Copper Partitioning Among Mineral and Organic Fractions in Biosolids

Environmental Context. Only a portion of the total amount of heavy metals present in sewage biosolids is accessible to organisms, including plants, and therefore only that portion presents any possible toxicity threat. However, metals such as copper, which are commonly associated to a large degree with the organic fraction, may become more accessible over time as organic components degrade. Determining the extent of partitioning of Cu between the organic and inorganic fractions may provide an indication of any long-term risks associated with utilisation of biosolids in agriculture.

Abstract. Metal partitioning between organic and mineral fractions in biosolids may provide an indication of the long-term risks associated with land application of the material. For example, metals found to partition into the organic phase may pose a potential threat when the organic matter is decomposed, whereas metals bound in the mineral fraction would be expected to remain stable over time (given no changes in other environmental conditions) owing to the stability of mineral components. Therefore the question of which components bind copper in biosolids, and whether the sorption capacity is maintained over time, was addressed in the present study. Biosolids incubated for 21 months and non-incubated controls were examined. The solid–solution distribution coefficient (K_d) for Cu was measured in whole biosolids and in biosolid organic and mineral fractions via batch experiments employing the radioactive isotope ⁶⁴Cu. The mineral fraction was isolated by NaOCl oxidation, whereas the organic fraction was isolated using HF. Results found the relative importance of mineral and organic fractions to Cu sorption varies between biosolids, and in some cases can vary over time. Reduction in sorption capacity caused by losses of organic matter in some biosolids suggests the possibility of increased availability of biosolid metals over time.