Study On The Effects Of Natural Organic Matter On Adsorption Of PCBs By Carbon Adsorbents

Abstract:PCBs is an important class of persistent organic pollutants,and due to its long-term residual,non-volatile,high toxicity and accumulation in an organism characteristicsin the environment,it has great harm to human health and ecological environment.Traditional treatment methods of PCBs have various limitations,therefore the study of new treatment technology is imminent.Natural organic matter(NOM)can be widely found in water and soil,and it can form a complex with the environmental pollutants,thus affecting the biochar's adsorption effects of environmental pollutants.In this paper,we use LDPE membrane passive sampling technology to study the effect of natural organic matter on the adsorption of PCBs by carbonaceous adsorbents and its mechanism.The main research results are as follows:(1)Analysis of physical and chemical properties of three kinds of carbon adsorbents(plant ash,nut-shell activated carbon and coal-based activated carbon):the plant ash is honeycomb structure,and the BET specific surface area is the smallest,it's 13.53m2/g,the hole and large holes is the main structure,ant their contribution rate is 96.6%.Nut-shell activated carbon is the intestinal structure,and the wall has a small hole.The BET specific surface area is 204.16m2/g,the contribution rate of mesopores and micropores is 90.5%.Coal-based activated carbon is the flocculent structure,the contribution rate of mesopores and micropores is 91.7%.The BET specific surface area of the largest,388.71m2/g.The surface functional groups of the three kinds of carbon adsorbents are not very different,the common functional groups are-CH3,-OH,-NO2,=C-O-C and-C-X,The plant ash contains benzene ring;nut-shell activated carbon contains-C-O,-C-H,and coal-based activated carbon contains-N-H.(2)The natural organic matter NOM mainly contains fulvic acid,and the average molecular weight of NOM is about 4760Da.Three kinds of carbon adsorbents have a certain ability to adsorb NOM,the order of adsorption capacity is:coal-based activated carbon>nut-shell activated carbon>plant ash.The study of adsorption kinetics shows that the adsorption of NOM by carbon adsorbents is not a simple process,but a slow-fast-slow complex process.(3)Passive sampling LDPE film and PCB membrane-water partition coefficient lgKpe and PCB molecular octanol water partition coefficient lgKow showed a negativecorrelation.When there is NOM in the water environment,the Kpe value has a certain impact,but its impact is not significant.The adsorption isotherm of the three carbon adsorbents for PCBs can be well fitted by the Freundlich adsorption isotherm equation.When adsorption time is 12 months and the system does not exist NOM,the adsorption capacity is one to three orders of magnitude higher than that of 1 month.The adsorption capacity of coal-based activated carbon and nut-shell activated carbon are one to two orders of magnitude higher than that of plant ash.It's indicated that the adsorption did not reach equilibrium for one month,and the surface area of the adsorbent was the main factor affecting the adsorption capacity.When the adsorption time is one month,a small amount of NOM can slightly promote the adsorption of PCBs by three kinds of carbon adsorbents.With the increase of NOM concentration,the inhibitory effect of NOM was increased on the plant ash and nut-shell activated carbon.And NOM could promote the adsorption of small molecule PCBs and inhibit the adsorption of big molecule PCBs by coal-based activated carbon.When adsorption time extended to 12 months,NOM inhibited the adsorption of PCBs by three kinds of carbon adsorbents,and the inhibitory effect of nut-shell activated carbon and coal-based activated carbon almost disappeared.It is shown that when the pore structure of the carbonaceous adsorbent matches the molecular size of the adsorbed PCBs(ie,mainly in micropores),the PCBs molecules can penetrate NOM into the pores when the adsorption time is long enough.(4)The second-order kinetic equation is the best for the adsorption of PCBs by three carbon adsorbents,indicating that the adsorption process is a complex process involving a variety of adsorption reaction.When the NOM is not present in the system,the order of the second-order kinetic parameter qe is coal-based activated carbon>nut-shell activated carbon>plant ash.The results are similar to those of the adsorption isothermal experiment.The apparent diffusion rate of plant ash was slightly larger than that of nut-shell activated carbon and coal-based activated carbon.The average b value of plant ash is slightly smaller than that of nut-shell activated carbon and coal-based activated carbon,indicating that the physical and chemical properties of carbon adsorbents are differents,the adsorption process is not the same.NOM has similar effects on the kinetic curves of adsorbed PCBs with three kinds of carbon adsorbents.With the increase of NOM concentration,the diffusion rate increases slightly,and the transition from slow reaction to the slow reaction is less significant.The reason why the NOM attached to the surface of the carbonaceous adsorbent increases the driving force of the diffusion of PCBs molecules into the adsorbent,while blocking the passage of PCBs molecules into the interior of the adsorbent.The experiment indicating that the surface area and pore size distribution of the carbon adsorbent,the adsorption time,the concentration and nature of the natural organic matter can affect the adsorption process.