Effect of treated zeolite, iron waste, and liming on phytoavailability of Zn, Cu, and Ni in long-term biosolids-amended soils

Two metal-contaminated biosolids-amended soils (moderately and highly contaminated) from the Bromley Sewage Treatment Farm, Christchurch, New Zealand, were used to evaluate the effect of remedial treatments on Ni, Zn, and Cu phytoavailability to sunflower (Helianthus annus L.). Two different chemical treatments (iron waste and treated zeolite), at 2 rates of application (5% and 10% w/w), in combination with 3 rates of a liming material (Ca(OH)2 at 0%, 0.33%, and 0.66% w/w) were evaluated for their metal remediation potential using pot experiments. Under the moderately acidic pH conditions of the original soils (pH 5.4–5.7), neither of the materials had substantial effects on plant metal concentrations, and the application of treated zeolite resulted in a large decrease in plant yield (>60% reduction). However, in the presence of Ca(OH)2, both materials showed some potential for reducing Ni and Zn concentrations in sunflowers compared with Ca(OH)2 alone. The best combinations of zeolite or iron waste with Ca(OH)2 resulted in reductions in shoot Ni concentrations to below the detection limit. For Zn, the best combinations of materials resulted in reductions in sunflower shoot Zn concentrations of 91% for the moderately contaminated soil and 75% for the highly contaminated soil. Combinations of iron waste and Ca(OH)2 in particular resulted in substantial decreases in soluble soil Zn concentrations (>90% reduction) and increases in plant yield (63% increase for highly contaminated soil), attributed to the remediation of Zn toxicity. There was little effect of any treatment on Cu concentration in the sunflower plants.