Chapter 12
Constructed Wetland For Waste Water Treatment
REMEDIATION OF METAL-CONTAMINATED GROUNDWATER USING APATITE
For several years MSE Technology Applications, Inc. has studied the material ApatiteII™ (Patent Number 6,217.775; PIMS NW, Inc.; Drs James Conca and Judith Wright, developers) for the removal of metals, metalloids and radionuclides. Apatite II™ is a form of apatite that appears to have higher reactivity than natural apatite and is lower cost than commercially available bone char. ApatiteII™ is very effective in removing uranium. Several mechanisms for uranium removal by apatite are possible: uranium exchange for the calcium anions in the apatite matrix, non-specific adsorption to the apatite material, or the dissolution of apatite will create condi-tions at the boundary layer that exceed the Ksp for autunite (Ca(UO2)2(PO4)2 _ 10H2O) and this U-bearing crystalline phase will form. Laboratory batch studies observed the presence of meta-autunite (Ca(UO2)2(PO4)2 _ 6H2O) within the apatite material, suggesting the formation of autunite. Meta-autunite formation was due to the drying step before analysis by X-ray Powder Diffraction.Additionally, investigated the removal of soluble cadmium from syn-thetic and authentic contaminated groundwater. Cadmium removal by apatite is probably due to one or all of three mechanisms: ion exchange, nonspecific adsorption, and pre-cipitation of cadmium phosphate minerals. Previous studies have observed a cadmium phosphate mineral, otavite. However, the removal of cadmium only occurs at about 5 wt.% of the apatite added, as opposed to 10 to 20 wt.% in the case of uranium and lead. initial studies with this material showed that uranium was rapidly removed from solution in batch shake flasks. Bacterial growth was associated with the incubation of this material at room temperature however, sterilized controls vs. unsterilized flasks did not show significant difference in the rate of uranium uptake by the apatite. Subse-quently, batch studies were undertaken to determine whether surface area had an effect on uranium removal. Previous studies had used a very fine (surface area = 2.75 m 2 /g) powdered apatite that would not be as easily implemented in a canister or barrier situationin the field. Various crushed sizes were tested and it was shown that for the range tested (0.425 to 4.75 mm) there was no significant difference in the rate of uranium uptake.The object of these studies was to follow up the batch studies with column studies to determine if the apatite material could be used in permeable reactive barrier systems or canister type configurations for removal of uranium and cadmium from contaminated groundwater
Most recently we have begun studies to determine if apatite can be added in a soil-mixing scenario to remove metals from contaminated soils. Batch and column studies have been performed by MSE to determine the efficacy of metal cation removal from aqueous solutions by an organic apatite material. results have shown that the Apatite II™ media rapidly removes uranium (C0=5mg/L) from solution to below detection levels, and moderately removes cadmium.
The mechanism of uranium removal appears to be by reprecipitation as the uranium phosphate autunite. Cadmium removal appears to be via several mechanisms including adsorption and phosphate mineral precipitation. Currently, a soil-mixing study is being carried out to test immobilization of metal cations in contaminated soils.
HEAVY METAL REMOVAL BY ZERO-VALENT IRON
Zero-valent iron Fe(0) has been investigated as a potential remediation agent for the removal of a whole series of heavy metals. Possible removal mechanisms, depending on the metal of interest, are reduction, cementation, surface complexation and (co)precipitation The basis for any of these reactions is the corrosion of iron. The first corrosion product is amorphous ferrous hydroxide, which is predicted thermodynamically to convert to magnetite (Fe3O4) . Mixed valent iron salts, known as green rusts may also form. Their subsequent oxidation can lead to the formation of magnetite, maghemite, goethite and lepidocrocite . As a result of these reactions, the iron surface is coated by a layer of iron oxides and oxyhydroxides, similar to natural oxide solid phases, where heavy metals may interact.The chemistry of metal ions in natural waters strongly depends on the formation of complexes. Complexes reduce the concentration of the free metal species in solution, and affect the solubility, the toxicity, and the mobility of the metal . Polluted groundwaters, especially in the case of landfill leachate contaminated groundwater, contain elevated concentrations of inorganic and organic ligands. Three possible scenarios in aqueous systems containing both humic acids and heavy metals, in contact with hematite. Firstly, the binding of humic acids to the oxide surface may block the binding sites and compete with metal removal. Alternatively, complexes between metals and humic acids may remain in solution and prevent the metal from adsorbing to the oxide. Thirdly, metal adsorption can be enhanced by the formation of ternary humic acids-metal-surface complexes. The enhanced metal binding can result from the adsorption of metal-humic acid complexes, or through metal complexation by the sorbed humic acids. The overall effect from addition of humic acids on metal binding depends on the stability of the numerous interactions involved.
The results of this study indicate that zero-valent iron is a promising reactive agent for the in-situ removal of mixed heavy metals (i.e. Ni, Zn and Cr(VI)) from contaminated groundwater. Humic substances bind heavy metals and influence their fate and mobility. We found that humic acids formed metal-humate complexes remaining in solution, which prevented the removal of Zn and Ni in batch kinetic experiments.Chromate removal was not affected. Sorption of humic acids to the iron surface hindered the metal removal reactions in a longer-term column test, especially for Ni, probably through blocking of reactive sites.
Humic acids impact the efficiency and lifetime of a reactive iron barrier for in-situ removal of heavy metals.
The presence of dissolved organic matter in groundwater should therefore be considered in the design of a permeable barrier.A laboratory study was set-up to investigate the effects of Aldrich humic acids on the removal of zinc, nickel and chromate in zero-valent iron systems. The rate and the extent of zinc and nickel removal were negatively impacted by the presence of the humic acids in batch kinetic experiments. Chromate removal was not affected. It was hypothesized that the formation of humic acid–heavy metal complexes prevented the removal reactions at the iron surface. Two parallel column systems were set-up with the mixed heavy metals (5 mg/l each). The first system acted as reference without humic acids, while the second system was fed with humic acids (20 mg/l). Both column systems efficiently (> 90%) removed the heavy metal mixture during the first 27 weeks. When the input heavy metal concentration was increased to 8-10 mg/l, a significant breakthrough of nickel and zinc occurred in the column system with humic acids. Conversely, chromate and humic acids did not significantly break through. The accumulated humic acids on theiron surface (approx. 5 mg/g) probably hindered the nickel and zinc removal. After 60 weeks, the effect of humic acids on leaching of the accumulated metals (approx. 2 mg/g) was investigated. No significant leaching was observed. The results of present study indicate that the impact of dissolved organic matter on the efficiency and lifetime of a reactive iron barrier for in-situ removal of heavy metals should be considered in the design of the barrier.