Strategies to enhance the reactivity of zero-valent iron for environmental remediation: A review.

Application of zero-valent iron (ZVI) has become one of the most promising innovative technologies for the remediation of environmental pollutants. However, ZVI may suffer from the low intrinsic reactivity toward refractory pollutants, which seriously restricts its practical application in fields. Therefore, strategies have been developing to enhance the reactivity of ZVI. Until now, the most commonly used strategies include pretreatment of ZVI, synthesis of highly-active ZVI-based materials and additional auxiliary measures. In this review, a systematic and comprehensive summary of these commonly used strategies has been conducted for the following purposes: (1) to understand the fundamental mechanisms of the selected approaches; (2) to point out their advantages and shortcomings; (3) to illustrate the main problems of their large-scale application; (4) to forecast the future trend of developing ZVI technologies. Overall, this review is devoted to providing a fundamental understanding on the mechanism for enhancing the reactivity of ZVI and facilitating the practical application of ZVI technologies in fields.

Pilot-scale study on an advanced Fe-Cu process for refractory wastewater pretreatment.

The extreme pH, high color, and poor biodegradability of refractory wastewater have severe impacts on its biological treatment. To address this issue, an advanced Fe-Cu process with redox reaction and spontaneous coagulation was investigated and applied for pilot-scale (wastewater flow rate of 2000 m3·day-1) pretreatment of separately discharged acidic chemicals and alkaline dyeing wastewater. The advanced Fe-Cu process had five functions: (1) increasing the pH of chemical wastewater to 5.0 and above, with an influent pH of approximately 2.0; (2) transforming refractory organics of chemical wastewater with 10.0% chemical oxygen demand (COD) and 30.8% color removal, thereby enhancing the ratio of biological oxygen demand after five days (BOD5) to COD (B/C) from 0.21 to 0.38; (3) neutralizing the pH of the pretreated chemical wastewater for coagulation application with alkaline dyeing wastewater to avoid adding alkaline chemical; (4) achieving average nascent Fe(II) concentrations of 925.6 mg∙L-1 using Fe-Cu internal electrolysis for mixed wastewater coagulation, resulting in an average of 70.3% color removal and 49.5% COD removal; (5) providing more efficient COD removal and B/C enhancement than FeSO4∙7 H2O coagulation while avoiding secondary pollution. The green process offers an effective, easy-implemented solution for the pretreatment of separately discharged acidic and alkaline refractory wastewater.

Occurrence and distribution of organochlorine pesticides (DDT and HCH) in sediments from the middle and lower reaches of the Yellow River, China.

Sediments used in this study were selected from 23 stations in the middle and lower reaches of the Yellow River and its tributaries in November 2005. The levels and distribution patterns of selected organochlorine pesticides (OCPs = p,p(')-DDT, o,p (')-DDT, p,p(')-DDE, p,p(')-DDD, alpha-, beta-, gamma-, and delta-HCH) in samples were investigated by analysis of gas chromatography coupled with micro-electronic capture detector. Concentration of OCPs in the sediments from the Yellow River ranged from 0.05 to 5.03 ng g(-1) (mean, 1.02 ng g(-1)) for summation operatorDDT, 0.09-12.89 ng g(-1) (mean, 1.08 ng g(-1)) for summation operatorHCH. The concentration distribution of summation operatorDDT and summation operatorHCH varied significantly with different sampling station, indicating their different contamination sources. Composition analyses demonstrated that residues of DDTs in sediments came from the previous inputs of organochlorine pesticides, while beta-HCH and gamma-HCH significantly dominated in the sediments for HCHs.

Simultaneous nitrogen and phosphor removal in an aerobic submerged membrane bioreactor.

Simultaneous nitrification and denitrification (SND) effect and phosphor removal were investigated in a one-staged aerobic submerged membrane bioreactor on pilot-scale with mixed liquor suspended solids (MLSS) 19-20 g/L. The effects of DO concentration, sludge floc size distribution on SND were studied. Test results suggested that SND was successfully performed in the membrane bioreactor (MBR) and about 70% total nitrogen removal efficiency was achieved when DO concentration was set to 0.2-0.3 mg/L. The main mechanisms governing SND were the suitable sludge floc size and the low DO concentration which was caused by low oxygen transfer rate with such a high MLSS concentration in the MBR. In the meantime, phosphor removal was also studied with polymer ferric sulfate (PFS) addition and 14 mg/L dosage of PFS was proper for the MBR to remove phosphor. PFS addition also benefited the MBR operation owing to its reduction of extracellular polymer substances (EPS) of mixed liquor.

The removal of organic precursors of DBPs during three advanced water treatment processes including ultrafiltration, biofiltration, and ozonation.

The removal efficiency of organic matter, the formation potential of trihalomethanes (THMFP), and the formation potential of haloacetic acids (HAAFP) in each unit of three advanced treatment processes were investigated in this paper. The molecular weight distribution and the components of organic matter in water samples were also determined to study the transformation of organic matter during these advanced treatments. Low-molecular-weight matter was the predominant fraction in raw water, and it could not be removed effectively by ultrafiltration and biofiltration. The dominant species of disinfection by-product formation potential (DBPFP) in raw water were chloroform and monochloroacetic acid (MCAA), with average concentrations of 107.3 and 125.9 µg/L, respectively. However, the formation potential of chloroform and MCAA decreased to 36.2 and 11.5 µg/L after ultrafiltration. Similarly, biological pretreatment obtained high removal efficiency for DBPFP. The total THMFP decreased from 173.8 to 81.8 µg/L, and the total HAAFP decreased from 211.9 to 84.2 µg/L. Separate ozonation had an adverse effect on DBPFP, especially for chlorinated HAAFP. Numerous low-molecular-weight compounds such as aldehydes, ketones, and alcohols were generated during the ozonation, which have been proven to be important precursors of HAAs. However, the ozonation/biological activated carbon (BAC) combined process had a better removal efficiency for DBPFP. The total DBPFP decreased remarkably from 338.7 to 113.3 µg/L after the O3/BAC process, far below the separated BAC of process B (189.1 µg/L).

Event mean concentration and first flush effect from different drainage systems and functional areas during storms.

This study aimed to investigate the characteristics of the event mean concentration (EMC) and first flush effect (FFE) during typical rainfall events in outfalls from different drainage systems and functional areas. Stormwater outfall quality data were collected from five outfalls throughout Fuzhou City (China) during 2011-2012. Samples were analyzed for water quality parameters, such as COD, NH3-N, TP, and SS. Analysis of values indicated that the order of the event mean concentrations (EMCs) in outfalls was intercepting combined system > direct emission combined system > separated system. Most of the rainfall events showed the FFE in all outfalls. The order of strength of the FFE was residential area of direct emission combined system > commercial area of separated system > residential area of intercepting combined system > office area of separated system > residential area of separated system. Results will serve as guide in managing water quality to reduce pollution from drainage systems.

Simultaneously degradation of 2,4-dichlorophenol and EDTA in aqueous solution by the bimetallic Cu-Fe/O2 system.

Oxidative degradation of aqueous organic contaminants 2,4-dichlorophenol (2,4-DCP) using ethylenediaminetetraacetic acid (EDTA)-enhanced bimetallic Cu-Fe system in the presence of dissolved oxygen was investigated. The proposed process was applied for the pH range of 3~7 with the degradation efficiency of 2,4-DCP and EDTA varying within 10 %, and achieved at 100 % degradation of 40 mg L(-1) 2,4-DCP in 1 h, at the initial pH of 3, 25 g L(-1) of bimetallic Fe-Cu powder (WCu/WFe = 0.01289) and initial EDTA of 0.57 mM. However, the removal efficiency of 2,4-DCP in control tests were 7.52 % (Cu-Fe/O2 system) and 84.32 % (EDTA-enhanced Fe/O2 process), respectively, after 3 h, reaction. The proposed main mechanism, involves the in situ generation of H2O2 by the electron transfer from Fe(0) to O2 which was enhanced by ethylenediaminetetraacetic acid (EDTA), and the in situ generation of ·OH via advanced oxidation reaction. Accordingly, 2,4-DCP was attacked by ·OH to achieve complete dechlorination and low molecular weight organic acids, even mineralized. Systematic studies on the effects of initial EDTA and 2,4-DCP concentration, Cu-Fe dosing, Cu content, and pH revealed that these effects need to be optimized to avoid the excessive consumption of ·OH and new EDTA and heavy metal Cu pollution.

Assessment of the service performance of drainage system and transformation of pipeline network based on urban combined sewer system model.

In recent years, due to global climate change and rapid urbanization, extreme weather events occur to the city at an increasing frequency. Waterlogging is common because of heavy rains. In this case, the urban drainage system can no longer meet the original design requirements, resulting in traffic jams and even paralysis and post a threat to urban safety. Therefore, it provides a necessary foundation for urban drainage planning and design to accurately assess the capacity of the drainage system and correctly simulate the transport effect of drainage network and the carrying capacity of drainage facilities. This study adopts InfoWorks Integrated Catchment Management (ICM) to present the two combined sewer drainage systems in Yangpu District, Shanghai (China). The model can assist the design of the drainage system. Model calibration is performed based on the historical rainfall events. The calibrated model is used for the assessment of the outlet drainage and pipe loads for the storm scenario currently existing or possibly occurring in the future. The study found that the simulation and analysis results of the drainage system model were reliable. They could fully reflect the service performance of the drainage system in the study area and provide decision-making support for regional flood control and transformation of pipeline network.

[Investigation of enhanced low carbon wastewater denitrification by catalytic iron].

The nitrogen removal efficiency and N2O production during the process of coupling catalytic iron and biological denitrification for low C/N ratio wastewater were studied. The results showed that biological denitrification coupled with catalytic iron could significantly improve nitrate transforming efficiency, but led to nitrite accumulation, resulting in little difference in total nitrogen removal efficiency, compared with traditional biological denitrification. N2O production from the catalytic iron group was obviously higher than the traditional control group, among which chemical reaction between ferrous oxide and nitrite had great contribution. However, the highest amount of accumulated N2O was less than 8% of the removed nitrate, and N2O was further reduced to N2 by microorganisms. Furthermore, catalytic iron group could eliminate dissolved oxygen and lower the oxidation reduction potential, which is beneficial for keeping anoxic conditions during denitrification.

The pretreatment by the Fe-Cu process for enhancing biological degradability of the mixed wastewater.

The Fe-Cu process in combination with cyclic activated sludge system (CASS) was used to treat the mixed wastewater composed of industry wastewater and urban sewage in this work. The results showed that the pretreatment by the Fe-Cu process removed 20% of COD(cr) and 32% of total phosphorus (TP), which reduced the loading rate of the subsequent biological treatment. Mean while, biodegradability of the wastewater was enhanced, which created favorable condition for the subsequent biological treatment. The formation of heavy, lumpy or granular, absorbent, enriched with microorganisms bio-ferric activated sludge with good setting performance promoted degradation of various refractory organic contaminants. The increase by 10 times of nitrifying and denitrifying bacteria counts (total biofilm biomass increased by 59%) and 0.4 of pH value enhanced the biological nitrification and denitrification to ensure the final effluent NH(3)-N and TN to be 8 and 20mg/L, respectively. Agglomeration, passivation and clogging of iron were not observed in three months of continuous operation. Furthermore, the consumption of iron was low. All these led to an easy maintenance and low operating cost.

[Decolorization of the azo dye reactive red X-3B by an Al-Cu bimetallic system].

The decoloration mechanism and kinetics of the azo dye reactive red X-3B by an Al-Cu bimetallic system were investigated by measuring the dye removal, the TOC removal and the aniline concentration, and by adding EDTA as control experiments. The results showed the colority removal rate of X-3B reached 83% in the near neutral pH medium for 30 min and 96.4% for 120 min, in which, about 34% was due to the X-3B reduced to aniline, and about 20% and 30% was due to the flocculating of aluminum ions and surface adsorption of aluminum-fillings respectively. The decolorization of dyeing wastewater is a gradual reaction process, which first adsorbs a large number of dyeing ingredients and then carries out inner electrolysis reduction, improved effectively by the flocculating action of aluminum ions. The decolorization reaction appears to be a pseudo first-order reaction and increases with rising temperature.

[Catalytic reduction of CCl4 in water by Fe0 and amended Fe0].

Various bimetallic reductants of Cu/Fe, Ag/Fe, Pd/Fe and Ni/Fe were prepared by plating Cu, Ag, Pd and Ni on the surface of Fe0. Reductive dechlorination of toxic pollutants of CCl4 in water with Fe0 and amended Fe0 by batch experiments was investigated. Results show that CCl4 in water can be rapidly dechlorinated by above five catalytic reductants, and the presence of Cu, Ag, Pd can enhance the dechlorination rate dramatically. The reaction of CCl4 with various reductants was followed the pseudo first order kinetics, and the dechlorination rate constant of CCl4 in water by Fe0, Cu/Fe, Ag/Fe, Pd/Fe and Ni/Fe was 0.0393, 0.0925, 0.158, 0.0496 and 0.0533 min(-1) respectively. The byproducts and pathway of dechlorination of CCl4 by Fe0 and amended Fe0 was identified by GC/MS. Results indicate that the products and dechlorination rate of CCl4 by various bimetallic reductants are varied. The main products are chloroform and dichloromethane in Cu/Fe and Ag/Fe system, and that is methane in Pd/Fe system. Hydrogenolysis is the dominant reaction pathway of CCl4 by Fe0 and amended Fe0.

[Reductive dechlorination of chlorinated hydrocarbons in water by Ag/Fe catalytic reduction system].

Dechlorination of many kinds of chlorinated hydrocarbons including trichloromethane (CF), tetrachloromethane (CT), 1, 1,1-trichloroethane (1, 1, 1-TCA), 1, 1, 2, 2-tetrachloroethane (1, 1, 2, 2-TeCA), hexachloroethane (HCA), trichloroethylene (TCE), perchloroethylene (PCE) in water by Fe and Ag/Fe were investigated. Results show that the existence of Ag on Fe can enhance the dechlorination rate of chlorinated hydrocarbons. When the surface area concentration of iron is 150 m2 x L(-1), the dechlorination rate constant of CF, CT, 1, 1, 1-TCA, 1, 1, 2, 2-TeCA, HCA are 0.084 h(-1), 2.358 h(-1), 0.417 h(-1), 0.215 h(-1), 1.098 h(-1) by single Fe0, in this condition, no dechlorination of TCE and PCE during 10h, while the dechlorination rate constant of CF, CT, 1, 1, 1-TCA, 1, 1, 2, 2-TeCA, HCA, TCE, PCE are up to 1.850 h(-1), 9.504 h(-1), 1.624 h(-1), 1.778 h(-1), 2.842 h(-1), 0.463 h(-1), 1.251 h(-1) by Ag/Fe. The dechlorination rate constant of CF can increase by the factor of 20. The reaction pathways of chlorinated hydrocarbons with Fe and Ag/Fe were mainly including hydrogenolysis, reductive elimination and dehydrochlorination. The hydrogenolysis is the main reaction pathway for CF, TCE and PCE. The dechlorination pathway of 1, 1, 1-TCA are both hydrogenolysis and dehydrochlorination. Reductive beta elimination to form DCE is the dominant reaction pathway of 1, 1, 2, 2-TeCA, and HCA is via reductive beta elimination to form PCE firstly, then via hydrogenolysis to DCE.

[Electrochemical reduction characteristics and mechanism of chlorinated hydrocarbon at the copper electrode].

The electrochemical reduction characteristics of chlorinated hydrocarbons were investigated by applying cyclic voltammetry technique. The reduction mechanism and reactivity of the chlorinated hydrocarbons at the copper electrodes were explored. The relation between the reductive reactivity at the copper electrode and the structures of this kind of compounds was discussed. The experimental results show that chlorinated paraffin hydrocarbons and a portion of chlorinated aromatic hydrocarbons could be reduced directly at the copper electrode; however, chlorinated aromatic hydrocarbons aren't easy to reduced directly at the copper electrode. The results provide a theoretical basis for the catalyzed iron inner electrolysis method.