Photocatalytic, Oxidative Cleavage of C-C Bond in Lignin Models and Native Lignin.

It is challenging to realize the selective C-C bond cleavage of lignin ß-O-4 linkages for production of high-value aromatic chemicals due to its intrinsic inertness and complex structure. Here we report a light-driven, chlorine-radical-based protocol to realize the oxidative C-C bond cleavage in various lignin model compounds catalyzed by commercially available TPT and CaCl2, achieving high conversion and good to high product yields at room temperature. Mechanistic studies reveal that the preferential activation of Cß-H bond facilitates the oxidation and C-C bond cleavage of lignin ß-O-4 model via chlorine radical. Furthermore, this method is also applicable to the depolymerization of natural lignin extracts, furnishing the aromatic oxygenates from the cleavage of Cα-Cß bonds. This study provides experimental foundations to the depolymerization and valorization of lignin into high value-added aromatic compounds.

Anaerobic oxidation of arsenite by bioreduced nontronite.

The redox state of arsenic controls its toxicity and mobility in the subsurface environment. Understanding the redox reactions of arsenic is particularly important for addressing its environmental behavior. Clay minerals are commonly found in soils and sediments, which are an important host for arsenic. However, limited information is known about the redox reactions between arsenic and structural Fe in clay minerals. In this study, the redox reactions between As(III)/As(V) and structural Fe in nontronite NAu-2 were investigated in anaerobic batch experiments. No oxidation of As(III) was observed by the native Fe(III)-NAu-2. Interestingly, anaerobic oxidation of As(III) to As(V) occurred after Fe(III)-NAu-2 was bioreduced. Furthermore, anaerobic oxidization of As(III) by bioreduced NAu-2 was significantly promoted by increasing Fe(III)-NAu-2 reduction extent and initial As(III) concentrations. Bioreduction of Fe(III)-NAu-2 generated reactive Fe(III)-O-Fe(II) moieties at clay mineral edge sites. Anaerobic oxidation of As(III) was attributed to the strong oxidation activity of the structural Fe(III) within the Fe(III)-O-Fe(II) moieties. Our results provide a potential explanation for the presence of As(V) in the anaerobic subsurface environment. Our findings also highlight that clay minerals can play an important role in controlling the redox state of arsenic in the natural environment.

Concentrations and Exposure Evaluation of Metals in Diverse Food Items from Chengdu, China.

A total of 520 food samples belonging to 29 food types and 63 drinking water were collected in Chengdu market of China in 2014 to investigate the concentrations of 11 metals, and to assess the related exposure to the local consumers by estimating the hazard quotient and carcinogenic risk (CR). The results showed that metals concentrations in drinking water were below the limit values suggested by the Ministry of Health of the People's Republic of China, and FAO/WHO (Food and Agriculture Organization of the United Nations/World Health Organization). While As, Cd, and Cr were found at concentrations higher than the limit values in some of the foodstuffs. Children in Chengdu intake more metals compared to adults, with the same order of Mn > Zn > Cu > Sr > Cr > Ni > As > Cd > Pb > Co > Sb. Among all of the diverse food, rice, flour, and fish and seafood were the primary sources to intake metals for Chengdu residents. Residents in Chengdu are subjected to both carcinogenic and non-carcinogenic risks based on the calculated HI and CR values, especially for children. Finally, total daily metals intakes for both children and adults were calculated based on the current study and our previous studies, including consumption of food and drinking water and intake of outdoor and indoor dust. Dietary exposure is the predominant exposure route to metals for Chengdu residents, accounting for more than 75.8% of the total daily metals intakes for both children and adults.

Degradation of methylmercury into Hg(0) by the oxidation of iron(II) minerals.

Mercury contamination is a global concern, and the degradation and detoxification of methylmercury have gained significant attention due to its neurotoxicity and biomagnification within the food chain. However, the currently known pathways of abiotic demethylation are limited to light-induced photodegradation process and little is known about light-independent abiotic demethylation of methylmercury. In this study, we reported a novel abiotic pathway for the degradation of methylmercury through the oxidation of both mineral structural iron(II) and surface-adsorbed iron(II) in the absence of light. Our findings reveal that methylmercury can be oxidatively degraded by reactive oxygen species, specifically hydroxyl and superoxide radicals, which are generated from the oxidation of iron(II) minerals under dark conditions. Surprisingly, Hg(0) trapping experiments demonstrated that inorganic Hg(II) resulting from the oxidative degradation of methylmercury was rapidly reduced to gaseous Hg(0) by iron(II) minerals. The demethylation of methylmercury, coupled with the generation of Hg(0), suggests a potential natural attenuation process for methylmercury. Our results highlight the underappreciated roles of iron(II) minerals in the abiotic degradation of methylmercury and the release of gaseous Hg(0) into the atmosphere.

Capacity, stability and energy requirement of divalent mercury uptake by non-methylating/non-demethylating bacteria.

Methylmercury (MeHg) uptake by demethylating bacteria and inorganic divalent mercury [Hg(II)] uptake by methylating bacteria have been extensively investigated because uptake is the initial step of the intracellular Hg transformation. However, MeHg and Hg(II) uptake by non-methylating/non-demethylating bacteria is overlooked, which may play an important role in the biogeochemical cycling of mercury concerning their ubiquitous presence in the environment. Here we report that Shewanella oneidensis MR-1, a model strain of non-methylating/non-demethylating bacteria, can take up and immobilize MeHg and Hg(II) rapidly without intracellular transformation. In addition, when taken up into MR-1 cells, the intracellular MeHg and Hg(II) were proved to be hardly exported over time. In contrast, adsorbed mercury on cell surface was observed to be easily desorbed or remobilized. Moreover, inactivated MR-1 cells (starved and CCCP-treated) were still capable of taking up nonnegligible amounts of MeHg and Hg(II) over an extended period in the absence and presence of cysteine, suggesting that active metabolism may be not required for both MeHg and Hg(II) uptake. Our results provide an improved understanding of divalent mercury uptake by non-methylating/non-demethylating bacteria and highlight the possible broader involvement of these bacteria in mercury cycling in natural environments.

Spatial distribution, pollution assessment, and source identification of heavy metals in the Yellow River.

The discharge of pollutants into the Yellow River has been strictly controlled since 2013 due to the severe pollution. Thus, the overall water quality of the Yellow River has been becoming better year by year. However, the contamination status and source identification of heavy metals from the entire Yellow River remains unclear. Our results demonstrated that heavy metal contents in sediments showed little changes over time, whereas significant alleviation was observed in surface water compared to the reported metal concentrations before 2013. No heavy metal contamination was observed in surface water, and the distribution of all heavy metals in surface water fluctuated along the mainstream without a significant spatial difference. Heavy metals in sediments were assessed as low to moderate contamination degree. The majority of heavy metal concentrations were higher in the upstream and midstream than that in the downstream. Besides anthropogenic activities, the natural contribution from soil erosion of the Loess Plateau was also an important source of heavy metals in the Yellow River sediments. Our results highlight that control of anthropogenic activities and soil erosion of the Loess Plateau are necessary measures to reduce heavy metals in the Yellow River.