Remediation of heavy metal contamination in biosolids-amended soils

Abstract

The Bromley Sewage Treatment Farm (BSTF) at Christchurch, NZ, has received biosolids since the late 1960's, resulting in heavy metal accumulation in the soil. The purpose of this study was to determine whether the bioavailability/mobility of heavy metals can be reduced by remediation methods involving addition of inexpensive materials to the biosolids-amended soils.

Ten soils from the BSTF with a range of metal concentrations (Cu, Cr, Cd, Ni, Pb and Zn) were initially examined. Most sites have already reached the maximum recommended* heavy metal concentrations (Cu, Cd and Zn), and a significant percentage of the total metals were associated non-residual, potentially labile fractions.

Three soils, representing low, medium and high levels of metal contamination (Zn, Ni, Cu and Cd) were selected for further study. Iron oxide, peat, zeolite, iron waste and treated zeolite were selected as potential amendments for reducing metal mobility/bioavailability.

A comparison of metal sorption by the biosolids-amended soils and the various amendments suggested that peat, treated zeolite and iron oxide had the potential to reduce metal solubility in biosolids-amended soils. However, metal sorption increased strongly with an increase in pH, and with some amendments it may be necessary to increase soil pH by liming to obtain the most effective metal retention.

Results from an incubation study revealed that, in many cases, addition of the amendments successfully reduced soluble metal concentrations in the soils. However, soluble metal concentrations in the control lowest contaminated soil were often near or below the detection limits to begin with. For the medium and highly contaminated soils, lime was the most effective material for increasing soil pH, and lime, treated zeolite and iron waste were the best amendments for immobilizing Zn, Cu and Ni, especially when the two amendments were combined with lime. Only the iron oxide reduced Cd concentrations significantly in the medium soil.

A preliminary glasshouse study, showed that sunflowers have a relatively high ability to absorb heavy metals from the soil compared to a range of other plant species, and therefore sunflowers were used as the test plant in the main experiment. Using the medium and highly contaminated soils, it was demonstrated that, combinations of iron waste with lime (0.66% w/w) increased root production significantly. However, application of treated zeolite on its own suppressed dry matter production and shoot Ni, Cu and Zn uptake. The addition of lime alone (0.33% w/w) significantly increased shoot growth for the highly contaminated soil.

Decreases in plant Ni and Zn concentration/uptake were observed due to application of treated zeolite or iron waste together with lime, and with some exceptions, lime or iron waste additions on their own. The proportional decreases were higher for the medium soil treatments relative to the highly contaminated soil. There were minimal effects of the various treatments on plant Cu concentration or uptake. In a root study experiment, it was shown that water-extractable Ni, Cu and Zn in rhizosphere soil generally decreased as a result of amendment with iron waste, with or without lime, and with lime alone.