Assessment of the speciation and bioavailability of arsenic in acid mine drainage

Ramaqele, Mathabo
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The mining industry is a major economic development factor for many countries however, industrialization is one of the major causes of environmental pollution worldwide. The Witwatersrand basin in South Africa is the largest gold basin in the whole world. The main ore occurs in association with many other minerals of which some such as arsenic are highly toxic even at trace levels. The mining processes in the region have brought to the surface the minor minerals in the form of mine dumps and mine tailing dams which contribute to acid mine drainage (AMD). Though the problem has manifested itself mostly along these gold fields, it has been seen in other mining provinces too. This study presents data pertaining to speciation of arsenic present in the Witwatersrand mining area and also discuss the conditions that impact on arsenic bioavailability and toxicity. Electrochemical methods such as differential pulse anodic stripping voltammetry (DPASV) and other techniques such as inductively coupled plasma-optical emission spectroscopy (ICP-OES), inductively coupled plasma-mass spectrometry (ICP-MS) and ion chromatography (IC) were applied in this work. Geochemical modelling using a PHREEQC program was also applied to study the distribution of arsenic species. The DPASV method was optimized using 100 µg L-1 As(III) standard solution for parameters including deposition potential (DP), deposition time (DT), rotation speed (RS) and modulation amplitude (MA). The optimum conditions were obtained at -0.4 V, 180 s, 500 rpm and 0.04V for DP, DT, RS and MA, respectively. Acid mine drainage and Fleurhof dam water were used to assess the method viability. The results show that DPASV was more sensitive in dam water with recovery percentage of 81.6% and 123% for As(III) and As(V), respectively. While in AMD, the interference of various trace metals and anions was observed. Thus making this method more effective for determination of arsenic species in natural water. The total arsenic concentrations were determined to be 0.2786 mg L-1 and 0.0120 mg L-1 by spectrometric techniques in AMD and Fleurhof dam samples, respectively. PHREEQC modelling was applied to simulate the environment and determine the species distribution in AMD. The method was also utilized to study iii how parameters such as pH and temperature may influence the species distribution. Acidic conditions which are characteristic of AMD and neutral conditions representing natural waters were simulated. It was discovered that acidic conditions favour dissolution of both As(III) and As(V) species but As(III) was the more dominant species. At neutral pH, it was discovered that As(V) was the dominant species but the overall dissolved metals were shown to be limited by precipitation. Only slight contribution of temperature was observed as both simulations for cold and warm temperatures did not show much difference in terms of species distribution.
A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements of the Master of Science degree University of the Witwatersrand Johannesburg 2019