Effect of aging biosolids with soils of contrasting pH on subsequent concentrations of Cu and Zn in pore water and on their plant uptake.

PURPOSE: The study examines if a short period of reaction after addition of biosolids to soils can reduce the solubility and potential phytotoxicity of biosolid-borne Zn and Cu. METHODS: The effects of period of aging (zero, 60, and 120 days) of biosolids (applied at 0, 10, 20, and 30 g kg⁻¹) with an acid, neutral, or alkaline soil on pH and concentrations of Zn, Cu, and dissolved organic C in solution over a 60-day growth period of spinach were investigated using Rhizon pore water samplers. RESULTS: In the acid and neutral soils, increasing aging period markedly reduced the concentrations of Zn and Cu in solution and there were concomitant increases in solution pH. The effect was much less pronounced in the alkaline soil. Soluble Zn and Cu concentrations were generally positively correlated with dissolved organic C concentrations, negatively correlated with pH in the alkaline and neutral soils but positively correlated with pH in the acid soil. Spinach yields were lower in the acid than neutral and alkaline soils and tended to increase with increasing rates of biosolids in all three soils. The concentrations of tissue Zn and Cu were notably high in shoots of plants grown in the acid soil. For all biosolid-amended soils, the concentrations of tissue Cu were lower in plants grown after 60 days rather than no aging. CONCLUSIONS: Following biosolids applications to soils, an aging period of only a few months is likely to lower the solubility, and potential phytotoxicity, of biosolid-borne Zn and Cu particularly in acid and neutral soils.

Comparison of organic and inorganic amendments for enhancing soil lead phytoextraction by wheat (Triticum aestivum L.).

Phytoextraction has received increasing attention as a promising, cost-effective alternative to conventional engineering-based remediation methods for metal contaminated soils. In order to enhance the phytoremediative ability of green plants chelating agents are commonly used. Our study aims to evaluate whether, citric acid (CA) or elemental sulfur (S) should be used as an alternative to the ethylene diamine tetraacetic acid (EDTA)for chemically enhanced phytoextraction. Results showed that EDTA was more efficient than CA and S in solubilizing lead (Pb) from the soil. The application of EDTA and S increased the shoot biomass of wheat. However, application of CA at higher rates (30 mmol kg(-1)) resulted in significantly lower wheat biomass. Photosynthesis and transpiration rates increased with EDTA and S application, whereas these parameters were decreased with the application of CA. Elemental sulfur was ineffective for enhancing the concentration of Pb in wheat shoots. Although CA did not increase the Pb solubility measured at the end of experiment, however, it was more effective than EDTA in enhancing the concentration of Pb in the shoots of Triticum aestivum L. It was assumed that increase in Mn concentration to toxic levels in soil with CA addition might have resulted in unusual Pb concentration in wheat plants. The results of the present study suggest that under the conditions used in this experiment, CA at the highest dose was the best amendment for enhanced phytoextraction of Pb using wheat compared to either EDTA or S.

EDTA-assisted Pb phytoextraction.

Pb is one of the most widespread and metal pollutants in soil. It is generally concentrated in surface layers with only a minor portion of the total metal found in soil solution. Phytoextraction has been proposed as an inexpensive, sustainable, in situ plant-based technology that makes use of natural hyperaccumulators as well as high biomass producing crops to help rehabilitate soils contaminated with heavy metals without destructive effects on soil properties. The success of phytoextraction is determined by the amount of biomass, concentration of heavy metals in plant, and bioavailable fraction of heavy metals in the rooting medium. In general, metal hyperaccumulators are low biomass, slow growing plant species that are highly metal specific. For some metals such as Pb, there are no hyperaccumulator plant species known to date. Although high biomass-yielding non-hyperaccumulator plants lack an inherent ability to accumulate unusual concentrations of Pb, soil application of chelating agents such as EDTA has been proposed to enhance the metal concentration in above-ground harvestable plant parts through enhancing the metal solubility and translocation from roots to shoots. Leaching of metals due to enhanced mobility during EDTA-assisted phytoextraction has been demonstrated as one of the potential hazards associated with this technology. Due to environmental persistence of EDTA in combination with its strong chelating abilities, the scientific community is moving away from the use of EDTA in phytoextraction and is turning to less aggressive alternative strategies such as the use of organic acids or more degradable APCAs (aminopolycarboxylic acids). We have therefore arrived at a point in phytoremediation research history in which we need to distance ourselves from EDTA as a proposed soil amendment within the context of phytoextraction. However, valuable lessons are to be learned from over a decade of EDTA-assisted phytoremediation research when considering the implementation of more degradable alternatives in assisted phytoextraction practices.