Evaluation of proton and europium (III) binding reactions in a sewage sludge humic acid

Abstract

Recent increases in land application of sewage sludge have raised concerns over the potential introduction of contaminants into the human food chain. The establishment of sound regulation requires knowledge on the fate and distribution of contaminants in sludge amended soils. Humic substances play a crucial role in trace element bioavailability and transport; thus research is needed to elucidate their mechanisms of reaction. This paper reports the results of a study conducted with a sewage sludge humic acid. The objectives were to evaluate potential differences in composition between this humic acid and its soil analogs and to ascertain their impact on its reactivity. Higher nitrogen and sulfur contents, as well as lower oxygen levels were detected in the sewage sludge humic acid compared to those of its soil counterparts. Determinations of proton and Eu³⁺ affinity constants suggest that the ion binding reactions in this humic acid are controlled mainly by a class of functional groups possessing carboxyl-like reactivities. This finding contrasts with the general notion that ion binding reactions in humic substances are controlled by equivalent fractions of carboxyl and phenolic-type groups, and it confirms the heterogeneous nature of these compounds.