A multi-technique investigation of copper and zinc distribution, speciation and potential bioavailability in biosolids

Abstract

The use of biosolids in agriculture continues to be debated, largely in relation to their metal contents. Our knowledge regarding the speciation and bioavailability of biosolids metals is still far from complete. In this study, a multi-technique approach was used to investigate copper and zinc speciation and partitioning in one contemporary and two historical biosolids used extensively in previous research and field trials. Using wet chemistry and synchrotron spectroscopy techniques it was shown that copper/zinc speciation in the biosolids was largely equivalent despite the biosolids being derived from different countries over a 50 year period. Furthermore, copper speciation was consistently dominated by sorption to organic matter whereas Zn partitioned mainly to iron oxides. These data suggest that the results of historical field trials are still relevant for modern biosolids and that further risk assessment studies should concentrate particularly on Cu as this metal is associated with the mineralisable biosolids fraction.

Introduction

The application of municipal wastewater biosolids (treated sewage sludge) to agricultural land contributes useful constituents such as macro-nutrients and organic matter to the soil, but also adds metals such as copper (Cu) and zinc (Zn). Copper and Zn are essential micronutrients required for plant nutrition and their addition with biosolids may be beneficial, but they are also potential environmental contaminants and will persist in the soil long-term if added to excess. Their potential role as environmental and food chain contaminants has been a matter of extensive debate (e.g. Beckett et al., 1979; McBride, 1995; Basta et al., 2005). Previous experiments have included laboratory incubation experiments (e.g. Stacey et al., 2001), large-scale, long-term field trials (e.g. McGrath, 1987; Heemsbergen et al., 2010) and mechanistic studies (e.g. Nagoshi et al., 2005; Hettiarachchi et al., 2006). Despite these efforts, questions regarding the bioavailability and ecotoxicity of biosolids-derived metals persist, and research is ongoing (e.g. Smolders et al., 2012; Donner et al., 2011). The lack of determinative answers is due partly to the complexity of the biosolids materials themselves and partly to the complexity and variety of soil types they may be added to.

The interpretation and use of results from previous field trials and laboratory based experiments is complicated by the fact that earlier research was typically carried out using highly metal contaminated biosolids (e.g. Mulchi et al., 1987; McGrath, 1987; Brown et al., 1996). These are far from representative of contemporary biosolids currently being accepted for use in agriculture. Nowadays, regulation of the biosolids land disposal pathway has been widely implemented (PSD, 2009), and this, together with increasingly stringent wastewater discharge permitting for industrial facilities has led to a general trend of improving biosolids quality. Biosolids have also increasingly been subjected to further treatment stages such as composting in order to reduce pathogenic risks prior to agricultural use, and their chemical properties are known to change as a result (Amir et al., 2005). For example, Donner et al. (2011) recently reported consistent differences in Cu and Zn speciation and distribution in fresh vs. aged biosolids. As this kind of mechanistic information can help predict the fate of metals in land applied biosolids, it is of interest to know whether metal speciation in earlier biosolids materials containing higher metal contents differs significantly from that of contemporary materials.

This paper presents a detailed examination of three biosolids products representing both historic and contemporary biosolids. All three biosolids have been used in multiple experiments, including long-term field trials, to investigate the effects of biosolids application on soil health and crop metal uptake. For the current work, a multi-technique approach incorporating a range of experimental methods has been used to investigate Cu and Zn chemistry in these materials; from the lability of the metals to their partitioning and speciation. The methods employed include isotopic dilution (E-values), sequential extraction, sodium polytungstate density separation, and extended x-ray absorption fine structure spectroscopy (EXAFS). The technique of differential individual particle analysis (DIPA, Hunt and Johnson, 2011) in association with micro x-ray fluorescence (μ-XRF) element mapping before and after sodium hypochlorite treatment is also explored in this paper. To our knowledge this is the first published experiment to demonstrate the use of this technique in association with μ-XRF analysis. The multi-technique characterisation of these biosolids will facilitate further interpretation of the field trials in which they have been employed and will provide a comparison between the speciation of historical and modern biosolids.