David McConchie

Redox stratification and heavy metal partitioning in Avicennia-dominated mangrove sediments: a geochemical model

Abstract Mangrove forest sediments can provide a sink for trace metals because the mangroves create a baffle that promotes the accumulation of fine-grained organic matter-rich sediment, which is usually sulphidic due to the presence of sulphate-reducing bacteria. Direct adsorption, complexing with organic matter, and the formation of insoluble sulphides all contribute to the trapping of metals. The concentration and chemical speciation of the metals are influenced by the distribution of geochemically distinct horizons within the sediment. In horizons with a pH>7 and an Eh +100 mV (oxidation horizons), most metals are present as exchangeable or oxide-bound species. In most cores, two oxidation and two reduction horizons can be recognised, but dark mottles of low Eh ( +100 mV) sediment can be found around mangrove roots and burrows in the reduction horizons. The depth to each horizon, differs between cores and can change in response to seasonal shifts in the position of the water table. A model is presented that accounts for the development of the oxidation and reduction horizons within the Avicennia -dominated mangrove forest sediment and describes the major controls on metal cycling within the sediment.

Adsorption of arsenate from water using neutralized red mud

The potential for using seawater-neutralized red mud (Bauxsol), a waste from aluminum manufacturing, as an adsorbent for removing As(V) (arsenate) from water is studied. Herein, adsorption characteristics are investigated and it is shown that adsorption follows the Langmuir model, with the adsorption constants indicating the feasibility of the process. Furthermore, the adsorption is found to increase with decreasing pH (i.e., ligand-like adsorption), higher adsorbent dosages, and lower initial arsenate concentrations. The effects of ions are also tested and it is shown that the adsorption of arsenate decreases in the presence of HCO3-, while Cl- has little effect, and Ca2+ increases the adsorption. Water quality assessment after treatment with Bauxsol indicates that none of the trace elements tested are released from the adsorbent. A TCLP leaching test also reveals that the used adsorbent is not toxic. It is foreseen that Bauxsol may be developed into an efficient low-cost adsorbent for (pre-) treating arsenate contaminated waters.

Chemistry of seawater neutralization of bauxite refinery residues (red mud)

Bayer process red muds are highly alkaline, and their disposal and potential for reuse are complicated by high concentrations of sodium, high pH, and high alkalinity. Several methods of neutralization of red muds have been reported including: infiltration of rainwater and atmospheric CO2, treatment with strong acids, gypsum addition, and seawater neutralization. Using seawater to neutralize red mud at the Queensland Alumina Ltd. (QAL) refinery at Gladstone, QLD, began as a fresh water conservation measure but led to many benefits, including reduced freshwater use at the refinery, increased settling rates at the pond and reduced alkalinity and sodicity in the solid wastes and entrained liquor. The study reported here describes and quantifies the geochemical consequences of seawater neutralization of bauxite refinery residues. Methods and results presented allow description of the chemical, mineralogical, and physical characteristics of seawater-neutralized red mud. Experiments then identify and quantify th...

A modified chromium-reducible sulfur method for reduced inorganic sulfur: optimum reaction time for acid sulfate soil.

Reaction times for 16 acid sulfate soil materials analysed using a modified chromium-reducible sulfur method varied between 10 and 15 min, regardless of whether the samples had been dried and ground prior to analysis or were analysed without pretreatment. The reaction time for a ground (<63 mm) pyritic rock sample was 20 min. An optimum reaction time of 20 min is recommended for analysing acid sulfate soil using the modified method; this reaction time is much less than the 1 h reaction time used in previous methods.

Phosphate Removal from Aqueous Solutions using Neutralised Bauxite Refinery Residues (Bauxsol

Environmental Context.Eutrophication of freshwater and marine ecosystems is a global problem, which is frequently linked to high phosphorus concentrations. The present study investigated the use of Bauxsol™, a modified bauxite refinery residue, to remove dissolved phosphate from water, and has shown that it can be used as a cost-effective adsorbent for treating phosphate-contaminated waters. The results provide water and environmental managers with a new technique for decreasing the phosphate loads in water and wastewater. Environmental benefits include improved water quality, minimisation of excessive plant growth, including potentially toxic blue green algae, and the utilisation of an industrial residue for environmental remediation. Abstract.Phosphate (PO43–) removal by Bauxsol™, a neutralised bauxite refinery residue, was investigated as a function of time, pH, ionic strength, adsorbent dosage, competing ions, and initial phosphate concentration. The results of adsorption and desorption studies indicate that adsorption of PO43– by Bauxsol™ is based on a ligand-exchange mechanism, although the low reversibility pH-independent desorption observed in acid-treated Bauxsol™ indicates a dominance of chemisorption. It was shown that PO43– adsorption onto both Bauxsol™ and acid-treated Bauxsol™ followed a Langmuir isotherm model, with adsorption capacities of 0.21 and 0.48 mmol g−1 at pH 9.0 and 5.2 respectively. Adsorption of PO43– by Bauxsol™ increased with decreasing pH, with maximum adsorption efficiencies obtained at pH 5.2 ± 0.1 (the lowest pH investigated), higher Bauxsol™ to initial phosphate concentration ratios, and increased time. Studies of the effects of competing ions on the adsorption of PO43– by Bauxsol™ indicated that adsorption decreased in the presence of HCO3− ions, whereas SO42–and Cl− ions had little effect, and Ca2+ and Mg2+ ions increased adsorption. These findings suggest that Bauxsol™ could be used as an efficient low-cost adsorbent for treating phosphate-contaminated waters.

Selective chemical extraction and grainsize normalisation for environmental assessment of anoxic sediments: validation of an integrated procedure

An integrated selective extraction and size normalisation procedure for use in metal partitioning and diagenetic studies of anoxic sediments and soils is presented. Data obtained by this procedure can readily be combined with other primary data (e.g. sulfur concentrations, carbonate concentrations, cation exchange capacity, etc.) and derived parameters (e.g. degrees of pyritisation and sulfidisation) that enhance interpretation of the behaviour of trace metals in anoxic sediments. Achieving size normalisation during extraction, allows direct comparison of sediments from dissimilar sedimentary environments, and simplifies assessment of the processes that determine whether a sediment is a source of or a sink for trace metals. Aspects of a study of trace metals in sediments from the Brisbane River estuary, Australia, are used to illustrate applications of the integrated procedure.

Carbon Capture and the Aluminium Industry: Preliminary Studies

Environmental Context. Carbon dioxide concentrations in the atmosphere are rising every year by 1.5–3.0 ppm and there is now a general acceptance that increased efforts must be made to reduce industrial sources of this greenhouse gas. Carbonation of red mud wastes produced by aluminium refineries has been carried out to study the capacity of these wastes to capture carbon dioxide. Removal is very rapid, with the added carbon dioxide recorded as a large increase in bicarbonate alkalinity. Although these results can only be considered preliminary, the experiments indicate that these wastes can potentially remove up to 15 million tonnes of carbon dioxide produced in Australia per annum. Furthermore, the carbonated waste can be used in other industrial processes to add further value to these waste materials. Abstract. Carbonation of raw red mud produced by aluminium refineries and a chemically and physically neutralized red mud (Bauxsol™) has been carried out to study the capacity of these wastes to capture carbon dioxide. After only 5 min of carbonation of raw red mud, total alkalinity dropped 85%. Hydroxide alkalinity was almost totally consumed, carbonate alkalinity dropped by 88%, and bicarbonate alkalinity increased to 728 mg L–1. After 24 min carbonation, the bicarbonate alkalinity reached its maximum value of 2377 mg L–1, and hydroxide and carbonate alkalinity were virtually absent. After 30 and 60 min carbonation, bicarbonate alkalinity started to decrease slightly as the pH of the slurry increased. After 5 min carbonation of Bauxsol™, total and bicarbonate alkalinity dropped 89% and 9%, respectively. After 20 min carbonation, bicarbonate alkalinity dropped another 11%, but after 30 min carbonation bicarbonate alkalinity increased 26% to levels found in the original Bauxsol material, and pH was stable. Based on these experiments, a calculation of the amount of carbon dioxide that could be removed annually at aluminium refineries in Australia is potentially 15 million tonnes, and suggests that further studies are necessary to maximize this carbon removal process. Furthermore, carbonation produces a product, which can potentially be used in other industrial and agricultural activities to remove toxic metals and nutrients.

The application of sediment capping agents on phosphorus speciation and mobility in a sub-tropical dunal lake

Experimental sediment cores from Lake Ainsworth, Australia, were exposed to an induced 46-day, anoxic/oxic cycle in the laboratory, mimicking the seasonal thermal stratification cycle commonly observed in the lake’s waters every summer. Under oxic conditions the supply of phosphorus (P) and iron (Fe) to the overlying water was slow, however, induced anoxia led to an enhanced release of P and Fe from the sediments to the water column. An inverse relationship between total P, Fe and redox potential suggests that Lake Ainsworth sediments are redox sensitive. Phosphorus speciation analysis of Lake Ainsworth sediments revealed the presence of a large pool of organic P, reactive Fe-bound P, and CaCO3-bound P, the latter fraction decreasing during anoxic conditions. Two sediment-capping agents, a lanthanum modified bentonite clay and Bauxsol (a waste product from the aluminium smelting industry) were assessed for their ability to reduce the levels of P released from Lake Ainsworth sediments during the 46-day, anoxic/oxic cycle. The bentonite clay was highly effective at reducing plant available P in anoxic/oxic conditions, but levels of dissolved Fe were enhanced with its use. Although the use of Bauxsol to remove plant available P is not recommended in anoxic waters, its use in suspension in oxic waters warrants further study.

An improved analytical procedure for determination of total actual acidity (TAA) in acid sulfate soils

An improved analytical procedure is proposed for the determination of total actual acidity (TAA) in acid sulfate soils. The proposed method involves the use of a superior extracting solution, 0.5 M BaCl2, instead of the 1 M NaCl used by Konsten et al. (Konsten CJM, Brinkman R, Andriesse W. A field laboratory method to determine total potential and actual acidity in acid sulfate soils. In: Dost H, editor. Selected papers of the Dakar Symposium on Acid Sulfate Soils. Wageningen: ILRI Publication 44, 1988:106-134.) and improved experimental design to obtain correction factors for calculating TAA. The introduction of a multi-choice procedure also enables increased accuracy of analytical results to be obtained if more accurate TAA estimation is required.

Effects of Bauxsol on the immobilisation of soluble acid and environmentally significant metals in acid sulfate soils

The effects of Bauxsol, an abundant industrial by-product, on the immobilisation of soluble acid and a range of potentially environmentally toxic metals in artificial and natural acid sulfate soils were investigated. The acid neutralising capacity of Bauxsol increased with decreasing pH, which is probably provided not only by basic metal hydroxides, carbonates, and hydroxycarbonates but also by protonation of variably charged particles (e.g. gibbsite and hematite) present in Bauxsol. Simulation experiment results show that the removal of 9 tested environmentally significant heavy metals can be enhanced by addition of BauxsolTM; an exception was Co. The removal of the added soluble heavy metals by the BauxsolTM-soil mixtures shows a preferential order of Pb > Fe > Cr > Cu > Zn > Ni > Cd > Co > Mn. For the natural acid sulfate soil without added synthesised metal solution, the retention of the investigated environmentally significant metals is in the following decreasing order : Al > Zn > Fe > Co > Mn.