Research

My current work focuses on exploring the adsorption behaviors of lead (Pb) and arsenic (As) on metal oxides by using a combination of spectroscopic techniques (i.e., synchrotron-based X-ray absorption and in-situ infrared spectroscopy) and computational modeling (i.e., density functional theory and surface complexation modeling). Please see the projects below.

Arsenic removal by co-precipitation using ferric chloride or lime has been widely used due to its low cost and easy operation. Although this topic had been thoroughly studied over decades, we find something new during these procedures:

(1) We found that arsenic is incorporated into ferric oxides particles, which is a rapid process. However, the incorporated arsenic is not stable thermodynamically and therefore was released after several days of aging of the sludge because of the growth of ferric oxides. (please see our article: Shi, Q.; Jing, C.; Meng, X., Competing Interactions of As Adsorption and Fe(III) Polymerization during Ferric Coprecipitation Treatment. Environmental Science & Technology 2018, 52, 7343-7350.).

(2) During the removal procedures, arsenic and ferric ions form a dissolved complex (Fe(III)-As(V) complex). We employed x-ray absorption spectroscopy and mass spectroscopy to identify its possible molecular structure. The possible geometric structures are further verified by density functional theory calculation. (please see our article: Shi, Q., Sterbinsky, G. E., Zhang, S., Christodoulatos, C., Korfiatis, G. P., Meng, X. Formation of Fe (III)-As (V) complexes: effect on the solubility of ferric hydroxide precipitates and molecular structural identification. Environmental Science: Nano. 2020, 7, 1388-1398, (Selected as Back Cover).)

(3) Similar to ferric, calcium also forms a dissolved complex (Ca(II)-As(V) complex), which consequently affects the removal of arsenic by calcium at different pH. The molecular structure of the Ca(II)-As(V) complex has also been identified. (please see our article: Shi, Q.*, Zhang, S., Korfiatis, G. P., Christodoulatos, C., Meng, X. Identifying the existence and molecular structure of the dissolved HCO₃-Ca-As (V) complex in water. Science of The Total Environment, 2020, 724, 138216.)

Lead in tap water has caused lots of water crises in the United States. During these water crises, more than 100,000 Point-of-Use (POU) filters were distributed to households for lead removal from tap water and activated carbon (AC) is the most used adsorbent in these filters. Since 2016, we have conducted a systematic study to compare the commercial adsorbents and explore the removal mechanisms:

(1) We found that metal oxides adsorbents, including activated alumina (AA), titanium dioxides (TiO₂), and iron hydroxides (Fe(OH)₃), have a better lead removal ability than AC. Specifically, 20 mL AC reduced lead from 80 to less than 15 µg/L for 57 L tap water, while the same volume of metal oxides adsorbent can treat more than 700 L water. The co-existing calcium reduces the lead removal by AC while sulfate and phosphate promote the lead removal by AC. (please see our article:Shi, Q.; Terracciano, A.; Zhao, Y.; Wei, C.; Christodoulatos, C.; Meng, X., Evaluation of metal oxides and activated carbon for lead removal: Kinetics, isotherms, column tests, and the role of co-existing ions. Science of The Total Environment 2019, 648, 176-183.).

(2) The removal mechanism of lead by AC was found to be complexed with carboxyl and hydroxyl groups, forming bidentate and monodentate configurations, respectively. On the other hand, lead adsorbs on TiO₂ surface as a tridentate configuration. The co-existing sulfate and phosphate form ternary complexes with lead on the AC and TiO₂ surfaces. According to this information, we set up a surface complexation modeling, which can simulate and predict the lead removal by AC and TiO₂ under different conditions. (please see our articles: Shi, Q.; Sterbinsky, G. E.; Prigiobbe, V.; Meng, X., Mechanistic Study of Lead Adsorption on Activated Carbon. Langmuir 2018, 34, 13565-13573. Zhang, S.; Shi, Q. *; Chou, T.-M.; Christodoulatos, C.; Korfiatis, G. P.; Meng, X., Mechanistic Study of Pb(II) Removal by TiO₂ and Effect of PO₄. Langmuir 2020, 36, (46), 13918-13927.).

(3) Phosphate has been used as a corrosion inhibitor to prevent the release of lead from pipes in the drinking water system. Our study found that phosphate leads to a transformation from precipitated lead species to adsorbed species, indicating that the lead might be transferred from precipitated species on pipelines to adsorbed species on iron oxides particles in the tap water. This information is important because the iron oxides particles are widely found in tap water, which might carry lots of lead ions. (please see our article: Shi, Q., Zhang, S., Ge, J., Wei, J., Christodoulatos, C., Korfiatis, G. P., Meng, X. Lead immobilization by phosphate in the presence of iron oxides: Adsorption versus precipitation. Water Research, 2020, 179, 115853.)

Arsenic removal by co-precipitation using ferric chloride or lime has been widely used due to its low cost and easy operation. Although this topic had been thoroughly studied over decades, we find something new during these procedures:

(1) A membrane made of poly(vinyl alcohol)/poly(acrylic acid) (PVA/PAA) nanofibers has been synthesized to remove lead and cadmium ions from water. This membrane is water stable and shows promising lead and cadmium removal ability. A filtration system filled with a half inch PVA/PAA membrane was used to remove lead from water, which shows more than 100 times lead removal ability than the traditional adsorbents (AC and metal oxides adsorbents) in terms of removed lead per mass of the adsorbent. (please see our articles: Zhang, S.; Shi, Q.; Christodoulatos, C.; Meng, X., Lead and cadmium adsorption by electrospun PVA/PAA nanofibers: Batch, spectroscopic, and modeling study. Chemosphere 2019, 233, 405-413. Zhang, S.; Shi, Q.; Christodoulatos, C.; Korfiatis, G.; Meng, X., Adsorptive filtration of lead by electrospun PVA/PAA nanofiber membranes in a fixed-bed column. Chemical Engineering Journal 2019, 370, 1262-1273.)

(2) The poly(vinyl alcohol)/polyethylenimine (PVA/PEI) nanofiber was used to remove chromate (Cr(VI)) from water. We found that Cr(VI) was reduced to Cr(III) during the removal procedure. The amine group on this nanofiber is responsible for both adsorption and reduction of Cr(VI). Moreover, the sulfate and nitrate have significantly reduced the Cr(VI) removal ability by this nanofiber. The major driven force is the negatively charged Cr(VI) ions and positively charged fiber surface. After the adsorption and reduction, Cr(III) is coordinated with three nitrogen atoms in the amine group on PVA/PEI nanofiber. (please see our article: Zhang, S.; Shi, Q.*; Christodoulatos, C.; Korfiatis, G.; Wang, H.; Meng, X., Chromate Removal by Electrospun PVA/PEI nanofibers: Adsorption, Reduction, and Effects of Co-existing ions, Chemical Engineering Journal 2020, 387, 124179)

Arsenic and fluoride in China are serious public concerns because millions of people are exposed to groundwater with high levels of arsenic and fluoride. During my PhD, I have conducted a series of works to synthesize new lanthanum-based composited materials for the removal of both arsenic and fluoride. I also learned how to utilize advanced techniques to explore molecular structures and build modeling for the prediction.

(1) By using a simple coating method, nano lanthanum oxide hydroxide (LaOOH) was impregnated onto the activated alumina (AA) surface. The modification of LaOOH improves the removal ability of AA for both arsenic and fluoride. This adsorbent can be easily regenerated and still shows good removal ability after 5 cycles of regeneration. The lanthanum contributes adsorption sites for both arsenic and fluoride. Arsenic was adsorbed as a monodentate mononuclear complex on LaOOH surface according to X-ray absorption spectroscopic analysis. The proton number of adsorbed arsenate is 1 or 2. The surface complexation modeling has been set up for the simulation and prediction of arsenic by this adsorbent (please see our articles: Shi, Q.; Huang, Y.; Jing, C., Synthesis, characterization and application of lanthanum-impregnated activated alumina for F removal. Journal of Materials Chemistry A 2013, 1, 12797-12803. Shi, Q.; Yan, L.; Chan, T.; Jing, C., Arsenic Adsorption on Lanthanum-Impregnated Activated Alumina: Spectroscopic and DFT Study. ACS Applied Materials & Interfaces 2015, 7, 26735-26741.)

(2) Since graphene was successfully isolated in 2004, numerous studies regarding reduced graphene oxide/metal oxide (rGO/MO) have been reported, in which rGO/MO was usually considered as a catalyst. We found that that rGO/MO acts as an oxidant rather than a catalyst for the oxidation of adsorbed arsenite, because the oxidation occurs in the absence of other oxidants or radicals. The epoxy group on rGO is confirmed to be responsible for this oxidation, evidenced by the significant correlation between the consumption of epoxy group and the oxidation of arsenite. The mechanism of the adsorption-oxidation has been further proposed and confirmed by the computational calculation: the oxygen atom in the epoxy group is bonded to the adsorbed arsenite (AsO₃), which is consequently oxidized to arsenate (AsO₄). (please see our article: Shi, Q., Yan, L., Jing, C., Oxidation of Arsenite by Epoxy Group on Reduced Graphene Oxide/Metal Oxide Composite Materials. Advanced Science. 2020, 7, 2001928. https://doi.org/10.1002/advs.202001928 (DOI: 10.1002/advs.202001928).

Radium (Ra²⁺) is often found in shallow aquifers due to natural or anthropogenic leakage or spill of brine from deep subsurface. The major factor on the transport and fate of Ra²⁺ in water is the adsorption/desorption process onto gothite surface, which contributes the most adsorption ability in soil particles. By using density functional theory calculation found that the outer-sphere adsorption dominates Ra²⁺ complexation on goethite, suggesting a significant effect of salinity on Ra²⁺ transport in the subsurface. Based on these results, a surface complexation model was developed and validated successfully with literature data. This study provides insights into the mechanism of Ra²⁺ adsorption on soils containing goethite and provides chemical reactions of Ra-goethite surface interaction that can be coupled with a transport model to predict Ra²⁺ migration in the subsurface (Please see our article: Shi, Q., Meng, X., Prigiobbe, V. The Journal of Physical Chemistry C 2020 124 (1), 805-814 DOI: 10.1021/acs.jpcc.9b10451).