Yeqing Lan

Fe(III) photocatalytic reduction of Cr(VI) by low-molecular-weight organic acids with α-OH

The photochemical reduction of Cr(VI) by four low-molecular-weight organic acids (tartaric acid, citric acid, malic acid, and n-butyric acid) in the presence of either dissolved Fe(III) in dilute aqueous solution or adsorbed Fe(III) on clay mineral surfaces (kaolinite, montmorillonite and illite) was investigated using batch reactors at a pH range from 3.5 to 4.5 at 25 degrees C. The results indicate that Fe(III) photocatalytic reduction of Cr(VI) by organic acids with alpha-OH is extremely fast. During a reaction period when less than 80% initial Cr(VI) was consumed, the reaction can be described as pseudo-first-order with respect to Cr(VI) when organic acid in excess. By plotting ln[Cr(VI)] as a function of reaction time, rate constants of Cr(VI) reduction by organic acids are obtained. The rate constants involving the four acids are in the order: tartaric acid (with 2 carboxylic groups and 2 alpha-OH groups)>citric acid (with 3 carboxylic groups and 1 alpha-OH group) approximately malic acid (with 2 carboxylic groups and 1 alpha-OH group)>>n-butyric acid (with 1 carboxylic group and no alpha-OH group). This order suggests that the number of alpha-OH but not the number of carboxylic groups is an important determinant of kinetics. With light, the reduction of Cr(VI) by citric acid is accelerated by clay minerals. The enhancement of Cr(VI) reduction is attributed to the catalysis of Fe(III) adsorbed on clay mineral surfaces. However, such an acceleration is markedly suppressed by introducing NaF into the reaction system since NaF forms a complex with Fe(III). It is concluded that the complex formation between Fe(III) and organic acid is a key step for the photocatalytic reduction of Cr(VI) in the presence of Fe(III) and organic acids with alpha-OH.

Influence of complex reagents on removal of chromium(VI) by zero-valent iron

The removal of Cr(VI) by zero-valent iron (Fe(0)) and the effect of three complex reagents, ethylenediaminetetraacetic acid (EDTA), NaF and 1,10-phenanthroline, on this reaction were investigated using batch reactors at pH values of 4, 5 and 6. The results indicate that the removal of Cr(VI) by Fe(0) is slow at pH 5.0 and that three complex reagents play different roles in the reaction. EDTA and NaF significantly enhance the reaction rate. The zero-order rate constants at pH 5.0 were 5.44 microM min(-1) in the presence of 4mM EDTA and 0.99 micrM min(-1) in the presence of 8 mM NaF, respectively, whereas that of control was only 0.33 micrM min(-1), even at pH=4.0. This enhancement is attributed to the formation of complex compounds between EDTA/NaF and reaction products, such as Cr(III) and Fe(III), which eliminate the precipitates of Cr(III), Fe(III) hydroxides and Cr(x)Fe(1-)(x)(OH)(3) and thus reduce surface passivation of Fe(0). In contrast, 1,10-phenanthroline, a complex reagent for Fe(II), dramatically decreases Cr(VI) reduction by Fe(0). At pH=4.0, the zero-order rate constant in the presence of 1mM of 1,10-phenanthroline was 0.02 micrM min(-1), decreasing by 99.7% and 93.9%, respectively, compared with the results in the presence and absence of EDTA. The results suggest that a pathway of the reduction of Cr(VI) to Cr(III) by Fe(0) may involve dissolution of Fe(0) to produce Fe(II), followed by reduction of Cr(VI) by Fe(II), rather than the direct reaction between Cr(VI) and Fe(0), in which Fe(0) transfers electrons to Cr(VI).

Photodegradation mechanism and kinetics of methyl orange catalyzed by Fe(III) and citric acid

In this study, the photodegradation process of methyl orange (MO) catalyzed by Fe(III) and citric acid and the reaction kinetics were investigated in detail at pHs from 2 to 8. The results show that the photodegradation of MO is slow in the presence of Fe(III) or citric acid alone. However, it is markedly enhanced when Fe(III) and citric acid coexist. High initial citric acid or initial Fe(III) concentrations lead to increased photodegradation of MO. And Fe(III) citrate mediated photodegradation of MO is optimized at pH 6. The photoproduction of hydroxyl radicals (·OH) in different catalytic systems was determined by HPLC. And the concentrations of Fe(II) and citric acid concentration in the process of the reaction were analyzed. The photodegradation of MO obeys to pseudo-zero order kinetics with respect to MO and the degradation reaction occurs in two phases. At the initial initiation stage, degradation rate is relatively slow, and significantly increases at a later acceleration stage.

Photo-oxidation of Cr(III)-citrate complexes forms harmful Cr(VI).

Photo-oxidation is a potential pathway for the transformation of Cr(III) to Cr(VI) in natural environments. In this study, the Cr(III)-citrate complex (Cr(III)-cit) was prepared and its speciation was determined by high performance liquid chromatography (HPLC). Results showed that Cr(III)-cit existed in [Cr(III)-H-cit](+) and [Cr(III)-cit] species in a pH range of 3-5, in [Cr(III)-cit] only from pH 6-8, in [Cr(III)-cit] and [Cr(III)-OH-cit](-) from pH 9-11, and only in [Cr(III)-OH-cit](-) at pH 12. Additional experiments were conducted in batch systems with pHs of 5 to 12 at 25 °C, where aqueous Cr(III) and Cr(III)-cit were fully exposed to light from medium pressure mercury lamps and a xenon lamp mimicking solar light irradiation. Results demonstrated that oxidation of Cr(III) in Cr(III)-cit was much faster than that in aqueous Cr(III). Rates of Cr(III) photo-oxidation were not sensitive to pH in the range from 7 to 9 but increased significantly with further increases in pH, which was consistent with the distribution of Cr(III) forms. It appeared that [Cr(III)-cit-OH](-) was the most photochemically active form and Cr(II), resulting from a ligand-to-metal charge-transfer (LMCT) pathway after light absorption, was a precursor of the oxidation of Cr(III) to Cr(VI). Both dissolved oxygen and the hydroxyl radical ((•)OH), an intermediate, served as oxidants and facilitated the oxidation of Cr(II) to Cr(VI) via a multiple step pathway. The photoproduction of (•)OH was detected by HPLC using benzene as a probe, supporting the proposed reaction mechanism.

A comparative study of oxidation of Cr(III) in aqueous ions, complex ions and insoluble compounds by manganese-bearing mineral (birnessite).

Manganese oxides are considered to be main oxidants resulting in transformation of Cr(III) to Cr(VI) in soils. Oxidation of aqueous Cr(III), Cr(III)-EDTA and insoluble species of Cr(III), such as Cr(OH)3, CrFe(OH)6 and CrPO4, by delta-MnO2 was investigated in batch reaction systems at 25 degrees C and different pH values to predict the potential for Cr(III) oxidation in soil environments. Results indicate that Cr(III) can be rapidly oxidized to Cr(VI) at the beginning of the reaction; however, Mn(II) is produced and fills the adsorption sites on the manganese oxide surface. As a result, produced Mn(II) greatly slows Cr(III) oxidation by delta-MnO(2). Lower pH and higher concentration of manganese oxide markedly enhance the rate and extent of aqueous Cr(III) oxidation. The oxidation of Cr(III)-EDTA by manganese oxide is significantly affected by the chelating time between Cr(III) and EDTA and the molar ratio of EDTA to Cr(III). The formed complex ions of Cr(III)-EDTA are hardly oxidized by manganese oxide and no Cr(VI) was detected in a pH range of 5-6. The rate and extent of oxidation of Cr(OH)3 and CrFe(OH)6 by manganese oxide decrease with pH increasing from 2 to 4. No release of Cr(VI) was observed in the suspension of CrFe(OH)6 and manganese oxide at pH 4 and in the suspension of CrPO4 and manganese oxide at all pH levels tested. The results demonstrate that the order of stability of Cr(III) in these precipitates is CrPO4>CrFe(OH)6>Cr(OH)3 in the presence of manganese oxide.

Rapid reduction of Cr(VI) coupling with efficient removal of total chromium in the coexistence of Zn(0) and silica gel

The effect of silica gel on the efficiency enhancement of Zn(0) for the reduction of Cr(VI) and removal of total chromium was investigated in this study. The batch experiment was carried at 4 ≤ pH ≤ 10 with 50 μM initial Cr(VI) concentration, and mass loading of 0-40 g/L for silica gel and 0-8 g/L for Zn(0). Results showed limited Cr(VI) reduction in the Zn(0)/H(2)O system, which was attributed to the formation of passivating films on the Zn(0) surface. However, a complete reduction of Cr(VI) by Zn(0) in the presence of silica gel could be achieved at the all tested pHs. The rate of Cr(VI) reduction was markedly enhanced with a pH decrease, an increase of silica gel or Zn(0) loading, and specific surface area of silica gel. Almost complete removal of total chromium was also observed, suggesting that Cr(III) yielded from the reduction of Cr(VI) was adsorbed onto the silica gel and ZnO surface or existed in Zn-Cr mixed oxides or other Zn-Cr co-precipitates. The possible pathways for Cr(VI) reduction and total chromium removal were proposed in this study, revealing the potential mechanism responsible for the rapid reduction of Cr(VI) coupling with the efficient removal of total chromium in the coexistence of Zn(0) and silica gel.

Degradation of methyl orange by Zn(0) assisted with silica gel

The degradation of methyl orange (MO) by Zn(0) assisted with silica gel was investigated under different conditions. The results show that the degradation of MO by Zn(0) alone was slow and incomplete due to the rapid corrosion of Zn(0) particles on surface. However, the degradation of MO can be markedly enhanced when Zn(0) and silica gel coexist, even under neutral and alkaline conditions. MO removal was improved with a pH decrease, an increase of the initial amount of silica gel and Zn(0), and specific surface area of silica gel. The degradation of MO by Zn(0) assisted with silica gel can be described by a pseudo-first-order kinetic. And the surface of Zn(0) before and after the reaction was characterized by the microscopic analysis of morphology, revealing the potential mechanism responsible for the enhanced reactivity of silica gel.

Adsorption and mobility of Cr(III)–organic acid complexes in soils

The soluble Cr(III) is likely to be complexed with organic ligands in ligand-rich soil. Cr(VI) chemical reduction by organic acids and bioreduction by microorganisms can produce soluble Cr(III)-organic acids complexes. Thus, it is of great significance to investigate the absorption and mobility of Cr(III)-organic acid complexes in soils. In this study, Cr(III)-EDTA and Cr(III)-cit were prepared and purified, and then were examined for adsorption and mobility. The results demonstrated that Cr(III) was strongly bound to soil, while Cr(III)-organic acid complexes had no or slight interaction with soils since Cr(III)-EDTA and Cr(III)-cit complexes mainly existed as the forms of [Cr(III)-EDTA](-) and [Cr(III)-cit], respectively, under the tested conditions with initial pH 4.0-9.0. The adsorption of Cr(III) increased but that of Cr(III)-organic acid complexes decreased with the content of soil organic matter. Compared with Cr(III)-EDTA, the mobility of Cr(III)-cit in soil columns was reduced, due to the specific adsorption between soils and Cr(III)-cit which contained one free hydroxyl group.

Rapid degradation of aniline in aqueous solution by ozone in the presence of zero-valent zinc.

The effects of Zn(0) dosage from 0.1 to 1.3gL(-1), pH from 2 to 12 and temperature from 288 to 318K on the degradation of aniline in aqueous solution by ozone in the presence of Zn(0) were investigated through batch experiments. The results demonstrated that Zn(0) had a significantly synergistic role in the degradation of aniline by ozone. A complete decomposition of the initial aniline (10mgL(-1)) was achieved by ozone together with Zn(0) within 25min, and meanwhile nearly 70% of the total organic carbon in the solution was removed. The decomposition efficiency of aniline markedly increased with an increase of Zn(0) dosage. However, temperature exerted a slight impact on the degradation of aniline and the optimum removal efficiency of aniline was realized at 298K. Aniline was efficiently degraded at all the tested pHs except for 12. Free radicals were investigated by electron paramagnetic resonance technique and free radical scavengers. H2O2 concentration generated during the reactions was analyzed using a photometric method. Based on the results obtained in this study, it is proposed that O2(-) instead of OH is the dominant active species responsible for the degradation of aniline. It is concluded that ozone combined with Zn(0) is an effective and promising approach to the degradation of organic pollutants.

Photocatalytic reduction of Cr(VI) by small molecular weight organic acids over schwertmannite.

In this study, a series of bath experiments was carried out to investigate the photoreduction of Cr(VI) by small molecular weight organic acids (SOAs) over schwertmannite, a mineral found in acid mine drainage (AMD). The results demonstrated that schwertmannite or SOAs alone was unable to effectively transform Cr(VI) to Cr(III) even if exposed to an illumination of mimic solar light. However, an addition of schwertmannite significantly enhanced the reduction of Cr(VI) by SOAs under the same condition. For example, 100μM Cr(VI) was almost completely removed within 50min in the presence of both schwertmannite (0.6gL(-1)) and oxalic acid (300μM) at pH 3.0. The photocatalytic reduction of Cr(VI) was strongly influenced by pH, the initial concentrations and the structures of SOAs. Of the tested three SOAs, the reaction rates of photocatalytic reduction of Cr(VI) were in the order of oxalic acid>citric acid>tartaric acid. The reaction obeyed to zero-order kinetics with respect to Cr(VI) with excess SOAs. A possible mechanism for photoreduction of Cr(VI) by SOAs over schwertmannite was proposed. Fe(III) on the surface of schwertmannite was dissolved by SOAs, and then Fe(III)-SOA complexes with high photochemical activity formed. Further, Fe(II) together with organic acid radicals, CO(2)(-) and O(2)(-), was generated through a metal-ligand-charge-transfer pathway (MLCT), leading to a rapid reduction of Cr(VI).