Jia Xin

Mechanism insights into enhanced trichloroethylene removal using xanthan gum-modified microscale zero-valent iron particles

This report focuses on the enhancement in trichloroethylene (TCE) removal from contaminated groundwater using xanthan gum (XG)-modified, microscale, zero-valent iron (mZVI). Compared with bare mZVI, XG-coated mZVI increased the TCE removal efficiency by 30.37% over a 480-h experimental period. Because the TCE removal is attributed to both sorption and reduction processes, the contributions from sorption and reduction were separately investigated to determine the mechanism of XG on TCE removal using mZVI. The results showed that the TCE sorption capacity of mZVI was lower in the presence of XG, whereas the TCE reduction capacity was significantly increased. The FTIR spectra confirmed that XG, which is rich in hydrophilic functional groups, was adsorbed onto the iron surface through intermolecular hydrogen bonds, which competitively repelled the sorption and mass transfer of TCE toward reactive sites. The variations in the pH, Eh, and Fe(2+) concentration as functions of the reaction time were recorded and indicated that XG buffered the solution pH, inhibited surface passivation, and promoted TCE reduction by mZVI. Overall, the XG-modified mZVI was considered to be potentially effective for the in-situ remediation of TCE contaminated groundwater due to its high stability and dechlorination reactivity.

Zero-valent aluminum for reductive removal of aqueous pollutants over a wide pH range: Performance and mechanism especially at near-neutral pH

Zero-valent aluminum (ZVAl) draws much attention due to its strong reducibility. However, under neutral pH conditions, the reduction ability of ZVAl for pollutant removal has still been suspected because of the formed compact surface film of Al-(hydr)oxide. In this study, unmodified ZVAl was employed to reductively remove aqueous pollutants over a wide pH range, and its performance and mechanism, especially at near-neutral pH, were systematically studied for the first time. Results demonstrated that ZVAl had a wide range of pH applicability from 2 to 12, even in neutral environment. Typical nitro compound nitrobenzene (NB), typical azo dye acid orange 7 (AO7), and typical inorganic heavy metal ion Cr(VI) can be effectively removed at initial pH 7. Based on the changes of pH, ORP, DO, Al ions and TOC of the reaction solution, and the determination of reduction products of NB by UV-Vis and GC-MS, we found that NB removal by ZVAl can be primarily attributed to the reduction role. NB was reduced to nitrosobenzene firstly, and to aniline finally. Meanwhile, the adsorption phenomenon existed in this system. Next, the surface reaction mechanism was deeply revealed through the characterization of ZVAl particles before and after reaction by SEM-EDS, TEM, HRTEM, XRD, and XPS. It was found that ZVAl powders with core/shell structure participated in the redox reaction, and that ZVAl core was corroded, generating Al-(hydr)oxide. ZVAl surface oxide film was not directly removed, instead of a rougher one. Finally, the proposed reductive mechanism of aqueous pollutants by ZVAl was speculated from the angle of electronic competition. In water environment, O2, H2O and pollutants, with a clear competitive relationship, can capture electrons released from ZVAl. When pollutants opportunities for getting electrons are enhanced, efficiently reductive reactions for pollutant removal can take place, even at near-neutral pH. In a word, ZVAl is a promising material to remove aqueous pollutants over a wide pH range, even in neutral environment, which exhibits its great potential as an effective and environment-friendly agent for pollutant removal from water.

Investigating the efficiency of microscale zero valent iron-based in situ reactive zone (mZVI-IRZ) for TCE removal in fresh and saline groundwater

In this study, long-term column experiments were conducted in three media (Milli-Q water, fresh groundwater and saline groundwater) to evaluate the trichloroethylene (TCE) removal performance, electron efficiency (EE), and permeability loss of a microscale zero valent iron-based in situ reactive zone (mZVI-IRZ) under different field conditions. A potential scenario of in situ contamination plume remediation was simulated by adding a TCE-containing influent to columns filled with mixed mZVI particles and silica sand at a flow rate of 4 mL h-1 for 6 months. Results showed that, over the course of 100 pore volumes (PV) for 6 months, mZVI displayed the lowest TCE breakthrough rate (0.0026 PV-1) and highest TCE removal capacity (43.72 mg) but the poorest EE value (25-40%) in saline groundwater. Mineral characterization (SEM, XRD), ion concentration analysis, and geochemical modeling corroborated that different dominant solid precipitates (magnetite, siderite, dolomite/magnetite) were identified inside the three columns. The column containing saline groundwater experienced the greatest porosity loss, approximately 30.23 mL over the course of 100 PVs. This study illustrates that, to improve designs of mZVI-IRZs, EE as well as hydraulic conductivity should be taken into consideration for predictive evaluations.

Selective enhancement and verification of woody biomass digestibility as a denitrification carbon source

The denitrification efficiency of woody biomass as carbon source is low because of its poor carbon availability. In this study, representative poplar sawdust was pretreated with lime and peracetic acid to enhance the biomass digestibility to different degrees; sawdust was then mixed with soil to investigate its denitrification efficiency. Under controllable conditions (25-95°C, 12-24h, varying dosages), sawdust digestibility (characterized by reducing sugar yield) was selectively enhanced 1.0-21.8 times over that of the raw sawdust (28.8mgeq.glucoseg-1 dry biomass). This increase was mainly attributed to the removal of lignin from the biomass. As a carbon source, the sawdust (digestibility enhanced by 5.4 times) increased the nitrate removal rate by 4.7 times, without N2O emission. However, the sawdust with high digestibility (12.6 or 18.0 times), despite releasing more dissolved organic carbon (DOC), did not exhibit further increase in denitrification efficiency, and emitted N2O.

Effects of alkali-treated agricultural residues on nitrate removal and N 2 O reduction of denitrification in unsaturated soil

Lignocellulosic agricultural residues were utilized as denitrification carbon substrates to improve the purification capacity of unsaturated soil and alleviate nitrate pollution of groundwater. In this study, corncob and wheat straw were treated by calcium hydroxide to improve biodegradability and enhance denitrification potential. Calcium hydroxide treatment decreased the contents of lignin (i.e., from 16.7 wt% to 15.2 wt% in corncob and from 21.9 wt% to 20.6 wt% in wheat straw), increased potential biodegradable carbon by 4.4-5.3 times, reached complete nitrate removal 7-14 days earlier and decreased N2O/(N2O+N2) ratios by 85-99%. The results provide an insight into the application of alkali-treated agricultural residues as denitrification carbon sources to alleviate nitrate transport to groundwater and reduce potential greenhouse effect.

Soluble organic nitrogen cycling in soils after application of chemical/organic amendments and groundwater pollution implications

Nitrogen (N) fertilizers have been extensively used to maintain soil fertility in intensively agricultural soils, creating serious environmental pollution. In this study, a 70-day incubation experiment was conducted to investigate the effects of different N fertilizers (urea, manure, straw) on N mineralization, soluble organic nitrogen (SON) dynamics and its leaching potential in typical agricultural soils of the Shandong Peninsula. The results showed that the addition of N fertilizers affected the SON pools and soil N mineralization in different ways owing to their various properties and interaction with soils. When comparing treatments, urea application was found to decrease SON content, whereas manure and straw addition increased the SON content after long-term incubation. Considering that SON content depended on a complicated formation process and consumption process, no direct link between SON content and N mineralization capacity was observed in different treatments. Additionally, we analyzed free amino acids (FAAs) in SON and found that FAA content was negatively correlated with N mineralization, except for the straw treatment. This suggested that FAAs were important substrates of N mineralization in soils. In addition, the composition of SON was determined by 3-dimensional excitation-emission matrix and ultraviolet-visible absorbance spectrophotometer after long-term incubation. The PIII+V/PI+II+IV ratio, SUVA254, and A253/A203 ratio decreased after fertilizer application. This indicated that fertilizer addition decreased the SON humification degree and increased SON leaching. Therefore, SON should be taken into account when optimizing fertilization management and evaluating the risk of N leaching in groundwater systems.