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The interacting effect of pH, phosphate and time on the release of arsenic (As) from As-rich river bed sediments was studied. Arsenic release edges and kinetic release experiments (pH range 3-10), in the absence and presence of phosphate, coupled with sequential extraction procedures, SEM/EDX analyses and geochemical calculations, were carried out to evaluate As remobilisation and to elucidate the mechanisms involved. The results showed that As release underwent pronounced kinetic effects, which were strongly influenced by pH and phosphate. Remobilisation of As after 24 h was low (between *1 and 5%) and varied slightly with pH, whereas alkaline conditions generally promoted As remobilisation after 168 h, with up to 12-21% of total As released. The results showed that depending on the pH and sediment considered, the release of As increased dramatically after *48-72 h, suggesting that different processes are involved at different reaction periods. The addition of phosphate (1 mM) increased both the amount of As released (between 2 and 8 times) and the rate of As release from the sediments within the entire pH range (3-10) and period (168 h) studied. Moreover, in some cases, it also affected the shape of the As release edges and kinetic profiles. The similarities in the release profiles and the positive correlations between As and some sediment components, especially Fe and Al hydroxides, and organic matter-which appears to play a key role at high pHsuggest that As release from the studied sediments may be associated with solid phase dissolution processes under both acid and alkaline pH, whereas desorption plays a key role in the short term and at natural pH conditions, especially in the presence of phosphate, which acts as an As-displacing ligand. Evaluation of As mobility based on short-time leaching experiments may seriously underestimate the mobilisation of As from sediments.

In this study, we investigated the effect of some potential alleviative compounds against the acute toxicity of arsenic (As(V), As(III) and DMA(V)) on Aliivibrio fischeri (formerly Vibrio fischeri), a bioluminescent model bacterium, through the Microtox bioassay. The compounds studied differed in their mechanism of action, and they included the following: phosphate and glycerol, as chemical analogues (and potential competitors) of As(V) or As(III), respectively; citrate, a weak natural organic ligand; and the antioxidant ascorbic acid. Special attention was paid to phosphate effects, a widespread pollutant in natural environments. As(V) was found to be more acutely toxic than As(III) to A. fischeri, in accordance with its higher interaction with the bacteria. Both As(V) and As(III) were found to be much more acutely toxic than DMA(V), which was essentially non-acutely toxic even at very high concentrations. Phosphate presence (at equimolar P/As ratios or higher) resulted in the almost total suppression of bioluminescence inhibition, suggesting it exerts an alleviative effect against As(V) acute toxicity on A. fischeri. Interestingly, the uptake and the percentage of extracellular As(V) were not affected by the addition of phosphate, suggesting that such protective effect does not result from the competition for their common transporters. In contrast, the acute toxicity of As(III) was essentially unaffected by phosphate. Glycerol did not decrease the acute toxicity or the uptake of As(III) by A. fischeri, denoting the likely occurrence of an additional mechanism for As(III) uptake in such bacteria. Similarly, citrate and ascorbic acid essentially did not caused alleviation of As(V) or As(III) acute toxicity. As for environmental and operational implications, P could beneficially protect aquatic microorganisms against acute detrimental effects of As(V), whilst its presence could mask the toxicity due to As(V) when assessed using the Microtox bioassay, thus leading to seriously underestimate the actual ecological and health risks.

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