The role of iron in hexavalent chromium reduction by municipal landfill leachate.

The function of iron (ferric (Fe(III)) and ferrous (Fe(II))) in the hexavalent chromium (Cr(VI)) reduction mechanism by bacteria in municipal landfill leachate (MLL) was assessed. Evidence of an "electron shuttle" mechanism was observed, whereby the Cr(VI) was reduced to trivalent chromium (Cr(III)) by Fe(II) with the resulting Fe(III) bacterially re-reduced to Fe(II). Typically, investigations on this electron shuttle mechanism have been performed in an artificial medium. As MLL comprises an elaborate mixture of bacteria, humic materials and organic and inorganic species, additional complexities were evident within the cycle in this study. Bioavailability of the Fe(III) for bacterial reduction, availability of bacterially produced Fe(II) for chemical Cr(VI) reduction and hydrolysis of Fe(II) and Fe(III) become prevalent during each phase of the shuttle cycle when MLL is present. Each of these factors contributes to the overall rate of bacterial Cr(VI) reduction in this media. This work highlights the need to consider local environmental conditions when assessing the bacterial reduction of Cr(VI).

Microbial reduction of hexavalent chromium by landfill leachate.

The reduction of hexavalent chromium (Cr(VI)) in municipal landfill leachates (MLL) and a non-putrescible landfill leachate (NPLL) was investigated. Complete Cr(VI) reduction was achieved within 17 days in a MLL when spiked with 100 mg l(-1) Cr(VI) or less. In the same period, negligible Cr(VI) reduction was observed in NPLL. In MLL, Cr(VI) reduction was demonstrated to be a function of initial Cr(VI) concentration and bacterial biomass and organic matter concentrations. The bacteria were observed to tolerate 250 mg l(-1) Cr(VI) in MLL and had an optimal growth activity at pH 7.4 in a growth medium. The MLL also possessed an ability to sequentially reduce Cr(VI) over three consecutive spiking cycles.

Effect of methanol quality on the ionisation of herbicides, insecticides and fungicides using gradient elution liquid chromatography.

This paper explores the changes in the electrospray signal response of 39 structurally different compounds caused by the quality of the methanol, when used as a component in a gradient elution mobile phase. When three batches of LC-MS grade methanol from one manufacturer were evaluated, the largest variation in the electrospray signal responses of the 39 compounds examined was 18%. However, significant enhancement of the electrospray signals of up to 106% were observed among different brands of LC-MS grade methanol from different manufacturers. The effect of methanol quality on signal response was found to be compound dependent. This study also demonstrated that the senescence of the methanol was important. Using an expired batch of LC-MS grade methanol, electrospray signals were suppressed by as much as 95% for all compounds measured using positive mode electrospray. Conversely, the negative mode electrospray signals of compounds such as 4-octyl benzoic acid showed an enhancement of up to 96% when using the same batch of methanol. Linuron was used as a model compound to examine the change in the electrospray response, during gradient elution, when the proportion of an expired batch of methanol was varied. An infusion experiment showed that the linuron signal intensity decreased as the proportion of expired methanol increased in the mobile phase, which was in direct contrast to the increase in linuron signal observed with a non-expired batch of methanol. A series of isocratic experiments also showed that the linuron signal decreased as the proportion of expired methanol increased in the mobile phase. The ion ratios of several of the compounds studied changed significantly when using the expired batch of LC-MS methanol. The change in the ion ratios accentuates the difficulty of identifying compounds from in-source spectral libraries. A protocol is recommended for assessing the quality of methanol for LC-MS applications.

Microbial transformation of arsenic species in municipal landfill leachate.

The microbial transformation of arsenic species in municipal landfill leachate (MLL) was investigated with the objective to highlight arsenic transformation in the landfill system. Across the 43 day incubation in MLL, more than 90% arsenate (iAs(V)) was found to reduce to arsenite (iAs(III)) within 20 days, while iAs(III) was comparably stable although a fraction of iAs(III) was temporarily oxidated to iAs(V) in the first 3 days. Transformation of monomethylarsonic acid (MMA(V)) to dimethylarsinic acid (DMA(V)) in MLL was slow with only 5% MMA(V) methylated to DMA(V) after 43 days incubation. A portion of DMA(V) and MMA(V) in MLL was demonstrated to transform into thiol-organoarsenic and monomethylarsonous acid (MMA(III)), which were identified to include dimethyldithioarsinic acid (DMDTA(V)), dimethylmonothioarsinic acid (DMMTA(V)) and monomethyldithioarsonic acid (MMDTA(V)) by HPLC-ICPMS and LC-ESI-MS/MS. The microbial formation of DMDTA(V), DMMTA(V) and MMDTA(V) is postulated to relate to hydrogen sulfide generated by bacteria in MLL. Differences in arsenic transformation in sterilised and non-sterilised MLLs demonstrate bacteria play a crucial role in arsenic transformation in the landfill body. This study reveals the complexity of arsenic speciation and highlights the potential risk of forming highly toxic thiol-organoarsenic and MMA(III) in the landfill environment.

Arsenic speciation in municipal landfill leachate.

Arsenic species in municipal landfill leachates (MLL) were investigated by HPLC-DRC-ICPMS and LC-ESI-MS/MS. Various arsenic species including arsenate (iAs(V)), arsenite (iAs(III)), monomethylarsonic acid (MMA(V)), dimethylarsinic acid (DMA(V)), as well as sulfur-containing organoarsenic species were detected. Two sulfur-containing arsenic species in a MLL were positively identified as dimethyldithioarsinic acid (DMDTA(V)) and dimethylmonothioarsinic acid (DMMTA(V)) by comparing their molecular ions, fragment patterns and sulfur/arsenic ratios with in-house synthesised thiol-organoarsenic compounds. The findings demonstrated the potential for transformation of DMA(V) to DMDTA(V) and DMMTA(V) in a DMA(V)-spiked MLL in a landfill leachate environment.

A novel approach for the determination of detection limits for metal analysis of environmental water samples.

Despite the widespread use of the USEPA method (U.S. Environmental Protection Agency, 40 CFR 136 Appendix B) for the determination of method detection limit (MDL), criticisms have been raised that the method does not account for measurement bias and outliers that subsequently lead to a common misunderstanding of the requirement for the determination of MDL. This paper demonstrates that it is difficult to follow the USEPA method for verifying the MDL for analysis involving multiple metals and proposes a precision and bias criterion for determining the MDL. A multiple-point fitted profile, based on the correlation between relative standard deviation (RSD) and concentration, is used to derive a robust MDL value. Representative examples of As, Ca, Cr, and Cu are used to illustrate this procedure. A procedure for identifying outliers is also discussed.

Selective determination of airborne hexavalent chromium using inductively coupled plasma mass spectrometry.

A new method for determining ultra-trace levels of hexavalent chromium in ambient air has been developed. The method involves a 24-h sampling of air into potassium hydroxide solution, followed by silica gel column separation of chromium (VI), then preconcentration by complexation and solvent extraction. The chromium (VI) complex was dissolved in nitric acid. The resultant chromium ions were determined by inductively coupled plasma mass spectrometry (ICP-MS) using a dynamic reaction cell (DRC) with ammonia as the reactive gas to reduce polyatomic interferences. The interconversion of chromium in potassium hydroxide solution and air sample matrix were investigated under ambient conditions. It was found that there was no conversion of chromium (VI) into chromium (III) species. However, it was observed that some chromium (III) species were converted into chromium (VI) species. For a KOH solution containing 100 mug l(-1) of chromium (III) species, the rate of conversion was found to be 3% after 24 h exposure, 8% after 48 h, 10% after 72 h and no further conversion was observed thereafter. However, in a solution containing air sample matrix, 9.3% of chromium (III) converted to chromium (VI) within 6 h, and during the course of a 11-day exposure period, 13% (range 8-17%) of chromium (III) converted to chromium (VI). The method detection limit (MDL) for chromium (VI) in potassium hydroxide solution (0.025 M) was found to be 2x10(-2) mug l(-1). This is equivalent to 0.2 ng m(-3) (for 23 m(3) air sampled into 200 ml of KOH solution over a 24-h period). The recovery of spiked chromium (VI) from solutions containing air sample matrix was 95+/-9% (n=8). Matrix related interferences were estimated to be less than 10% based on recovery studies. The concentration of airborne chromium (VI) in Sydney residential areas was found to be less than 0.2 ng m(-3), however, in industrial areas the concentrations ranged from 0.2 to 1.3 ng m(-3) using this analytical procedure.