Nutrition potential of biogas residues as organic fertilizer regarding the speciation and leachability of inorganic metal elements

Abstract

Biogas residues (BRs) are prospective organic fertilizer sources for agricultural cultivation. Besides N and P, however, other inorganic metal elements, such as K, Fe, Cu, Zn and so on, also affect the nutritional level of BRs significantly. In this study, a sequential extraction procedure (SEP) combined with a toxicity characteristic leaching procedure (TCLP) was conducted to investigate the speciation and leachability of metal components in BRs. The results showed that element K was the most effective nutrient component due to its largest available fraction and highest mobility factor (MF) of 78.4, whereas phytotoxic Al was the most stable and inert element in terms of its 96.68% residual fraction. Ca and Mg could be viewed as potential nutrient sources because their MFs exceeded 60. TCLP results revealed that these BRs could be classed as non-toxic organic waste but Cu and Zn should be paid more attention in that their total contents were beyond the permissible values. Meanwhile, more concerns should be given to Ni and Pb due to their large TCLP extractable fraction. In conclusion, these BRs can be used as a prospective nutrient pool for agricultural cultivation. SEP combined with TCLP can be effectively applied for assessing the nutrient level of the BRs as organic fertilizer for agricultural use.