Heavy metal complexation in sludge‐amended soil. The role of organic matter in metal retention

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Abstract
Soil columns which had received a range of applications of anaerobically digested sewage sludge were incubated for five months. The sludge application rates were equivalent to the one year average value and maximum value (equivalent to six years’ addition) permitted by UK guidelines. About 23–26% of total organic matter degraded in the soil columns during incubation. Organically bound metals were extracted with pyrophosphate following incubation. Over half of the cadmium was in the organic phase, compared to less than 20% of the lead. The proportion of lead extractable was unaffected by the rate of sludge addition, whereas for copper percentage extractability decreased with increasing sludge application and for nickel and zinc percentage extractability increased. Organic complexes of copper and cadmium were stronger than for the other metals, and the binding capacity of the organic matter for metals was greatest for copper and nickel.