Permeable Reactive Materials Research Articles

Assessment of zeolite and compost-zeolite mixture as permeable reactive materials for the removal of lead from a model acidic groundwater

Column experiments were performed to assess the effectiveness of zeolite and compost-zeolite mixture in removing dissolved lead (Pb2+) from acidic water of pH 2.4. The acid neutralizing ability and hydraulic performance of the materials were also studied. Fitting the advection-dispersion equation (ADE) and mathematical models (i.e. the Dose-Response, Adams-Bohart, and Yoon-Nelson models) to the Pb2+ experimental breakthrough curves (BTCs) was also performed. The compost-zeolite mixture proved to be better than zeolite alone both: in removing Pb2+ and in buffering the acidic pH. The maximum adsorption capacity, qo obtained for zeolite was 0.097 mg/g and 0.151 mg/g for the compost-zeolite mixture, respectively. Lead removal was attributed to ion exchange and adsorption. Observed Pb2+ BTCs demonstrated sorption-related nonequilibrium effects in the columns. The hydraulic conductivity of zeolite decreased by 2% and by 28.8% in the case of compost-zeolite mixture at the end of the experiment. The entire experimental BTC of Pb2+ was well described by the Dose-Response model while the Adams-Bohart model was better in describing only the initial part of the lead BTCs.

Uranium Removal from Groundwater by Permeable Reactive Barrier with Zero-Valent Iron and Organic Carbon Mixtures: Laboratory and Field Studies

Zhovty Vody city, located in south-central Ukraine, has long been an important center for the Ukrainian uranium and iron industries. Uranium and iron mining and processing activities during the Cold War resulted in poorly managed sources of radionuclides and heavy metals. Widespread groundwater and surface water contamination has occurred, which creates a significant risk to drinking water supplies. Hydrogeologic and geochemical conditions near large uranium mine tailings storage facility (TSF) were characterized to provide data to locate, design and install a permeable reactive barrier (PRB) to treat groundwater contaminated by leachate infiltrating from the TSF. The effectiveness of three different permeable reactive materials was investigated: zero-valent iron (ZVI) for reduction, sorption, and precipitation of redox-sensitive oxyanions; phosphate material to transform dissolved metals to less soluble phases; and organic carbon substrates to promote bioremediation processes. Batch and column experiments with Zhovty Vody site groundwater were conducted to evaluate reactivity of the materials. Reaction rates, residence time and comparison with site-specific clean-up standards were determined. Results of the study demonstrate the effectiveness of the use of the PRB for ground water protection near uranium mine TSF. The greatest decrease was obtained using ZVI-based reactive media and the combined media of ZVI/phosphate/organic carbon combinations.

The toxicity of cationic surfactant HDTMA-Br, desorbed from surfactant modified zeolite, towards faecal indicator and environmental microorganisms

Surfactant Modified Zeolite (SMZ) represents a versatile, cost-effective permeable reactive material, capable of treating multiple classes of contaminants. The potential for HDTMA-Br, a cationic surfactant commonly used to modify zeolite, to desorb from the zeolite surface has been identified as a potential issue for the ongoing use of SMZ in water remediation contexts. This paper investigates the toxicity of HDTMA-Br towards enteric virus surrogates, F-RNA bacteriophage MS2 and E. coli, Bacillus subtilis, and soil microflora. The concentration of surfactant desorbing from SMZ was quantified through a bioassay using E. coli.Results showed HDTMA-Br concentrations of ≥10−5M were toxic to MS2, ≥10−4M were toxic to E. coli and ≥10−6M were toxic to B. subtilis. No toxic relationship was established between HDTMA-Br and soil microflora. Desorption of ≥10−4M of HDTMA-Br was shown for the two SMZ samples under the mixing conditions used. Effects of this surfactant on total soil microflora were ambiguous since no toxic relationship could be established, however, HDTMA-Br, at concentrations desorbing from SMZ, were shown to impact the soil bacterium B. subtilis. Further research is required to determine the effect of this surfactant on microbial populations and species diversity in soils.

Effects of Permeable Reactive Composite Electrodes on Hexavalent Chromium in the Electrokinetic Remediation of Contaminated Soil

Electrokinetic transport processes have been shown to have potential for effective removal of heavy metals from soils. However, pH changes near the anode and cathode limit their widespread application in remediation of contaminated soils. Permeable reactive composite electrodes (PRCEs) were constructed by attaching reactive materials such as Fe(0) and zeolite to electrodes, and effects of composite electrodes on pH control, chromium (Cr) removal efficiency, and adjustment of speciation of Cr were studied in the electrokinetic remediation of Cr(VI)-spiked soil. Composite electrodes consisting of permeable reactive materials gave better pH control and Cr removal efficiency compared to application of traditional electrodes, and a reactive layer of Fe(0) and zeolite in the anode exhibited the best performance compared to zeolite or Fe(0) alone. After 5 d of electrokinetic remediation with a DC voltage of 2.0 V/cm, the Fe(0)+zeolite reactive layer buffered pH fluctuation, maintained soil pH in the range of 5.5–8.5, and increased Cr(VI) remediation efficiency up to 97% in each section of soil. Additionally, it resulted in lower Cr(III) residues, enhancement of the amount of Cr retention up to 8× and 1.8×, respectively, and transformation of 98% of the Cr(VI) to lower toxicity Cr(III). After remediation, heavy metal ions were removed from the remediation system simply by unplugging the composite electrodes, which theoretically needed no further treatment—a major advantage over traditional electrokinetic remediation methods. This study provides a theoretical basis for exploitation of PRCEs, which is a practical option for future applications.

Spectroscopic study of precipitates formed during removal of selenium from mine drainage spiked with selenate using permeable reactive materials

The potential for Se removal from mine drainage water using permeable reactive materials was evaluated by a laboratory column experiment. The column materials, organic carbon and zero valence iron (ZVI), were exposed to mine drainage containing 630 mg L-1 SO42-. The influent water was spiked with 40 mg L-1 Se(VI) to assess the potential for Se removal. This high Se(VI) concentration was selected to ensure that there would be a sufficient mass of Se-bearing reaction products available for mineralogical characterization. The experiment was conducted in an anaerobic chamber to replicate the anaerobic conditions that prevail in permeable reactive barrier systems. After loading 10.8 pore volumes of input solution, the column effluent contained <0.002 mg L-1 Se and <300 mg L-1 SO42-. After the column experiments was complete the reactive materials were sampled in the anaerobic chamber and examined using scanning electron microscopy (SEM) coupled with energy dispersion X-ray analysis (SEM-EDAX), X-ray photoelectron spectroscopy (XPS) and by Raman spectroscopy. Sulfate was reduced to sulfide and elemental sulfur, which accumulated on the surfaces of the column materials. Se-bearing precipitates were observed at the base of column. Scanning electron microscopy (SEM) showed the presence of elemental Se, suggesting that Se(VI) was partly reduced to metallic Se(0). The XPS results revealed that selenate was reduced mainly to iron selenide (FeSe and/or FeSe2) on the surface of the column substances. These observations suggest that both chemical reduction and biologically mediated reduction of Se(VI) occurred.

Identification of Sulfate- and Arsenate-Reducing Bacteria in Sheep Manure as Permeable Reactive Materials after Arsenic Immobilization in Groundwater

Permeable reactive barriers (PRBs) column tests were performed to investigate the contribution of anaerobic microbial community in sheep manure on the removal of As from groundwater. The column was packed with zero valent iron (ZVI), sheep manure, compost, wood chips, glass beads and gravels. All materials were sterilized except for sheep manure that contains anaerobic bacteria. Decrease in sulfate concentration was observed at the maximum rate of 0.26 mmol dm−3 day−1. In addition, the sulfur isotopic ratio of δ34S increased from the influent (−4.3 extperthousand) to the effluent (0.2 extperthousand), suggesting that there was sulfate-reducing activity in the microbial community. Arsenate was more effectively immobilized on ZVI than arsenite. Bacterial community analysis using polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) on 16S rRNA sequences suggested that majorities of bacteria were several Clostridium species and one species of Proteobacteria, Geobacter metallireducens GS-15, independently of PRBs column heights. Some of them might have contributed to the removal of arsenic.

Arsenic removal using steel manufacturing byproducts as permeable reactive materials in mine tailing containment systems

Steel manufacturing byproducts were tested as a means of treating mine tailing leachate with a high As concentration. Byproduct materials can be placed in situ as permeable reactive barriers to control the subsurface release of leachate from tailing containment systems. The tested materials had various compositions of elemental Fe, Fe oxides, Ca–Fe oxides and Ca hydroxides typical of different steel manufacturing processes. Among these materials, evaporation cooler dust (ECD), oxygen gas sludge (OGS), basic oxygen furnace slag (BOFS) and to a lesser degree, electrostatic precipitator dust (EPD) effectively removed both As(V) and As(III) during batch experiments. ECD, OGS and BOFS reduced As concentrations to <0.5 mg/l from 25 mg/l As(V) or As(III) solution in 72 h, exhibiting higher removal capacities than zero-valent iron. High Ca concentrations and alkaline conditions (pH ca. 12) provided by the dissolution of Ca hydroxides may promote the formation of stable, sparingly soluble Ca–As compounds. When initial pH conditions were adjusted to 4, As reduction was enhanced, probably by adsorption onto iron oxides. The elution rate of retained As from OGS and ECD decreased with treatment time, and increasing the residence time in a permeable barrier strategy would be beneficial for the immobilization of As. When applied to real tailing leachate, ECD was found to be the most efficient barrier material to increase pH and to remove As and dissolved metals.