A preliminary investigation into the stability of inorganic arsenic species in laboratory solutions simulating sediment pore water

Letitia Pillay,Andrew Kindness

doi:10.17159/0379-4350/2015/v69a2

Letitia Pillay, Andrew Kindness

Open Access

https://doi.org/10.17159/0379-4350/2015/v69a2

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Abstract

A simple method to preserve arsenic species in simulated pore water was investigated. Synthetic pore water containing high levels of Fe, Mn and S (as sulfide, S 2– ) were synthesized and spiked with different arsenic species. Arsenite [As(III)], arsenate [As(V)], dimethylarsinate [(CH 3 ) 2 AsO 2– , DMA], monomethylarsonate [(CH3)AsO 3 2– , MMA], monothioarsenate(V) species (AsO 3 S 3– ,MTA) and tetrathioarsenate(v) species (AsS 4 3– , TTA) in different combinations were used. Disodium ethylenediamine tetraacetate (EDTA) and temperature was used to preserve speciation. Solutions were analyzed at intervals of one day, week, month and 2 months after preparation. Samples were analyzed by HPLC-ICP-MS using an ion exchange column and ammonium carbonate buffer. Samples containing Fe and Mn spiked with As(III), As(V), DMA,MMAshowed adequate species preservation for two months when EDTA was added. The total As in samples containing Fe,Mn and S 2– was preserved over 60 days however, speciation was not preserved. Samples spiked with synthesized mono - and tetra -thioarsenate(V) species showed immediate degradation of the mono - and tetra -thioarsenic species into unidentified As-S species. The results show that while EDTA may be adequate to preserve As species containing high Fe and Mn concentrations, the combination of EDTA and temperature was not successful in preserving As speciation in samples containing sulfides. KEYWORDS Arsenic speciation, EDTA, HPLC-ICP-MS, thioarsenic species.

Highlights

The toxicity of As species is well documented.[1]
ethylenediamine tetraacetate (EDTA) can be effectively used to preserve the total As in solutions containing Fe, Mn and S2– at levels typically found in estuarine conditions
The addition of EDTA to complex Fe and Mn prevented the formation of Fe and Mn–As species

Summary

Introduction

The toxicity of As species is well documented.[1]. Inorganic forms of arsenic are generally more toxic than the organic; the trivalent form generally more toxic than the pentavalent. The measurement of As species present in pore water cannot be undertaken without an adequate preservation method since arsenic is light sensitive, redox and pH active.2c A number of detailed studies have been carried out to assess the success of complexing agents, ion exchange resins, acidification, temperature and opaque storage vessels as preservation agents for As species.[6] Most of these studies measure the stability of As(III), arsenate [As(V)], dimethylarsinate(V) [(CH3)2AsO2–, DMA], monomethylarsonate(V) [(CH3)AsO32–, MMA ] in natural or synthetic water or on acid mine drainage samples. The influence of EDTA and low storage temperature on the stability of inorganic As species in synthesized pore water was investigated These two parameters are simple to control and could be administered to samples in the field. The synthesized pore water contained high amounts of Fe and S2– (to mimic the upper levels found in estuaries) and their effect on As speciation was investigated These three elements were chosen as their presence in natural environments is key to As speciation and mobility.

Instrumentation HPLC separation of species was carried out using an Agilent

Results and Discussion

As Speciation in Pore Water Solution D Spiked with 100 μg L–1 MTA and TTA

Conclusions