Arsenic in leachates from mine waste rocks.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This research dealt with the leaching of arsenic containing waste rocks from a proposed gold mine site at Reefton, New Zealand. Then it established the acidity and concentrations of Fe, sulfate, As(V) and As(III) released as a result of leaching experiments which simulated the weathering process. It also determined the arsenic concentrations that were released from old mine workings into the Devils Creek sediments catchment and concentrated into sediments. The Devils Creek catchment is located downstream from the proposed mine. Acid mine drainage (AMD) results from oxidation of sulfide bearing rocks and deposits (e.g. FeS and FeS2). It is characterised by elevated concentrations of sulphate and iron and low pH. As(V) and As(III) may also be a component in AMD from the oxidation of FeAsS. Biologically As(III) is considered the more toxic form. Methods were developed for the measurement of arsenic species (As(V) and As(III)) in leachates derived from mine waste rocks. These were applied to freshwater samples and sediment extracts. These included: • A field method for the on-site fractionation of arsenic in fresh waters using XAD-4 and Chelex-Fe(III) resins. In this two-step process the XAD-4 resin was used to isolate and pre-concentrate natural organic matter (NOM) and therefore the arsenic fraction associated with it through iron bound to the NOM. In the second step Fe(III) modified Chelex 100 resin was used to selectively preconcentrate As(V) from water samples. Thus the combined method achieved discrimination between free and bound fractions of As(V) and the separation of As(V) from As(IU). • A flow injection analysis protocol based on spectrophotometric detection was modified to increase its sensitivity for measurement of arsenic species in leachates derived from waste rocks. The method was effective for the determination of arsenate. Arsenite was measured by prior oxidation with iodate. Study on the Devils Creek catchment established the existing baseline levels of arsenic in the fresh water and sediments. The water contains high levels of arsenic. Most of the arsenic in Devils Creek water is soluble. As(V) is the predominant species of which a minor fraction (13-28%) is bound to NOM. It was also established that arsenic was associated with Fe and A1 rich phases in the Devils Creek sediments. Due to the high accumulated arsenic load in the sediments, further uptake of arsenic from the water column is no longer viable because potential binding sites in the sediments have been saturated. Leaching simulation on mine basic waste rocks (GB 13) and an acidic sandstone were conducted using a variety of leach ants applicable to the field, then quantifying the Fe, As(III), As(V), heavy metals and acidity in the leachates. It was established that the sandstone rock had the greater potential to produce acid mine drainage (AMD). Its leachates give a high acidity (pH 2.9-3.4) compared to leachates of the GB 13 waste rock which have a weakly basic pH of7.2. The dominant arsenic species in leachates of both rock types is As(V). The concentrations of arsenic, soluble Fe and heavy metals leached from the sandstone were greater than those leached from GB 13. The interactions of arsenate with other components of AMD and natural organic matter were examined. It was established that ferrihydrite semi-quantitatively and rapidly removes arsenate from solution. The arsenate adsorption is inhibited by competing ions in the following order: sulfate> FA ≡ HA. Complexes of HA-Fe(III) and FA-Fe(III) bind arsenate as 1:1 complexes through the Fe(III). Investigations were made on the use of limestone for the control of arsenic in mine waste rock leachates. It was established that where acidic leachate contacts limestone the pH rises as a result of neutralisation reactions. Subsequently there is a precipitation of Fe(III) and retention of As(V) on the Fe(OH)3. As (III) is oxidised by Fe3+(aq) and Fe(OH)3. On contact with the limestone surface, As (III) is also retained and oxidised. Fe(III) impurities in the limestone are responsible for the uptake of both As(V) and As(III). Insoluble calcium arsenates or arsenites are not formed.