Douglas I. Stewart

Shrinkage and desiccation cracking in bentonite–sand landfill liners

Abstract Data are reported on the shrinkage and desiccation cracking exhibited by bentonite-enhanced sand mixtures (BES) upon air-drying. Mixtures containing 10 and 20% bentonite by dry weight, compacted at moisture contents ranging from 8 to 32%, were investigated. Hydraulic conductivity data for BES specimens saturated and tested immediately after compaction, and for similar specimens that had no visible damage after air-drying, are also presented. All the mixtures exhibited volumetric shrinkage upon air-drying with the amount of shrinkage increasing with increasing moisture content during compaction. At any initial moisture content mixtures containing 20% bentonite shrink more than those containing 10% bentonite, but the shrinkage is insensitive to the compactive effort. Compacted beds of BES containing 10 and 20% bentonite exhibit no visible desiccation cracking as the top surface is dried when compacted at 15 and 14% moisture content, respectively, and only minor cracking when compacted at initial moisture contents of 20 and 15%, respectively. For the range of mixtures tested, it appears that cracking only occurs when BES undergoes more than about 4% volumetric shrinkage when air-dried. The saturated hydraulic conductivity of intact BES specimens is unaffected by a drying episode prior to testing.

Effect of humic substances on Cu(II) solubility in kaolin-sand soil

The type and amount of organic matter present in industrially contaminated soils will influence the risk they pose. Previous studies have shown the importance of humic and fulvic acids (FAs) (important components of soil organic matter) in increasing the solubility of toxic metals but were not carried out using toxic metal levels and the pH range typical of industrially contaminated soils. This study investigated the influence of three humic substances (HSs: humates, fulvates and humins) on the solubility of copper(II) ions in kaolinitic soil spiked with Cu at levels representative of industrially contaminated soil. Humates, fulvates and humin were extracted from Irish moss peat, and controlled pH batch leaching tests were conducted on an artificial kaolin-sand soil that was spiked with each. Further leaching tests were conducted on soil spiked with each HS and copper nitrate. Dissolved organic contents were determined by titration and total and free aqueous copper concentrations in the leachate were measured using AAS and ion selective electrode (ISE) potentiometry respectively (dissolved complexed copper levels were determined by difference). It was found that humates and fulvates are partially sorbed by the soil, probably by chemisorption on positively charged gibbsite (Al-hydroxide) sites in the kaolinite. The addition of 340 mg/kg Cu(II) ions did not significantly affect the amount of humate or fulvate sorbed. Dissolved humates and fulvates form soluble complexes with copper over the pH range 3-11. However, in the presence of kaolinite, soluble copper humates and fulvates are unable to compete with the kaolinite for Cu ions at pH 6-7. Above pH 8, humate and fulvate complexes are the only forms of dissolved Cu. Humin is largely insoluble and has little effect on Cu mobility between pH 2 and 12. The implication of this study is that measurement of total soil organic content and water leaching tests should be a standard part of contaminated site investigation.

Behavior of Aluminum, Arsenic, and Vanadium during the Neutralization of Red Mud Leachate by HCl, Gypsum, or Seawater

Red mud leachate (pH 13) collected from Ajka, Hungary is neutralized to < pH 10 by HCl, gypsum, or seawater addition. During acid neutralization >99% Al is removed from solution during the formation of an amorphous boehmite-like precipitate and dawsonite. Minor amounts of As (24%) are also removed from solution via surface adsorption of As onto the Al oxyhydroxides. Gypsum addition to red mud leachate results in the precipitation of calcite, both in experiments and in field samples recovered from rivers treated with gypsum after the October 2010 red mud spill. Calcite precipitation results in 86% Al and 81% As removal from solution, and both are nonexchangeable with 0.1 mol L(-1) phosphate solution. Contrary to As associated with neoformed Al oxyhydroxides, EXAFS analysis of the calcite precipitates revealed only isolated arsenate tetrahedra with no evidence for surface adsorption or incorporation into the calcite structure, possibly as a result of very rapid As scavenging by the calcite precipitate. Seawater neutralization also resulted in carbonate precipitation, with >99% Al and 74% As removed from solution during the formation of a poorly ordered hydrotalcite phase and via surface adsorption to the neoformed precipitates, respectively. Half the bound As could be remobilized by phosphate addition, indicating that As was weakly bound, possibly in the hydrotalcite interlayer. Only 5-16% V was removed from solution during neutralization, demonstrating a lack of interaction with any of the neoformed precipitates. High V concentrations are therefore likely to be an intractable problem during the treatment of red mud leachates.

Stimulation of Microbially Mediated Chromate Reduction in Alkaline Soil-Water Systems

Acetate was added to two closed soil-water systems that are representative of the subsurface environment close to chromium ore processing residue disposal sites; one had a pH of 7.7, the other 9.3. Cr(VI) reduction occurred in both systems as part of a cascade of microbially mediated terminal electron-accepting processes, occurring between nitrate and iron reduction. Cr(VI) and subsequently iron reduction took longer to start and were slower in the more alkaline system. At the point when Cr(VI) reduction was essentially complete, the microbial populations in both systems showed an increase in species closely related to β-proteobacteria that are capable of nitrate reduction.

Chromate reduction in Fe(II)-containing soil affected by hyperalkaline leachate from chromite ore processing residue

Highly alkaline (pH 12.2) chromate contaminated leachate (990 μmol L(-1)) has been entering soils below a chromite ore processing residue disposal (COPR) site for over 100 years. The soil immediately beneath the waste has a pH of 11→12.5, contains 0.3→0.5% (w/w) chromium, and 45→75% of the microbially available iron is Fe(II). Despite elevated pH, a viable microbial consortium of Firmicutes dominated iron reducers was isolated from this COPR affected soil. Soil pH and Cr concentration decrease with distance from the waste. XAS analysis of soil samples indicated that Cr is present as a mixed Cr(III)-Fe(III) oxy-hydroxide phase, suggesting that the elevated soil Cr content is due to reductive precipitation of Cr(VI) by Fe(II). Microcosm results demonstrate the capacity of COPR affected soil to abiotically remove all Cr(VI) from the leachate within 40 days. In air oxidation experiments less than 2% of the total Cr in the soil was remobilised despite significant Fe(II) oxidation. XAS analysis after air oxidation showed no change in Cr-speciation, indicating the Cr(III)-containing phase is a stable long term host for Cr. This work suggests that reductive precipitation of Cr(VI) is an effective method of contaminant immobilisation in soils where microbially produced Fe(II) is present.

Extracellular Electron Transport-Mediated Fe(III) Reduction by a Community of Alkaliphilic Bacteria That Use Flavins as Electron Shuttles

ABSTRACT The biochemical and molecular mechanisms used by alkaliphilic bacterial communities to reduce metals in the environment are currently unknown. We demonstrate that an alkaliphilic (pH > 9) consortium dominated by Tissierella, Clostridium, and Alkaliphilus spp. is capable of using iron (Fe3+) as a final electron acceptor under anaerobic conditions. Iron reduction is associated with the production of a freely diffusible species that, upon rudimentary purification and subsequent spectroscopic, high-performance liquid chromatography, and electrochemical analysis, has been identified as a flavin species displaying properties indistinguishable from those of riboflavin. Due to the link between iron reduction and the onset of flavin production, it is likely that riboflavin has an import role in extracellular metal reduction by this alkaliphilic community.

Leaching behaviour of a chromium smelter waste heap.

This paper reports the results of geochemical sampling and modelling of leachates from a chromite ore processing residue (C.O.P.R.) pile under rainwater infiltration. The waste pile is located in the north of England and consists of 800,000 m3 of waste. The pH of fresh leachate is similar to that of a solution in equilibrium with portlandite Ca(OH)2, which is a major constituent of the waste. The in-gassing of CO2(g) causes the pH of the leachates to drop along the drainage ditch and calcite precipitation to occur. The extent of in-gassing is dependent upon the flow rate within the drainage ditch. The dissolution of solid solutions containing residual chromate is likely to control chromate concentrations within the leachate.

Biogeochemical reduction processes in a hyper-alkaline leachate affected soil profile

Hyperalkaline surface environments can occur naturally or because of contamination by hydroxide-rich wastes. The high pH produced in these areas has the potential to lead to highly specialized microbial communities and unusual biogeochemical processes. This article reports an investigation into the geochemical processes that are occurring in a buried, saturated, organic-rich soil layer at pH 12.3. The soil has been trapped beneath calcite precipitate (tufa) that is accumulating where highly alkaline leachate from a lime kiln waste tip is emerging to atmosphere. A population of anaerobic alkaliphilic bacteria dominated by a single, unidentified species within the Comamonadaceae family of β-proteobacteria has established itself near the top of the soil layer. This bacterial population appears to be capable of nitrate reduction using electron donors derived from the soil organic matter. Below the zone of nitrate reduction a significant proportion of the 0.5N HCl extractable iron (a proxy for microbial available iron) is in the Fe(II) oxidation state, indicating there is increasing anoxia with depth and suggesting that microbial iron reduction is occurring. Supplemental materials are available for this article. Go to the publishers online edition of Geomicrobiology Journal to view the free supplemental files.

Resistivity imaging of soil during electrokinetic transport

Abstract Electrical resistance imaging of soil specimens during electrokinetic treatment is reported. Column experiments were carried out on Speswhite kaolinite contaminated with lead nitrate to levels both above and below its cation exchange capacity (CEC). Post test chemical analyses of the specimens and their pore fluids show that resistivity variations correlate with changes in pore fluid chemistry but do not show the extent of decontamination. Regions of high resistivity correspond with precipitation zones within the specimens whereas regions of low resistivity correspond with regions of high pore fluid ionic strength. Where the contamination level is below the CEC, decontamination is slow as lead ions are mostly sorbed to the clay so most of the current is carried by electrolysis products and clay dissolution products. A broad resistive zone forms over the cathode half where hydroxyl and HCO − 3 ions formed in the cathode reservoir precipitate clay with dissolution products and other ions. Where the contamination level is above the CEC, lead ions are initially major charge carriers and decontamination over the bulk of the specimen is rapid. However, lead still precipitates immediately adjacent to the cathode reservoir to form a narrow resistive region.

Effect of Microbially Induced Anoxia on Cr(VI) Mobility at a Site Contaminated with Hyperalkaline Residue from Chromite Ore Processing

This paper reports an investigation of microbially mediated Cr(VI) reduction in a hyperalkaline, chromium-contaminated soil-water system representative of the conditions at a chromite ore processing residue (COPR) site. Soil from the former surface layer that has been buried beneath a COPR tip for over 100 years was shown to have an active microbial population despite a pH value of 10.5. This microbial population was able to reduce nitrate using an electron donor(s) that was probably derived from the soil organic matter. With the addition of acetate, nitrate reduction was followed in turn by removal of aqueous Cr(VI) from solution, and then iron reduction. Removal of ∼300 μM aqueous Cr(VI) from solution was microbially mediated, probably by reductive precipitation, and occured over a few months. Thus, in soil that has had time to acclimatize to the prevailing pH value and Cr(VI) concentration, microbially mediated Cr(VI) reduction can be stimulated at a pH of 10.5 on a time scale compatible with engineering intervention at COPR-contaminated sites.