Soil washing of arsenic from mixed contaminated abandoned mine soils and fate of arsenic after washing.

Arsenic contamination in abandoned soils is a global concern which warrants an effective method of remediation. In this study, two organic acids and one biodegradable chelating agent were used to treat arsenic (As) contaminated abandoned mine soils. The concentration of As was 19,100 and 75,350 (mg/kg) for Webbs Consols (WC) and Mole River (MR) samples, respectively. X-ray diffraction and scanning electron microscopy confirmed that tooeleite, arsenopyrite, scorodite and quartz were the major minerals in these soils. A major portion of the As was composed of amorphous and crystalline oxides of Fe and Al determined by sequential extraction. Among the three washing reagents (oxalic acid, citric acid and EDDS) oxalic acid showed the best performance for extracting As. Based on the batch experiment, 0.5 M oxalic acid and 3 h of washing was the most efficient treatment to extract As and other trace elements. Extraction of As, Fe, and Pb was 70, 55, and 48% respectively for WC, while 68, 45 and 63% respectively for MR soil. Oxalic acid extracted 75 and 83% of As and Fe, respectively from tooeleite. Leachability and bioaccessibility of As and Fe in the treated soil was reduced due to washing. However, bioaccessibility and leachability of Pb in soil and Fe and As in tooeleite increased in washed samples. Though the leachability and bioaccessibility of As and Fe in soil was reduced in the treated soil, As still exceeded the USEPA criteria (5 mg/L) which is needed to successfully remediate soil by washing. Soil washing and subsequent solidification/stabilization could be an alternative option to remediate extremely contaminated abandoned mine soil.

Arsenic bioaccessibility and fractionation in abandoned mine soils from selected sites in New South Wales, Australia and human health risk assessment.

Understanding the transport behaviour of arsenic (As) from soils to humans is critical when undertaking human health risk assessment and contamination control. This research examined As bioaccessibility in different As fractions and particle size fractions of As-enriched mine soils using different extractions. Bioaccessibility of As ranged from 0.24% to 32% for Solubility Bioaccessibility Research Consortium (SBRC) and Physiologically Based Extraction Test (PBET) methods, with extractable As (using 0.43 M HNO3) being 1.3-24.9%. The highest As bioaccessibility (19-32%) was consistently observed in the fine particle size fraction (< 53 µm) of all three extractions. Sequential extractions revealed that As fractions were mostly associated with crystalline (30-73%) and amorphous (9-59%) Fe/Al oxyhydroxides. The bioaccessibility of As in the gastric phase of SBRC and PBET methods highlighted a positive correlation (R2 = 0.83-0.88, p < 0.01) with exchangeable, surface and amorphous- bound As fractions, while the intestinal phase showed a strong positive correlation (R2 = 0.85-0.89, p < 0.01) with exchangeable and surface bound fractions. The study revealed that As bioaccessibility in soils can potentially be determined using the 0.43 M HNO3 extraction procedure. Health risk assessment confirmed that there was a strong increase in chronic daily intake, hazard quotient and cancer risk, with a reduction in particle size.

Geochemical fractionation and mineralogy of metal(loid)s in abandoned mine soils: Insights into arsenic behaviour and implications to remediation.

Trace element contamination from abandoned mine sites is a major threat to the environment. The distribution of trace elements in various particle size fractions of soils from abandoned mine sites plays a critical role in designing remediation approaches. This study investigated the geochemical distribution of trace element enrichment and mineralogical composition in various particle size fractions from contrasting abandoned mine sites (Webbs Consols, Halls Peak and Mole River, Australia). Results revealed that arsenic and other element concentrations increased with decreasing particle size for samples from Webbs Consols and Halls Peak. The highest arsenic (3.05%), lead (3.23%) and zinc (1110 mg/kg) were found in the finest fraction (<0.053 mm). In Mole River, the highest concentration of arsenic (10.8%), lead (209 mg/kg) and zinc (351 mg/kg) were observed in coarse fractions. Arsenic fractionation by sequential extraction showed that arsenic was strongly associated with the amorphous and crystalline iron phases. X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies revealed that tooeleite (a ferric arsenite mineral, also confirmed by Transmission electron microscopy (TEM)), arsenopyrite, scorodite and arsenolite were the dominant arsenic minerals. The study showed elevated levels of arsenic bearing minerals across particle sizes which has significant implications for remediation approaches at abandoned mine sites.