Xiongfei Huang

Relative distribution of Pb2+ sorption mechanisms by sludge-derived biochar

Lead sorption capacity and mechanisms by sludge-derived biochar (SDBC) were investigated to determine if treatment of acid mine drainage (AMD) containing metals with SDBC is feasible. It was found that the biochar derived from pyrolysis treatment of sewage sludge could effectively remove Pb(2+) from acidic solution with the capacities of 16.11, 20.11, 24.80, and 30.88mgg(-1) at initial pH 2, 3, 4 and 5, respectively. Lead sorption processes were pseudo-second order kinetic and faster at a higher pH. Furthermore, the relative contribution of both inorganic mineral composition and organic functional groups of SDBC for Pb(2+) removal mechanisms, was quantitatively studied at pH 2-5. The results showed that Pb sorption primarily involved the coordination with organic hydroxyl and carboxyl functional groups, which was 38.2-42.3% of the total sorbed Pb varying with pH, as well as the coprecipitation or complex on mineral surfaces, which accounted for 57.7-61.8% and led to a bulk of Ca(2+) and Mg(2+) release during sorption process. A new precipitate was solely observed on Pb-loaded SDBC as 5PbO·P(2)O(5)·SiO(2)(lead phosphate silicate) at initial pH 5, confirmed by XRD and SEM-EDX. The coordination of Pb(2+) with carboxyl and hydroxyl functional groups was demonstrated by FT-IR, and the contribution of free carboxyl was significant, ranging from 26.1% to 35.5%. Results from this study may suggest that the application of SDBC is a feasible strategy for removing metal contaminants from acid solutions.

Photocatalytic activity of polymer-modified ZnO under visible light irradiation

Photocatalytic removal of phenol, rhodamine B, and methyl orange was studied using the photocatalyst ZnO/poly-(fluorene-co-thiophene) (PFT) under visible light. After 2 h irradiation with three 1 W LED (light-emitting diode) lights, about 40% removal of both phenol and methyl orange was achieved; rhodamine B was completely degraded to rhodamine. Diffuse reflectance spectra showed that the absorbance range of PFT/ZnO was expanded from 387 nm (ZnO) to about 500 nm. Photoluminescent spectra and photoluminescent quantum efficiency indicated that electrons were transferred from PFT to the conduction band of ZnO. Electron spin resonance (ESR) signals of spin-trapped paramagnetic species with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) evidenced that the OH* radicals were indeed formed in the PFT/ZnO system under visible light irradiation. A working mechanism involving excitation of PFT, followed by charge injection into the ZnO conduction band is proposed.

Role of oxygen active species in the photocatalytic degradation of phenol using polymer sensitized TiO2 under visible light irradiation

The role of dissolved oxygen, and of active species generated by photo-induced reactions with oxygen, in the photocatalytic degradation of phenol was investigated using polymer [poly-(fluorene-co-thiophene) with thiophene content of 30%, so-called PFT30] sensitized TiO2 (PFT30/TiO2) under visible light irradiation. The photoluminescent (PL) quantum yield of PFT30/TiO2 was about 30% of that of PFT30/Al(2)O(3), proving that electron transfer took place between the polymer and TiO2. The result that photocatalytic degradation of phenol was almost stopped when the solution was saturated with N(2) proved the importance of O(2). Addition of NaN(3), an effective quencher of singlet oxygen ((1)O(2)), caused about a 40% decrease in the phenol degradation ratio. Addition of alcohols caused about a 60% decrease in the phenol photodegradation ratio, indicating that the hydroxyl radicals (OH), whose presence was confirmed by electron spin resonance (ESR) spectroscopy, was the predominant active species in aqueous solution. In anhydrous solution, singlet oxygen ((1)O(2)) was the predominant species. These results indicate that oxygen plays a very important role in the photocatalytic degradation of phenol.

Visible light induced photocatalytic reduction of Cr(VI) over polymer-sensitized TiO2 and its synergism with phenol oxidation

In this study, both Cr(VI) reduction and phenol oxidation induced by polymer-sensitized TiO(2) were investigated under visible light. Study of the reaction mechanism indicated that poly(fluorene-co-thiophene) (PFT) acted as a semiconductor and was by itself able to reduce Cr(VI) under visible light irradiation. When coupled with TiO(2), PFT served not only as the electron donor for Cr(VI) reduction, but also as a sensitizer. Upon irradiation by visible light, electrons in the sensitizing PFT polymer are excited and are transferred to the conduction band of TiO(2). PFT-catalyzed reduction of Cr(VI) was significantly promoted by the presence of phenol, and synergism between Cr(VI) reduction and phenol degradation was demonstrated both by analysis of the FT-IR spectrum of PFT/TiO(2) and by measuring the effect of repeated use of PFT/TiO(2) on its photocatalytic efficiency. The results provide a cost-effective method to remove organic and inorganic pollutants simultaneously in the complex wastewater.

Chelant extraction of heavy metals from contaminated soils using new selective EDTA derivatives

Soil washing is one of the few permanent treatment alternatives for removing metal contaminants. Ethylenediaminetetraacetic acid (EDTA) and its salts can substantially increase heavy metal removal from contaminated soils and have been extensively studied for soil washing. However, EDTA has a poor utilization ratio due to its low selectivity resulting from the competition between soil major cations and trace metal ions for chelation. The present study evaluated the potential for soil washing using EDTA and three of its derivatives: CDTA (trans-1,2-cyclohexanediaminetetraacetic acid), BDTA (benzyldiaminetetraacetic acid), and PDTA (phenyldiaminetetraacetic acid), which contain a cylcohexane ring, a benzyl group, and a phenyl group, respectively. Titration results showed that PDTA had the highest stability constants for Cu(2+) and Ni(2+) and the highest overall selectivity for trace metals over major cations. Equilibrium batch experiments were conducted to evaluate the efficacy of the EDTA derivatives at extracting Cu(2+), Zn(2+), Ni(2+), Pb(2+), Ca(2+), and Fe(3+) from a contaminated soil. At pH 7.0, PDTA extracted 1.5 times more Cu(2+) than did EDTA, but only 75% as much Ca(2+). Although CDTA was a strong chelator of heavy metal ions, its overall selectivity was lower and comparable to that of EDTA. BDTA was the least effective extractant because its stability constants with heavy metals were low. PDTA is potentially a practical washing agent for soils contaminated with trace metals.

Silane-based coatings on the pyrite for remediation of acid mine drainage

Acid mine drainage (AMD) resulting from the oxidation of pyrite and other metal sulfides has caused significant environmental problems, including acidification of rivers and streams as well as leaching of toxic metals. With the goal of controlling AMD at the source, we evaluated the potential of tetraethylorthosilicate (TEOS) and n-propyltrimethoxysilane (NPS) coatings to suppress pyrite oxidation. The release of total Fe and SO4(-2) from uncoated and coated pyrite in the presence of a chemical oxidizing agent (H2O2) or iron-oxidizing bacteria (Acidithiobacillus ferrooxidans) was measured. Results showed that TEOS- and NPS-based coatings reduced chemical oxidation of pyrite by as much as 59 and 96% (based on Fe release), respectively, while biological oxidation of pyrite was reduced by 69 and 95%, respectively. These results were attributed to the formation of a dense network of Fe-O-Si and Si-O-Si bonds on the pyrite surface that limited permeation of oxygen, water, and bacteria. Compared with results for TEOS-coated pyrite, higher pH and lower concentrations of total Fe and SO4(-2) were observed for oxidation of NPS-coated pyrite, which was attributed to its crack-free morphology and the presence of hydrophobic groups on the NPS-based coating surface. The silane-based NPS coating was shown to be highly effective in suppressing pyrite oxidation, making it a promising alternative for remediation of AMD at its source.

Characterization of Conjugated Polymer Poly(fluorene-co-thiophene) and Its Application as Photosensitizer of

The copolymer poly(fluorene-co-thiophene) (PFT) has strong absorption in the visible light region. In this paper, PFTs with four different thiophene contents were evaluated for their suitability as photosensitizers of Ti. All four of the PFTs were capable of being excited by light with wavelengths above 455 nm, and all had reductive potentials more negative than the conduction band potential of Ti. Consequently, it was deemed that transfer of electrons from the excited PFTs to the conduction band of Ti was thermodynamically possible. PFTs with higher thiophene content had more electron transitions in the excited state, resulting in increased photocatalytic activity of PFT-sensitized Ti. The method used to prepare the combined PFT/Ti photocatalyst affected its photocatalytic activity. Best results in this study were achieved by dropwise addition of a tetrahydrofuran solution of PFT to a stirred Ti/ethanol suspension.

PAHs Sorption and Desorption on Soil Influenced by Pine Needle Litter-Derived Dissolved Organic Matter

Study of the relationship between plant litter-derived dissolved organic matter (DOM) and organic pollutant transport in soil is important for understanding the role of forest litter carbon cycling in influencing pollutant behaviour and fate in forest soil. With the aim of providing insight into the capacity of plant litter-derived DOM to influence sorption and desorption of selected polycyclic aromatic hydrocarbons (PAHs) on soil, batch experiments were carried out with application of a sorption-desorption model incorporating DOM effects. Freshly fallen pine (Pinus elliottii) needles were used as the source of organic matter. Input of the pine needle litter-derived DOM was found to significantly decrease desorption hysteresis as well as soil adsorption capacity of phenanthrene (PHE) and fluoranthene (FLA). Addition of 1 728 mg L−1 dissolved organic carbon (DOC) lowered the organic carbon-normalized sorption distribution coefficient of PHE from 7 776 to 2 541 L kg−1 C and of FLA from 11 503 to 4 368 L kg−1 C. Decreases of the apparent sorption-desorption distribution coefficients of PHE and FLA with increased DOC concentration indicated that DOM favored desorption of PAHs from soil. Increases in the fraction of apparently dissolved PAHs were attributable to the dissolved PAH-DOM complexes, accounting for the dissolved proportions of 39% to 69% for PHE and 26% to 72% for FLA in the sorption and desorption processes as the concentration of the added DOM solution rose from 0 to 1 728 mg L−1. Our results suggest that pine needle litter-derived DOM can have a substantial effect of inhibiting PAHs sorption and promoting PAHs desorption, thus leading to enhanced leaching in soil, which should be taken into account in risk assessment of PAHs accumulated in forest soil.

Synergistical enhancement by Ni2+ and Tween-80 of nanoscale zerovalent iron dechlorination of 2,2’,5,5’-tetrachlorinated biphenyl in aqueous solution

Effective dehalogenation by nanoscale zerovalent iron (nZVI) has been reported. In this study, the effects of Ni2+, Cu2+ ions, and the nonionic surfactant Tween-80 on dechlorination of 2,2’,5,5’-tetrachlorinated biphenyl (PCB-52) by nZVI were investigated in aqueous solution. The rate of dechlorination was significantly enhanced by Ni2+, while Cu2+ had a less significant catalytic effect. Ni2+ and Tween-80 used in combination synergistically enhanced dechlorination of PCB-52 by nZVI, although the enhancement by Tween-80 was inhibitory in the presence of Cu2+ and insignificant in the absence of both metal ions. Congener specificity in the dechlorination pathway resulted from the preferential retention of ortho-chlorine, which restricted the formation of environmentally toxic coplanar PCB congeners. The application of nZVI dehalogenation enhanced by Ni2+ and Tween-80 is a promising technique for posttreatment of PCB-contaminated soil washing solutions.

Zerovalent iron in conjunction with surfactants to remediate sediments contaminated by polychlorinated biphenyls and nickel

Dredging and disposal is commonly used for cleanup of contaminated sediments, leaving the relocated sediments still in need of remediation. In this study, the feasibility of two approaches to using zerovalent iron (ZVI) in conjunction with surfactants to remediate sediments contaminated by polychlorinated biphenyls (PCBs) and Ni was investigated. Approach A is surfactant desorption followed by ZVI treatment and approach B is a simple mixture of ZVI and sediment in surfactant solution. Results of approach A show that 65.24% of PCBs and 2.12% of Ni were desorbed by 1% Envirosurf; however, the sequential ZVI-mediated reductive dechlorination (ZVI-RD) was ineffective due to micelle sequestration by high contents of surfactants while Ni could be almost completely removed. For approach B, less than 1% of coexisting Ni was released to aqueous solution, and 47.18%-76.31% PCBs could be dechlorinated by ZVI with the addition of 0.04% surfactants (Tween-80 and Envirosurf). Results of dechlorination kinetics and ZVI morphologies reveal that surfactants at the concentrations as low as 0.04% were able to enhance the contact of sediment-bound PCBs with ZVI, and also to alleviate ZVI passivation. The PCB mixtures in sediment were continuously desorbed and dechlorinated, yielding lower substituted homologues that are less toxic and less hydrophobic. Thus, a simple mixture of ZVI and contaminated sediments without dewatering appears to be a promising alternative to the remediation of PCBs-contaminated sediments.