Lin Duan

Resistant desorption of hydrophobic organic contaminants in typical Chinese soils: Implications for long-term fate and soil quality standards†

Soil contamination is an enormous problem in China and severely threatens environmental quality and food safety. Establishing realistic soil quality standards is important to the management and remediation of contaminated sites and must be based on thorough understanding of contaminant desorption from soil. In the present study, we evaluated sorption and desorption behaviors of naphthalene, phenanthrene, atrazine, and lindane (four common soil contaminants in China) in two of the most common Chinese soils. The desorption of these compounds exhibited clear biphasic pattern-a fraction of contaminants in soil was much less available to desorption and persisted much longer than what was predicted with the conventional desorption models. The unique thermodynamic characteristics associated with the resistant-desorption fraction likely have important implications for the mechanism(s) controlling resistant desorption. Experimental observations in the present study are consistent with our previous work with chlorinated compounds and different adsorbents and could be well modeled with a biphasic desorption isotherm. We therefore suggest that more accurate biphasic desorption models should be used to replace the conventional linear sorption/desorption model that is still widely adopted worldwide in contaminant fate prediction and soil quality standard calculations.

Colloidal stability of reduced graphene oxide materials prepared using different reducing agents

The environmental transport and fate of nanomaterials are largely dependent on their colloidal stability. To date, the stability of reduced graphene oxide (RGO) materials is not well understood. We examined the electrokinetic properties and aggregation kinetics of three RGOs synthesized by reducing graphene oxide (GO) with N2H4, NaBH4, and L-ascorbic acid. In NaCl solution, the critical coagulation concentrations of the materials correlated reasonably with their C/O ratios. However, the increased aggregation tendency of RGOs was caused mainly by their increased hydrophobicity rather than decreased surface charge negativity. For both monovalent and divalent cations, less densely hydrated cations were more effective in causing aggregation than more densely hydrated cations (e.g., K+vs. Na+, and Ca2+vs. Mg2+) owing to the greater efficiency of the former in neutralizing surface charges and the bridging effect in the case of Ca2+. The RGOs exhibited high stability in artificial surface water, whereas different degrees of aggregation were observed in artificial groundwater. Strikingly, in artificial groundwater the aggregation tendency of the materials correlated poorly with the degree of reduction but strongly with the types and concentrations of surface O functionalities, which determined the nature and strength of interactions between RGO and cations in solution. The findings further underline that the stability of nanomaterials is controlled by the complex interplay between nanomaterial surface properties and solution chemistry factors.

The oxidation capacity of Mn3O4 nanoparticles is significantly enhanced by anchoring them onto reduced graphene oxide to facilitate regeneration of surface-associated Mn(III)

Metal oxides are often anchored to graphene materials to achieve greater contaminant removal efficiency. To date, the enhanced performance has mainly been attributed to the role of graphene materials as a conductor for electron transfer. Herein, we report a new mechanism via which graphene materials enhance oxidation of organic contaminants by metal oxides. Specifically, Mn3O4-rGO nanocomposites (Mn3O4 nanoparticles anchored to reduced graphene oxide (rGO) nanosheets) enhanced oxidation of 1-naphthylamine (used here as a reaction probe) compared to bare Mn3O4. Spectroscopic analyses (X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy) show that the rGO component of Mn3O4-rGO was further reduced during the oxidation of 1-naphthylamine, although rGO reduction was not the result of direct interaction with 1-naphthylamine. We postulate that rGO improved the oxidation efficiency of anchored Mn3O4 by re-oxidizing Mn(II) formed from the reaction between Mn3O4 and 1-naphthylamine, thereby regenerating the surface-associated oxidant Mn(III). The proposed role of rGO was verified by separate experiments demonstrating its ability to oxidize dissolved Mn(II) to Mn(III), which subsequently can oxidize 1-naphthylamine. The role of dissolved oxygen in re-oxidizing Mn(II) was ruled out by anoxic (N2-purged) control experiments showing similar results as O2-sparged tests. Opposite pH effects on the oxidation efficiency of Mn3O4-rGO versus bare Mn3O4 were also observed, corroborating the proposed mechanism because higher pH facilitates oxidation of surface-associated Mn(II) even though it lowers the oxidation potential of Mn3O4. Overall, these findings may guide the development of novel metal oxide-graphene nanocomposites for contaminant removal.

Release of hexachlorocyclohexanes from historically and freshly contaminated soils in China : Implications for fate and regulation

Hexachlorocyclohexanes (HCHs) were produced and used in large quantity worldwide and are common soil pollutants. In this study, desorption of alpha-HCH and gamma-HCH from two soil samples collected from a historical pesticide plant in Tianjin, China, was examined. As a comparison, desorption of freshly sorbed gamma-HCH was examined, using five typical Chinese soils. Strong resistant desorption was observed for both historically contaminated and freshly contaminated soils, and desorption results were well modeled with a biphasic desorption isotherm. The unique thermodynamic characteristics associated with the desorption-resistant fraction indicated that physical constraint within soil organic matrices was likely the predominant mechanism controlling resistant desorption. Resistant desorption could have significant effects on fate and exposure of HCHs in soil environment. More accurate biphasic desorption models that take into account of the resistant desorption can be used to facilitate regulating, management and remediation of HCH-contaminated sites.

Reply to the ‘Comment on “Colloidal stability of reduced graphene oxide materials prepared using different reducing agents”’ by M. Moazzami Gudarzi, Environ. Sci.: Nano, 2017, 4, DOI: 10.1039/C6EN00424E

This article provides our response to the comment by Gudarzi regarding our interpretation of the results of particle aggregation in NaCl presented in our recent publication (Qi et al., Environ. Sci.: Nano, 2016, 3, 1062–1071) reporting on the colloidal stability of reduced graphene oxide materials.