Comparison of dissolved organic matter from sewage sludge and sludge compost as electron shuttles for enhancing Fe(III) bioreduction

Land utilization of sewage sludge and sludge compost is a common practice in many countries. Soils amended with sewage sludge and sludge compost display different physicochemical properties, especially in terms of dissolved organic matter (DOM) composition that affects the electron-donating capacity (EDC) of DOM in soils. The aim of this paper was to compare the EDC of DOM derived from sewage sludge and sludge compost for enhancing Fe(III) bioreduction. It is expected that this research could be helpful for further understanding of soil remediation in the future. Sludge and compost DOM were extracted from sewage sludge and sludge compost, respectively. Fractionation, CHNO/S analysis, Fourier-transform infrared and ultraviolet-visible spectroscopy, and cyclic voltammetry were then used to determine the degree of aromaticity and humification in the samples. To determine if their EDC was altered during composting, samples were reduced by the humic-reducing bacterium, Shewanella cinica D14(T), after which their EDC values were determined. The initial and potential EDC values of the samples were measured using FeCl3/Fe(citrate) as an electron acceptor. The insoluble Fe(III) oxide reduction by strain D14(T) was mediated by DOM. Both sludge and compost DOM contained redox-active functional groups that could shuttle electrons to insoluble Fe(III) oxide and accelerate Fe(III) bioreduction. When FeCl3 was used as an oxidizing agent, their potential EDC reached 0.25 and 0.50 meq/(mg C), respectively. In addition, their electron transfer ability could be recycled. The degree of humification of DOM increased during sludge composting, which resulted in the EDC of compost DOM being greater than that of sludge DOM. Furthermore, the rate of Fe(III) bioreduction mediated by DOM could be accelerated under anaerobic conditions, which has important implications for soil biogeochemistry because it may accelerate the rate of some kinds of toxic metals and recalcitrant organic pollutants transformation or degradation.