline soil. High levels of heavy metals in the soil do not mean similar concentrations and ratios in plants growing in contaminated soil.