Fixed-bed flow experiments with supported green nZVI for the remediation of contaminated waters: Effect of pH and background solution composition.

Aim of this study is to evaluate the performance of a nanocomposite material (R-nFe), consisting of nano zero valent iron (nZVI) and a cation exchange resin, for Cr(VI) reduction and heavy metals removal from contaminated water streams. The effect of pH and the background solution composition was investigated by conducting three columns tests. In Column I, the experiments were carried out using NaCl as background electrolyte while in Column II and III the background solution consisted of 50% tertiary effluents of a waste water treatment plant and 50% deionized water. The kinetics of Cr(VI) removal was found to be much faster at acidic pHs. Based on the experimental results it was calculated that the required contact time of the permeating solution with the RnFe beads for Cr(VI) removal at pH 7 is 6.5 times longer than at pH 4. The results also revealed the role of competing ions on the performance of RnFe. During the operation of the column with an inert electrolyte, such as NaCl, the RnFe bed removed an amount of Cr(VI) equivalent to 4200 mg/kg. When the column was fed with a water stream containing relatively high levels of nitrates, the RnFe removed efficiently only 190 mg/kg Cr(VI), due to the competitive consumption of nanoiron for the reduction of nitrates. The nanocomposite was found to be very efficient for the simultaneous removal of tested heavy metals, i.e. Cu, Zn, Ni, Cd and Pb, which were selectively retained in the cation exchange sites of the supporting resin matrix.

Hexavalent chromium reduction by gallic acid.

Cr(VI) is widely used in industry and often large quantities have been accidently or due to lack of precautions released at the environment. The presence of high concentrations of Cr(VI) in waste streams causes concern due to its toxicity and solubility. However the Cr(III) is a useful micronutrient in human diet. The reduction of Cr(VI) to Cr(III) is a promising technology for a clean environment. Polyphenols are commonly found in plants and they are considered as strong natural antioxidants. In this study the reduction of hexavalent to trivalent chromium using gallic acid (GA), a polyhydroxyphenolic compound, was evaluated. The effect of pH, temperature, and Cr(VI) concentration on the kinetics of Cr(VI) reduction by GA were examined by conducting batch tests. It was found that reduction of Cr(VI) by GA is a rapid reaction under mild acidic conditions. Reduction was found to follow a 2nd order kinetics with respect to Cr(VI) concentration in the pH range 3-5. The results of the present study indicated that the treatment of contaminated waters with initial concentration of Cr(VI) 100 µΜ by gallic acid, below the environmental limit of 0.96 µM, required 20 min at pH 3 and 40 min at pH 4. At higher pH a different reduction mechanism seems to prevail and the required time for Cr(VI) reduction was approximately 10 h at pH 5 and 42 h at pH 6. The increase of temperature25 °C to 35 °C, at pH 6, was slightly accelerated the reaction.

Chromium Removal Using Soil Loaded with Green Iron Nanoparticles.

Chromate is considered as a serious environmental problem due its toxicity. Iron nanoparticles produced by green tea polyphenols (GT-nZVI) is a powerful reductant, which can effectively reduce Cr(VI) to Cr(III). Nano ZVI suspension was initially conceived ideal for direct injection in the contaminated aquifers. However GT-nZVI presents limited mobility in calcareous aquifers. For this reason the incorporation of nanoiron in a permeable reactive barrier was investigated as an alternative mode of GT-nZVI application. Namely an amount of soil was loaded with nZVI (0.40 mmol/g of soil) and was evaluated for Cr(VI) removal by conducting batch and column tests. Batch tests were carried out by mixing soil samples, loaded with different levels of nZVI from 0.04 to 0.40 mmol per gram, with contaminated groundwater (GW) containing 1300 ppb Cr(VI). Cr(VI) concentration dropped below detection limit within 1 day using the highest nZVI dose. Soil pre-loaded with nZVI (S-nZVI) presented also high efficiency for chromates remediation, when tested under flow conditions by conducting column tests.

Chromium Removal with Environmentally Friendly Iron Nanoparticles in a Pilot Scale Study.

Nano zero valent iron (nZVI) is highly reactive, targets a wide range of contaminants and is considered as a promising material for the in situ remediation of contaminated aquifers. Injection of nZVI suspension is a remedial option for the in-situ reduction and immobilization of pollutants in aquifers. The goal of the present study was to evaluate the efficiency of iron nanoparticles (GT-nZVI), synthesised by a green synthesis route, for Cr(VI) removal in a pilot scale application. A tank with dimensions 2.5 m (width) × 3.75 m (length) × 1.5 m (height) was constructed and filled with 24 tons of soil. Firstly 3 m3 GT-nZVI suspension was injected and then the supply of contaminated water containing 5 mg/L Cr(VI) was initiated. Samples were analysed for pH, redox potential and electric conductivity, as well as Fetot and Cr(VI). The reactive zone operated efficiently for the treatment of contaminated water for 1 year.

Green Synthesis of Resin Supported Nanoiron and Evaluation of Efficiency for the Remediation of Cr(VI) Contaminated Groundwater by Batch Tests.

In this study the fixation of nano zero valent iron (nZVI) on a polymeric matrix applying an environmentally friendly technique and the performance of this material for the treatment of Cr(VI) contaminated groundwater were evaluated. The investigated method could be applied in a pump & treat groundwater remediation system. A macroreticular cationic resin was selected as porous host material. It was found that green tea polyphenols were able to penetrate within the macroporous resin matrix and obtain the reduction of Fe(III) to the elemental state. The effectiveness of this composite material in removing Cr(VI) contaminated waters was evaluated by conducting batch tests. It was found that the reduction of Cr(VI) follows a kinetics law of first order with respect to the concentration of Cr(VI) and to the amount of nZVI per liter of solution. The kinetic constant varied between 5·10-3 and 0.5·10-3 per min and per mM of nZVI in the pH range 3.5-7.5.

Incorporation of zero valent iron nanoparticles in the matrix of cationic resin beads for the remediation of Cr(VI) contaminated waters.

The objective of present study was to obtain the fixation of nano zero valent iron (nZVI) particles on a permeable matrix and evaluate the performance of this composite material for the removal of Cr(VI) from contaminated waters. The experiments were carried out using the cationic resin Dowex 50WX2 as porous support of the iron nanoparticles. The work was carried out in two phases. The first phase involved the fixation of nZVI on the resin matrix. The resin granules were initially mixed with a FeCl3 solution to obtain the adsorption of Fe(III). Then the Fe(III) loaded resin (RFe) was treated with polyphenol solutions to obtain the reduction of Fe(III) to the elemental state. Two polyphenol solutions were tested as reductants, i.e. green tea extract and gallic acid. Green tea was found to be inefficient, probably due to the relatively big size of the contained polyphenol molecules, but gallic acid molecules were able to reach adsorbed Fe(III) and reduce the cations to the elemental state. The second phase was focused on the investigation of Cr(VI) reduction kinetics using the nanoiron loaded resins (R-nFe). It was found that the reduction follows a kinetic law of first order with respect to Cr(VI) and to the embedded nanoiron. Compared to other similar products, this composite material was found to have comparable performance regarding reaction rates and higher degree of iron utilization. Namely the rate constant for the reduction of Cr(VI), in the presence of 1 mM nZVI, was equivalent to 1.4 h of half-life time at pH 3.2 and increased to 24 h at pH 8.5. The degree of iron utilization was as high as 0.8 mol of reduced Cr(VI) per mole of iron. It was also found that this composite material can be easily regenerated and reused for Cr(VI) reduction without significant loss of efficiency.