Effect of fulvic acid on liberation of acidity and arsenic form arsenic substituted schwertmannite
Vithana, CL, Sullivan, LA, Bush, RT & Burton, ED 2013, 'Effect of fulvic acid on liberation of acidity and arsenic form arsenic substituted schwertmannite', in Proceedings of the 5th International contaminated site remediation conference, Melbourne, Victoria, 15-18 September.
Schwertmannite is one of the main secondary iron hydroxy sulfate minerals contributing to acidity in acid sulfate soils (ASS). While it is an important source of acidity in ASS, schwertmannite is also known to be a potential sink for trace metals such as arsenic (As) and chromium (Cr) (Fukushi et al. 2003; Regenspurg and Peiffer. 2005).Trace metal adsorption/substitution is known to stabilize schwertmannite which in turn retards its transformation to goethite which is one of the main processes of acidity generation in ASS. Fulvic Acid (FA) is a subclass of natural organic materials that are ubiquitous in aquatic environments (Wang et al. 1997). Due to the presence of polyfunctional organic groups in FA, it may help to retard the reactivity of iron hydroxy sulfate minerals such as schwertmannite, thereby affecting the mobility of trace metals in the aquatic environments (Wang et al. 1997). Our objective in this study was to examine the effect of FA on the release of As which had been substituted in schwertmannite.
A 1:40 suspension of arsenic-substituted schwertmannite: artificial acidic water was titrated to pH 6.5 and 4.5 with 0.005M NaOH in the presence of different FA concentrations (0, 1, 10 25 mgL-1) for 48 hrs. FA solutions were prepared by diluting a stock solution of Suwannee River Fulvic Acid (SRFA). As-substituted schwertmannite was consisted of 3.6% As (weight). The cumulative acidity released by schwertmannite over 48 hrs was measured by the volume of 0.005M NaOH consumed. The two pH values used in this study span a pH range typically found in ASS during the drought and flooding seasons. The selected FA concentrations were also representative for many natural water systems. After 48 hrs, the extract was filtered and analysed for As and the residue was examined using XRD to identify any mineralogical changes that may have occurred in schwertmannite. The variability between the effects of different FA concentrations on As release was statistically evaluated using the Analysis of Variance (ANOVA)-single factor at 90% confidence interval.
RESULTS AND DISCUSSION
At pH 6.5 and under moderate to lower FA concentrations (1-10 mgL-1) the acidity liberated from As-substituted schwertmannite showed only slight initial increase compared to that in the absence of FA, and in the presence of higher FA concentrations (25 mgL-1) (Figure 1a). Alternatively, at acidic pH (4.5), all three FA concentrations retarded the acidity release from As- substituted schwertmannite (Figure 1b). The P values determined at 90% confidence interval (i.e. Į =0.1) for the concentrations of As released at pH 4.5 and at 6.5 (at different FA concentrations) were 0.11 and 0.06 respectively. Since the calculated P< 0.1 at pH 6.5, it is apparent that there was significance variability between the effect of FA concentrations on As release from As-substituted schwertmannite. The data shows that the presence of FA increased the release of As from schwertmannite at both pH’s (Figure 2). Interestingly at pH 6.5, the lowest FA concentration (1 mgL-1) had the potential to remove twice as much as As from schwertmannite as was released in the absence of FA.