Adsorption And Reductive Reaction Of Organic Contamintants On Carbonaceous Materials

Owing to their large specific surface area, high surface hydrophobicity, and rich surface chemistry, carbonaceous materials can strongly absorb various hydrophobic organic contaminants as well as catalyze their transformation, which have attracted tremendous interest in many environmental applications including environmental analysis and pollution control.On the other hand, the increasing production, application, and disposal of engineered carbonaceous materials (e.g., activated carbons and carbon nanomaterials) will inevitably lead to their release into the natural environment. The released carbonaceous materials will strongly interact with coexisting organic contaminants, significantly influencing their reactivity, mobility, and bioavailability. Thus, understanding the adsorption behaviour and reactivity of organic contaminants on carbonaceous materials is important for predicting the environmental fate of organic contaminants, assessing environmental risks of carbonaceous materials, and expanding their environmental applications.In the present dissertation, the adsorption behaviour and reductive reactions of three typical categories of organic contaminants (chlorinated aliphatic hydrocarbons, mono-aromatic compounds, and pharmaceuticals) on various carbonaceous materials (including activated carbon, graphite, graphene oxide, and multi-walled carbon nanotube) were examined under environmentally relevant aquatic conditions. The structural and surface properties of carbonaceous materials were carefully characterized by complimentary techniques. Combined with batch adsorption and reaction experiment data, the impact of structure and surface chemistry of carbonaceous materials on the adsorption and reductive reaction of organic contaminants was investigated. The main findings achieved were summarized as follows:(1) Our results demonstrated that the pore structure of carbonaceous materials was a key parameter controlling their adsorption of organic contaminants. The adsorption of small-sized mono-aromatic compounds (i.e., phenol and nitrobenzene) on activated carbons was dominated by micropore-filling effect. Thus the sorption affinity was positively correlated with the microporosity of four activated carbons. Contrarily, the adsorption of large-sized pharmaceuticals (i.e., tetracycline and tylosin) on activated carbons increased with increasing mesopore volume due to the size-exclusion effect. Inspired by aforementioned sorption pore effect, we designed a novel silicalite-1 (MFI-type zeolite) coating for solid-phase microextraction. With the help of surface modification, the pore structure of silicalite-1 coating can be fine-tuned for the target contaminant, offering distinct shape-selectivity.(2) We reported, for the first time, that carbon nanomaterials would undergo transformation under natural reducing environments, which consequently altered their colloidal behaviour. The structure changes of graphene oxide (GO) in sodium sulphide (Na2S, a natural reductant) solution were investigated using FTTR, Raman, XPS, and other characterization techniques. GO was found to be reduced and destabilized in the presence of Na2S at environmentally relevant low concentration (0.5 mM). After reacting with Na2S, the surface oxygen-containing groups (mainly epoxy groups) of GO was significantly reduced. The reductive de-oxygenation process increased the surface hydrophobicity of GO and markedly reduced GO colloidal stability.(3) We reported that carbon nanomaterials were able to mediate the reduction of nitrobenzene and hexachloroethane by Na2S. In the presence of 5 mg/L GO, the observed pseudo-first-order rate constant (kobs) for nitrobenzene reduction was raised by nearly 2 orders of magnitude. Consistently, the presence of 10 mg/L GO or CNT enhanced the kobs for the reductive dechlorination of hexachloroethane by a factor of 2 and 4, respectively. The mediation efficiency of GO and CNT was found be much higher than that of graphite (microscale carbonaceous materials) and humic acid (the most common and widely distributed natural mediator). Based on the experimental data and simulation results, we proposed that the strong mediation capacity of carbon nanomaterials was attributed to the enhanced electron transfer by graphene basal planes, as well as the activation of reactant molecules by carbon atoms at the zigzag edges. The influence of solution chemistry (solution pH and dissolved humic acid) on the carbonaceous material-mediated reactions were also investigated. Our findings provide key knowledge for better predicting the transformation and degradation of organic contaminants on carbonaceous materials in complex natural environments.