Bryan Bilyeu

A review of chemical, electrochemical and biological methods for aqueous Cr(VI) reduction.

Hexavalent chromium is of particular environmental concern due to its toxicity and mobility and is challenging to remove from industrial wastewater. It is a strong oxidizing agent that is carcinogenic and mutagenic and diffuses quickly through soil and aquatic environments. It does not form insoluble compounds in aqueous solutions, so separation by precipitation is not feasible. While Cr(VI) oxyanions are very mobile and toxic in the environment, Cr(III) cations are not. Like many metal cations, Cr(III) forms insoluble precipitates. Thus, reducing Cr(VI) to Cr(III) simplifies its removal from effluent and also reduces its toxicity and mobility. In this review, we describe the environmental implications of Cr(VI) presence in aqueous solutions, the chemical species that could be present and then we describe the technologies available to efficiently reduce hexavalent chromium.

A combined electrocoagulation-electrooxidation treatment for industrial wastewater.

This study addresses the elimination of persistent organic compounds in industrial wastewater using a synergistic combination of electrocoagulation and electrooxidation. Electrocoagulation is a relatively quick process (30 min), which is very effective in removing colloidal and suspended particles, as seen in changes in coliforms, turbidity, and color and in the general absorbance by UV-vis spectroscopy. However, it is relatively ineffective in eliminating stable persistent organic compounds--in this work, only half of the COD was eliminated from wastewater and an oxidation peak in the cyclic voltammetry scan remained. Electrooxidation is very effective in breaking down organic compounds through oxidation as reflected in the elimination of COD, BOD(5), and oxidative peak in cyclic voltammetry, but requires so much time (21 h) that it has very limited practicality, especially when colloidal and suspended particles are present. Electrooxidative mineralization of electrocoagulated wastewater, in which most of the colloids and charged species have been removed, takes less than 2h. In the coupled technique, electrocoagulation quickly coagulates and removes the colloidal and suspended particles, as well as many charged species, then electrooxidation oxidizes the remaining organics. The coupled process eliminates COD, BOD(5), color, turbidity, and coliforms in a practical amount of time (2h).

Biosorption of Cr(III) and Fe(III) in single and binary systems onto pretreated orange peel.

Trivalent chromium and iron are the products of the traditional reduction of hexavalent chromium by ferrous salts in industrial wastewater. Although there have been a few studies of Cr(III) adsorption, none have considered the effect of Fe(III) on Cr(III) adsorption in a binary system representing expected products of hexavalent chromium in industrial wastewater. The biosorption of Cr(III) and Fe(III) ions onto pretreated ground orange peel in single and binary systems was studied in batch experiments using a variety of techniques. The kinetic results showed a rapid rate of biosorption of Cr(III) and Fe(III) in single and binary systems and mutual interference effects in the competitive binary Cr(III)-Fe(III) system. Second order kinetic models showed the best fit for all systems. The behavior of competitive Cr(III)-Fe(III) biosorption were successfully described by the multicomponent Langmuir model, obtaining maximum capacities for Cr(III) and Fe(III) of 9.43 and 18.19 mg/g respectively. SEM/EDS results confirmed that the metals adsorb on the surface and FTIR identified the hydroxyl groups on the carboxylic acids as the active binding sites.

A comparative study of natural, formaldehyde-treated and copolymer-grafted orange peel for Pb(II) adsorption under batch and continuous mode

Natural, formaldehyde-treated and copolymer-grafted orange peels were evaluated as adsorbents to remove lead ions from aqueous solutions. The optimum pH for lead adsorption was found to be pH 5. The adsorption process was fast, reaching 99% of sorbent capacity in 10 min for the natural and treated biomasses and 20 min for the grafted material. The treated biomass showed the highest sorption rate and capacity in the batch experiments, with the results fitting well to a pseudo-first order rate equation. In the continuous test with the treated biomass, the capacity at complete exhaustion was 46.61 mg g(-1) for an initial concentration of 150 mg L(-1). Scanning electronic microscopy and energy dispersive X-ray spectroscopy indicated that the materials had a rough surface, and that the adsorption of the metal took place on the surface. Fourier transform infrared spectroscopy revealed that the functional groups responsible for metallic biosorption were the -OH, -COOH and -NH(2) groups on the surface. Finally, the thermogravimetric analysis indicates that a mass reduction of 80% can be achieved at 600 degrees C.

Use of pH-sensitive polymer hydrogels in lead removal from aqueous solution.

Three gamma crosslinked polymeric hydrogels were synthesized and evaluated as lead ion sorbents. A crosslinked poly(acrylic acid) hydrogel was compared with two 4-vinylpiridine-grafted poly(acrylic acid) hydrogels (26.74 and 48.1% 4-vinylpiridine). The retention properties for Pb(II) from aqueous solutions of these three polymers were investigated by batch equilibrium procedure. The effects of pH, contact time and Pb(II) concentration were evaluated. The optimal pH range for all polymers was 4-6. The lightly grafted polymer (PAAc-g-4VP at 26.74%) exhibited a Pb(II) removal close to 80% at 5h and above 90% at 24h. The maximum Pb(II) removal was 117.9mg g(-1) of polymer and followed the Freundlich adsorption model. XPS characterization indicates that the carboxyl groups are involved in the Pb(II) removal.

Physicochemical Aspects of Electrocoagulation

Electrocoagulation is an electrochemical technique which is being applied to the removal of a number of water pollutants: metals, organic contaminants, dyes and pigments, colloidal solids and particles, soluble inorganic pollutants, and others. Both the electrochemical reactions that take place at the electrode and the effect on the bulk solution are explained. The influence of the metal cation concentration (iron or aluminum) as well as the pH of the aqueous solution is shown. The advantages and disadvantages of electrocoagulation compared to traditional coagulation methods are reviewed. Some considerations about the mathematical modeling of the process and actual reactor design are described. Some techniques that are derived from electrocoagulation—electro-Fenton and peroxi-coagulatio—are explained. Finally, the efficiency of removal for a number of pollutants from wastewater is presented.

Removal of organic pollutants in industrial wastewater with an integrated system of copper electrocoagulation and electrogenerated H2O2

The effectiveness of organics removal of an integrated electrochemical process, namely, electrocoagulation with copper ions followed by the use of electrogenerated hydrogen peroxide was evaluated with an industrial wastewater. The copper (II) ions addition into the wastewater using electro-dissolution of copper electrodes, reduces the chemical oxygen demand (COD) by 56% after 30 min of treatment, under optimal conditions of pH 2,8 and 14.2 mA cm(-2) of current density. The integrated electrochemical process reduces the COD by 78%, BOD₅ by 81%, color by 97% and fecal coliforms by 99.9%. The wastewater quality was monitored using UV-Vis spectrometry and Z-potential in order to characterize raw and treated wastewater.

Cr(VI) reduction in wastewater using a bimetallic galvanic reactor.

The electrochemical reduction of Cr(VI)-Cr(III) in wastewater by iron and copper-iron bimetallic plates was evaluated and optimized. Iron has been used as a reducing agent, but in this work a copper-iron galvanic system in the form of bimetallic plates is applied to reducing hexavalent chromium. The optimal pH (2) and ratio of copper to iron surface areas (3.5:1) were determined in batch studies, achieving a 100% reduction in about 25 min. The Cr(VI) reduction kinetics for the bimetallic system fit a first order mechanism with a correlation of 0.9935. Thermodynamic analysis shows that the Cr(VI) reduction is possible at any pH value. However, at pH values above 3.0 for iron and 5.5 for chromium insoluble species appear, indicating that the reaction will be hindered. Continuous column studies indicate that the bimetallic copper-iron galvanic system has a reduction capacity of 9.5890 mg Cr(VI) cm(-2) iron, whereas iron alone only has a capacity of 0.1269 mg Cr(VI) cm(-2). The bimetallic copper-iron galvanic system is much more effective in reducing hexavalent chromium than iron alone. The exhausted plates were analyzed by SEM, EDS, and XRD to determine the mechanism and the surface effects, especially surface fouling.

Synthesis and Thermal Cross-Linking Study of Partially-Aminated Epoxidized Linseed Oil

The reaction of diamines or triamines with epoxidized oils leads to different reactions of intra/intermolecular cross-linking and the disruption of the ester linkage. However, by controlling the reaction conditions of temperature, stoichiometry and catalyst, production of oxirane rings without cross-linking and/or disruption of ester group can be achieved. In this work, the synthesis of cross-linking aminated–epoxidized linseed oil resins in two stages is presented. The first one started with epoxidized linseed oil (55.4%) which was subjected to a partial aminolysis using three different amines (ethylenediamine, p-xylylenediamine and triethylenetetramine) and a ZnCl2 catalyst. Products were characterized by FT-IR, 1H-NMR and DSC. In the second stage, the thermal cross-linking reaction (thermal cure) for obtaining cross-linked polymers was studied by calorimetry using isothermal experiments. These results were extrapolated for producing sheet molding thermally cross-linked polymers.

Scratch resistance of different silica filled resins for obturation materials

Abstract The authors have synthesised new silica filled polymeric resins for use as dental obturation materials. In contrast to earlier materials prepared for similar purposes, the present ones contain up to 80% silica nanoparticles providing the materials with improved mechanical properties. Scratch testing results show low values of the penetration depths. The ceramic concentrations used exceed a threshold concentration needed to change the mechanical properties from a soft polymer to a rigid ceramic. Further addition of silica has only insignificant effects on tribology. The penetration depths vary more or less linearly with the load, while viscoelastic recovery has a more complicated dependence on the load. These new materials have scratch resistance values in the range appropriate for applications as obturation materials.