Abstract
The influence of epipsammic biofilms on As release from river sediments was evaluated in a microcosm experiment where biofilms were grown on sediments containing 106 mg kg−1As, collected in the Anllóns River, and compared with control systems without biofilms. The As transfer to the water column was low (<0.11% of total As in the sediment) and was further reduced by 64% in the presence of biofilms.AsVwas the predominant species in the overlying water in both systems.AsIIIconcentration was higher (up to 12% of total dissolved As) in the control systems than in the systems with biofilms, where this species was almost absent. This fact is of toxicological relevance due to the usually higher mobility and toxicity of the reducedAsIIIspecies. Control systems exhibited higher As mobility in water, in sulphate solution, and in weak acid medium and higher bioavailability in diffusive gradient in thin films (DGT) devices. Arsenic retained by the biofilm was equally distributed between extracellular and intracellular compartments. Inside the cells, significant concentrations ofAsIII, monomethylarsonic acid (MMAV), and dimethylarsinic acid (DMAV) were detected, suggesting that active methylation (detoxification) processes are occurring in the intracellular compartment.
Highlights
Arsenic (As) is a toxic element widely distributed in aquatic environments
The river water and sediment samples used in this study were collected in the Anllons River (Galicia, NW Spain) at a point known as Xavarido, just downstream from an area where gold mining operations have been performed throughout history and where high As concentrations have been previously detected in the riverbed sediment [4]
The literature on the interactions between epipsammic biofilms and As is limited and mainly focused on As toxicity [43] and on the effect of biofilms on the retention and speciation of As in the sediments [12, 13], while little is known about the effect of epipsammic biofilms on As transfer from As-rich sediments, which has been addressed in this work with the aim of contributing to the knowledge of the role of biofilms in As biogeochemistry in riverine systems
Summary
Arsenic (As) is a toxic element widely distributed in aquatic environments. Its presence is often attributed to lithogenic origin exacerbated by human activities [1]. Gold mining activities, carried out throughout history, resulted in the removal of As associated with Au mineralizations and its accumulation in sediments in the lower reaches of the river [4, 5]. Arsenic concentrations in these sediments reached up to 264 mg kg−1 [4], which exceed up to 5 times the reference levels of Galician soils defined at 50 mg kg−1 [6]. It has been shown that As solubility in these sediments increases in conditions of high salinity, extreme pH, or high P concentrations, as well as during high-flow resuspension events [7, 8]
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