In situ immobilization of cadmium and zinc in contaminated soils : fiction or fixation?

Keywords: beringite, cadmium, DOC, DOM, earthworms, immobilization, leaching, lime, manganese oxides, metal binding, metal uptake, organic matter partitioning, pH, soil contamination, remediation, sorption, Swiss chard, zeolites, zinc.

It is generally assumed that a decrease in metal concentration in the soil solution reduces metal leaching, and metal uptake by and toxicity to plants and soil organisms. In situ immobilization is a soil remediation technique that aims at reducing the metal concentration in the soil solution by adding a binding material to the soil. Application of this technique requires understanding of underlying mechanisms and potential side effects.

Both laboratory experiments and model calculations have been performed to gain insight in immobilizing processes. It is essential to quantify metal binding to natural organic matter. The NICA-Donnan model was designed to calculate metal binding by organic materials, but specific Zn parameters were not available due to a lack of analytical data. The Wageningen Donnan Membrane technique (WDMT) was therefore further developed to measure free Zn concentrations in humic acid solutions.

Many immobilizing materials increase the soil pH. This results in an increased negative charge of soil particles, and hence in a decreased metal mobility. In some cases, the addition of alkaline materials simultaneously increases the dissolved organic matter (DOM) concentration in the soil solution, resulting in increased leaching of metal-DOM complexes. We showed that alkaline soil amendments need to contain enough Ca to suppress the dispersion of organic matter as induced by a pH increase. We also quantified the dispersion of organic matter. It appeared that the Donnan potential of the organic matter, as calculated by the NICA-Donnan model, correlated very well with the DOM concentration.

The addition of alkaline materials strongly decreased the metal concentration in Swiss chard ( Beta vulgaris L. var. cicla ). In contrast, the uptake of Cd and Zn by earthworms ( Lumbricus rubellus and Eisenia veneta ) was hardly influenced by the addition of alkaline materials. Another experiment showed that the addition of MnO ₂ , which did not affect soil pH, resulted in a decreased Cd concentration in the earthworm tissue. Apparently, next to dermal uptake, pH independent Cd uptake via the intestine was an important uptake route. Cd uptake by earthworms could be estimated by a soil extraction with 0.1 M triethanolamine and 0.01 M CaCl ₂ adjusted to pH 7.2.