EFFECTS OF NUTRIENTS AND REDOX TRANSITIONS ON ARSENIC DYNAMICS IN WETLANDS, AND MICROBIALLY MEDIATED PYRRHOTITE FORMATION

Abstract

Constructed wetlands are economically viable and highly efficient in the treatment of high arsenic (As) waters discharged from chemical and power industry as well as agriculture. However, As dynamics and bioaccumulation coupled to nutrients and redox transitions are confounding. We investigated the effect of phosphate (PO43-), sulfate (SO42-) and iron (Fe(III)) reduction on As dynamics and its bioaccumulation in wetlands using greenhouse mesocosms. Results showed that high Fe (50µM ferrihydrite/g solid medium) and SO42- (5mM) treatments are most favorable for As sequestration in solid medium in the presence of wetland plants (Scirpus actus), probably because root exudates facilitate the microbial reduction of Fe(III), SO42-, and As(V) to sequester As(III) by incorporation into iron sulfides and/or plant uptake. Whereas, in the Fe(III)-rich solid medium with plants, high PO43- (100 µM) loading strongly enhanced As release into pore water under anoxic conditions, possibly due to the competitive sorption between PO43- and As(V) as well as the reductive dissolution of Fe(III) and As(V).

A majority of As mass (>80%) was sequestered in the solid medium and wetland plants for all the treatments. As retention in soils and accumulation in plants were mainly controlled by SO42- and PO43- rather than Fe levels in our study. An As speciation analyses in pore water showed that 19% more dissolved As was reduced under high SO42- than low SO42- levels, and 30% more As(III) was detected under high PO43- than low PO43- levels, which is consistent with the fact that more dissimilatory arsenate-respiring bacteria were found under high SO42- and high PO43- levels.

The formation processes of pyrrhotite at ambient temperature are poorly understood, because of the interplay of microbially mediated and abiotic reactions in the biogeochemical iron and sulfur cycle. We observed microbially mediated pyrrhotite formation suggested by X-ray photoelectron spectroscopy (XPS), which is not the common product (mackinawite and greigite) of biological sulfate reduction. Quantitative XPS analyses of Fe2p3/2 and S2p spectra revealed the possible sequential mechanism to explain the iron sulfides formation as Fe(II)+S2-  FeS  marcasite/pyrite-like iron sulfide  pyrrhotite-like iron sulfide. Further study needs to be done to confirm the mechanism.