Study On The Co-pyrolysis Of Sludge-Rice Hull And The Adsorption Properties Of Biochar

With the gradual development of urbanization in China,sewage treatment facilities are becoming more and more refined,and sludge production is increasing year by year.Harmful substances such as toxic organic substances,heavy metals,pathogenic microorganisms and so on contained in sludge will cause serious pollution to air,soil and water if they are discharged directly without proper treatment.China's“13th Five Year renewable energy development plan”clearly points out that urban sewage sludge should be treated Steadily and harmlessly,and the utilization of resources is encouraged.Sludge disposal and resource utilization have become an important focus of social attention.Pyrolysis,as an effective way of sludge disposal,can not only eliminate harmful organic matter and pathogenic microorganisms in sludge,but also stabilize heavy metals in sludge.The addition of rice husk can increase the stabilization effect of heavy metals.Biochar,the pyrolysis product,is a kind of material with high specific surface area,porous structure and rich oxygen-containing functional groups,which is widely used in the treatment of heavy metals and organic polluted water.The biochar prepared by synergistic pyrolysis of sludge and rice husk not only treat sludge innocuously,but also realize the resource utilization of sludge and rice husk,and realize the environmental protection theme of"treating waste by waste".In this paper,the co-pyrolysis characteristics of sludge and rice husk were studied.The physical and chemical properties of co-pyrolysis solid products(biochar)and the moving-transiting rule of heavy metals in biochar were studied.The preparing conditions of the biochar were optimized by the RSM for obtaining the best biochar that have the highest adsorption capacity for direct red 23(DR23).The sludge-rice husk biochar which has the best adsorption capacity was modified to improve the adsorption capacity.And the adsorption process and mechanism of sludge-rice husk biochar for DR23 were discussed.The main research contents and achievements are as follows.(1)Through the TG-MS analysis of the pyrolysis process of sludge,rice husk and sludge-rice husk mixture with different rice husk content,it was found that the addition of rice husk promoted the decomposition of volatile substances,increased the initial pyrolysis temperature and reaction activity,and decreased the final pyrolysis temperature.The main gas products of co-pyrolysis of sludge and rice husk were H₂,CH₄C₂H₂and CO₂in the order:CH₄>CO₂>H₂>C₂H₂.the increase of rice husk inhabited the H₂,CH₄and C₂H₂release.The interaction between rice husk and sludge suppressed gases generation but made more hydrocarbon enrich in the biochar,which increased the possibility of the residue as an absorbent.It worth noting when the content of rice husk is 30%,the synergistic effect is the most significant.(2)By Comparising and analysing the physicochemical properties of sludge biochar and co-pyrolysis biochar with different rice husk content,It was found that the addtion of rice husk increased specific surface area and pore structure of biochar,and also increased the carbonation,aromatization and active sites of biochar.The synergistic effect between rice husk and sludge promote more hydrocarbons enrich in S7R3C,which made the adsorption potential of S7R3C the most remarkable.Due to the decrease of pyrolysis yield,the total contents of Cu,Pb,Ni,Zn in biochars were higher than those of original sludge,but the leaching amount were obviously lower than those of original sludge,all of which did not exceed the first class standard of sewage discharge.The leaching amount of Cu,Zn in sludge and Cu in biochar which derived from pure sludge exceeded the first class standard of sewage discharge,The leaching amout of Cu,Zn,Ni and Pb in all the sludge-rice husk biochar with different rice husk ratio did not exceed the first class standard of sewage discharge.The results of Tessier extraction showed that pyrolysis can make the heavy metals in the sludge transfer to a more stable speciation,and the addition of rice husk made the speciation of heavy metals more stable.The more rice husks are added,the more heavy metals will be transformed into the high binding state.The RAC results showed that the Pb and Zn in the original sludge showed middle risk.The four heavy metals in all the biochar with different rice husk ratio all showed low risk or no risk,and the RAC index was inversely proportional to the amount of rice hull addition.(3)The preparation process of sludge-rice husk biochar was optimized by response surface method(RSM).The optimized preparation conditions were obtained as follows:pyrolysis temperature 651.356?,heating rate 10?/min,retention time4.677 h.The test of normal distribution,residual and predicted fitting,predicted and actuall fitting showed that the fitting result was excellent.The biochar prepared under the optimal conditions and non-optimal biochar were compared and analyzed in the aspects of element content,surface morphology,appearance structure and surface functional group.The results showed that the increase of pyrolysis temperature and retention time was beneficial to the increase of specific surface area and pore number of biochar,inceased the formatiion of?-?bonds between biochar and molecular of dye,thus increasing the adsorption capacity,meanwhile reduced the surface functional groups,which reduced the interaction between surface functional groups on biochar and that in dye molecules,thus negatively affecting the adsorption capacity.The promotion and inhibition of these two factors made the optimal preparation conditions.(4)The biochar(named OS7R3C)prepared in the chapter 3 was modified by oxalic acid,tartaric acid,KOH,KMn O₄,H₂O₂,tetradecyl-n,n-trimethylammonium bromide,and Zr(OC₃H₇)₄solution respectively.the results showed that the biochar modified by oxalic acid(OAOS7R3C)and Zr(OC₃H₇)₄solution(ZOS7R3C)had the best adsorption capacity on DR23 solution.it was found that oxalic acid solution partially dissolved the mineral components in OS7R3C,resulting in the increase of pore structure,specific surface area and pore size.The esterification of carboxyl in oxalic acid and hydroxyl groups in OS7R3C formed a new ester group and alsl introduced more carboxyl.These changes in physical and chemical structure improved the adsorption capacity of DR23 on OAOS7R3C.Zr(OC₃H₇)₄formed Zr O₂nanoparticles on the surface of OS7R3C,which made the adsorption capacity of DR23 onto ZOS7R3C higher.(5)The adsorption capacity of DR23 onto OS7R3C and OAOS7R3C exceeded most of the adsorbents reported in previous literatures.The adsorption capacities of DR23 onto the two biochars were related to p H,particle size,initial dye concentration,adsorption temperature and biochar dose.The adsorption capacities of DR23 onto the two biochars can be improved by setting these adsorption conditions reasonably.The experimental results showed that the lower p H is more beneficial to the adsorption of DR23,and the increase of temperature was beneficial to the improvement of adsorption capacity,but the increase is not obvious.Infrared spectra,Zeta potentials and XPS spectra changed obviously before and after adsorption of DR23,indicating that a large number of DR23 were attached to the surfaces of the two biochars by chemical adsorption.-OH,-COOH on OS7R3C surface and-OH,-COOH and-COOR on OAOS7R3C surface played an important role in the adsorption of DR23.The adsorption data were best fitted with iostherm langmuir model and kinetic pseudo-second-order model.The thermodynamic model confirmed that the adsorption were spontaneous and endothermic,and the adsorption processes were mainly chemical adsorption.The adsorption mechanism of OS7R3C and OAOS7R3C for DR23 mainly included surface participation,attachment onto minerals,electrostatic interaction,functional group interaction and?-?bond binding.