Research On The Removal Of Pollutants In Water By Graphene Oxide-based Nano-adsorbent

With the development of modern industry and agriculture,water pollution is becoming more and more serious.Meanwhile,the types of contaminants are increasingly diversified,and research focus have gradually transferred from traditional pollutants to emerging organic contaminants（EOCs）.In the real water systems,various sorts of contaminants coexist and interact with each other by a variety of interactions,making the forms of contaminants more complicated.At present,manufactured nanomaterials have shown great application potential in the field of water remediation.One of the most typical and well-known nanomaterials is graphene oxides（GOs）;they have been widely employed in water environmental remediation due to their unique characteristics,such as a variety of oxygen-containing functional groups,large specific surface area and relatively low cost of synthesis process.In this study,the adsorption and removal of typical antibiotics-heavy metal combined pollutants,small molecule antibiotics and endocrine disrupting chemicals are systematically studied,by using GO and a new temperature-sensitive modified GO based adsorbent,respectively.（1）Practices like livestock poultry wastewater and industrial wastewater centralizing lead to combined pollution of antibiotics and heavy metal in real waters.Here,GO was applied as an adsorbent to remove both single and combined contaminants of typical antibiotics（sulfadiazine（SDZ）and tetracycline（TC））,heavy metals（Cu（II）and Zn（II））from water.The interface interactions among the adsorbent,antibiotics and heavy metals were clarified through both experimental characterizations and theoretical computations.Interaction strength of contaminants towards GO followed the order of Cu（II）>Zn（II）?TC>SDZ（coordination of heavy metal-GO in sp³ regions;π-πstacking of antibiotic-GO in sp² and H-bonding in sp³regions）.In the combined contaminants,heavy metals demonstrated enhancement effect for the adsorption capacities（Q_e）for antibiotics,but antibiotics only displayed slight promotion to Q_e for heavy metals.Coexisting salt ions,especially Ca²⁺,inhibited the adsorption.The common coexisting natural organic matter（NOM）humic acid provided more sites for heavy metal uptake but competed with antibiotics for adsorption.After six adsorption release cycles,re-adsorption capacities still kept high（100 mg/L of Ca Cl₂ in mixed solvent of alcohol and water（1:1 in volume）was used as eluent）,implying the feasibility of GO in the removal of such sort of combined contaminants.（2）Through the above studies,it was found that GO had potential for the removal of combined contaminants,yet it still had limitations,including the following points:（i）GO had good dispersibility and was difficult to regenerate after use,which increased the cost of the adsorption and secondary water pollution risk.（ii）When desorbing pollutants,especially organic pollutants,in order to weaken the binding interactions between adsorbents and pollutants,it was necessary to use organic solvents,which further increased the environmental risk.Consequently,in the second part of the work,thermosensitive polymer PNNPAM was anchored onto GO sheets using a“graft-to”strategy to synthesize a new sort of facily recyclable adsorbents PNNPAM-g-GO（P-GO）,which was systematically investigated for selective adsorption of two small molecular EOCs（norfloxacin（NOR）and bisphenol A（BPA））with significant differences in octane-water distribution coefficients.The results of interfacial interactions and theoretical calculation showed that the original adsorption sites with higher binding energy for NOR could be replaced by PNNPAM with relatively smaller binding energy through a"adsorption site replacement"strategy;in addition the large adsorption capacity for NOR was maintained by increasing the number of adsorption sites on PNNPAM chains.For BPA,the abundant H-bonding and hydrophobic association sites on PNNPAM had a stronger binding with BPA,which increased the adsorption capacity of BPA.Finally,by using the temperature-sensitive characteristics of the new adsorbent,regeneration only using water as eluent and via adjusting the temperature,without additional chemicals,was achieved.Therefore,water treatment cost and environmental risk were both reduced.In summary,GO-based materials have shown considerable removal efficiency for multiple pollutants and have great application potential.The modification of GO by polymer chains not only enriches the understanding of the effect of molecular structure of adsorbents on removal of pollutants,but also provides practical guidance for the regulation of adsorbents parameters in practical operation.We sincerely hope that this study can do some help to the development and application of GO-based environmental functional materials.