Conversion of Chromium(III) Propionate to Chromate/dichromate(VI) by the Advanced Oxidation Process. Pretreatment of a Biomimetic Complex for Metal Analysis

Water Decontamination from Cr(III)-Organic Complexes Based on Pyrite/H2O2: Performance, Mechanism, and Validation

Fenton reaction is a widely used pretreatment technology to degrade toxic metal-organic complexes. However, its efficiency is greatly compromised for Cr(III)-organic complexes due to accumulation of more toxic Cr(VI) and pH dependence. Herein, we proposed a combined pyrite/H₂O₂-precipitation process to efficiently remove Cr(III) (initially at 10.4 mg Cr/L) complexed by various ligands (citrate, EDTA, oxalate, and tartrate). Negligible Cr(VI) and <0.3 mg/L Cr were detected in the effluent treated by pyrite/H₂O₂-precipitation over a wide pH range of 3-9. In contrast, > 0.5 mg/L Cr(VI) and >5 mg/L Cr remained after treatment by the ZVI/H₂O₂-precipitaion process at pH₀ > 5. As for the mechanisms, pyrite/H₂O₂ produced a considerable amount of aqueous Fe(II) to initiate Fenton reaction, concurrently releasing massive H⁺ to keep the reaction pH at ∼3.0 irrespective of the initial pHs. The generated •OH radicals oxidized Cr(III) into Cr(VI) and thereby releasing the organic ligands for further mineralization. The generated Cr(VI) was in situ reduced back to Cr(III) by aqueous Fe(II) and FeS₂. Subsequently, all the free metal ions including Cr(III), Fe(III), and Fe(II) were removed via precipitation. Kinetic modeling of the pyrite/H₂O₂ process involving 17 reactions was performed to verify the proposed mechanism. Additionally, the effectiveness of the combined process was further validated by its satisfactory performance in treating authentic tannery wastewater.

Efficient removal of Cr(III)-organic complexes from water using UV/Fe(III) system: Negligible Cr(VI) accumulation and mechanism

Most available processes are incapable of removing Cr(III)-organic complexes from water due to their high solubility, extremely slow decomplexation rate, and possible formation of more toxic Cr(VI) during oxidation. Herein, we proposed a new combined process, i.e., UV/Fe(III) followed by alkaline precipitation (namely UV/Fe(III)+OH), to achieve highly efficient and environmentally benign removal of Cr(III)-organic complexes from water. The combined process could remove Cr(III)-citrate from 10.4 mg Cr/L to 0.36 mg Cr/L and ∼60% total organic carbon as well. More attractively, negligible Cr(VI) (<0.06 mg/L) was formed during the process. In the viewpoint of mechanism, the added Fe(III) generates ·OH radicals to transform Cr(III) into Cr(VI) and simultaneously released the citrate ligand to form Fe(III)-citrate simultaneously. Then, the photolysis of Fe(III)-citrate under UV irradiation involved the citrate degradation and the production of massive Fe(II) species, which in turn transformed the formed Cr(VI) back to Cr(III). The free metal ions, including Cr(III), Fe(II) and Fe(III) were removed by the subsequent alkaline precipitation. Also, the combined process is applicable to other Cr(III) complexes with EDTA, tartrate, oxalate, acetate. The applicability of the combined process was further demonstrated by treating two real tanning effluents, resulting in the residual Cr(III) below 1.5 mg/L (the discharge standard of China) and negligible formation of Cr(VI) (<0.004 mg/L) as well. In general, the combined process has a great potential for efficient removal of Cr(III) complexes from contaminated waters.

Novel Pretreatments of Whole Blood Using Fenton-like Processes for Trace Metal Analysis

Whole blood is a complex mixture of biological and chemical species. Its pretreatment, which is often conducted by dry ashing, is needed before the analyses of trace metals in whole blood. Recently photo-Fenton Advanced Oxidation Process (AOP) process has been used in the pretreatment of whole blood. Two new AOP processes using simple heating and microwave irradiation have been developed in the current work to pretreat blood samples. The treatments are based on a Fenton-like AOP with acid deactivation of the enzyme catalase. The first treatment is performed with a lab oven over 5 h, while the second uses microwave irradiation for 6 min. These methods allow for either cost-effective pretreatment through the use of the lab oven, or time savings through the use of the microwave oven. The degradations of blood and pure hemoglobin samples are compared through UV/visible spectroscopy, and the copper concentration in the treated samples were analyzed via anodic stripping voltammetry as a demonstration of analyzing trace metals in the pretreated whole blood.

Adsorption of chromium from aqueous solutions using crosslinked chitosan-diethylenetriaminepentaacetic acid

Chitosan (CH) and its derivatives have been the focus of attention for researchers as potential adsorbents for heavy metal removal. The adsorption potential of chitosan cross-linked with diethylenetriaminepentaacetic acid (CD) for Cr6+ was investigated. CD was characterized by FTIR, XRD, TGA, XPS and ESR techniques. Batch experiments were conducted to optimize the parameters affecting the adsorption of chromium. The optimum pH was found to be 3 and the adsorption process was found to be exothermic. Adsorption isotherms were determined and the maximum adsorption capacity of CD for chromium was found to be 192.3 mg/g which was higher than the adsorption capacity of the adsorbents reported in literature. The thermodynamic parameters, such as Gibbs free energy, changes in enthalpy and changes in entropy change were also evaluated. XPS and ESR studies revealed that Cr6+ adsorbed onto CD was reduced to Cr3+. The efficacy of CD for removal of Cr6+ from chrome plating effluent was demonstrated.

Pretreatment of whole blood using hydrogen peroxide and UV irradiation. Design of the advanced oxidation process

A new process to pretreat blood samples has been developed. This process combines the Advanced Oxidation Process (AOP) treatment (using H(2)O(2) and UV irradiation) with acid deactivation of the enzyme catalase in blood. A four-cell reactor has been designed and built in house. The effect of pH on the AOP process has been investigated. The kinetics of the pretreatment process shows that at high C(H(2)O(2),t=0), the reaction is zeroth order with respect to C(H(2)O(2)) and first order with respect to C(blood). The rate limiting process is photon flux from the UV lamp. Degradation of whole blood has been compared with that of pure hemoglobin samples. The AOP pretreatment of the blood samples has led to the subsequent determination of chromium and zinc concentrations in the samples using electrochemical methods.

Photoredox pathways of Cr(III)-tartrate complexes and their impacting factors

In the present study, exposure of Cr(III)-tar to full light of medium pressure mercury lamps and a xenon lamp was conducted in batch reaction systems at 25°C and different pH values to predict the potential for Cr(III) oxidation. The results indicated that the more intense irradiation and higher pH facilitated Cr(III)-tar oxidation. It appears that a ligand-to-metal charge-transfer occurs for Cr(III)-tar after irradiation, leading to the generation of Cr(II) and tar· or ·OH. The accompanying photochemical decomposition of tar·/or tar, together with O(2), further caused the formation of ·OH through multiple pathways, which ultimately converted Cr(II) to Cr(VI) step by step. H(2)O(2), a direct source of ·OH under irradiation, significantly enhanced photooxidation of Cr(III)-tar, but not obviously of aqueous Cr(III) or Cr(III)-tar in dark, implying that Cr(II) acts as a precursor of Cr(III) oxidization to Cr(VI).

Inorganic sensing using organofunctional sol-gel materials

This Account describes recent work in the development and applications of sol-gel sensors for concentrated strong acids and bases and metal ions. The use of sol-gel films doped with organic indicators for the optical sensing of concentrated strong acids (HCl, 1-10 M) and bases (NaOH, 1-10 M) has been explored, and the development of dual optical sensor approaches for ternary systems (HCl-salt-H 2O and NaOH-alcohol-H 2O) to give acid and salt, as well as base and alcohol, concentrations is discussed. The preparation of transparent, ligand-grafted sol-gel monoliths is also described, and their use in the analysis of both metal cations (Cu (2+)) and metal anions [Cr(VI)] is presented. A new model using both metal ion diffusion and immobilization by the ligands in such monoliths has been developed to give metal concentrations using the optical monolith sensors. In addition to optical sensing, a method utilizing ligand-grafted sol-gel films for analyte preconcentration in the electrochemical determination of Cr(VI) has been explored and is discussed.

Quantitative analysis of trace chromium in blood samples. Combination of the advanced oxidation process with catalytic adsorptive stripping voltammetry

A new method for pretreating blood samples for trace Cr analysis is described. The advanced oxidation process (AOP with H2O2 and 5.5-W UV irradiation for 60 min) is used to remove biological/organic species for subsequent analysis. Prior to the AOP pretreatment, acid (HNO3) is used at pH 3.0 to inhibit the enzyme catalase in the blood samples. Catalytic adsorptive stripping voltammetry at a bismuth film electrode gives a Cr concentration of 6.0 +/- 0.3 ppb in the blood samples. This concentration was confirmed by dry-ashing the blood samples and subsequent analysis by atomic absorption spectroscopy. This current method may be used to monitor chromium, a trace metal in humans, and the efficacy and safety of chromium supplements as adjuvant therapy for diabetes.