Preparation Of Ferric-activated Sluge-based Adsorbent And Its Efficiency On The Removal Of Contaminants From Wastewaters

Sewgae sludge is a byproduct of wastewater treatment processes.The quantity of sludge are increasingly produced all over the world,which will cause the pollution problem in the environment in turn,if proper treatment and disposal are not implemented.The conversion of sewage sludge to adsorbents through pyrolysis might be a promising method,not merely to ecofriendly solve the problem of secondary pollution,but also to allow its reuse in water treatment applications.The purpose of this study is to evaluate the feasibility of preparing SBA from biological sludge by ferric activation,and its application for removing pollutants(heavy metals,phosphate and antibiotics)from wastewater.The potential of ferric-activated SBA adsorption for the sewage treatment was also investgated.Ferric activation was used to produce ferric-acitvated sludge-based adsorbent(SBA)from sewage sludge through pyrolysis.When the pyrolysis temperature increased from 200 °C to 850 °C,the yield of adsorbents decreased from about 90% to 40%,which was due to the release of volatiles and organic matters gasification during pyrolysis.As the mass ratio(activator/dried sludge)increased,the yield of adsorbents reduced,while heating rate and dwell time had less effect on the yield.Ferric activation and higher pyrolysis temperature could enhance the methylene blue adsorption value and iodine value of adsorbents.Faster heating rate and longer dwell time were able to promote the methylene blue adsorption value as well.Ferric activator could facilitate the sludge pyrolysis process,which was related to the promoted fragmentation of sludge organic substances by ferric activator during the pyrolysis,and the formation of iron compounds in the solid products.The ferricactivated sludge derived adsorbents were mesoporous materials.The surface area increased gradually with increasing pyrolysis temperature,and reached a maximum value at 750 °C.The surface area and pore volume of adsorbents increased initially with mass ratio,decreased when the mass ratio exceeded 0.5,and the surface area increased from 2.3 to 126.9 m2/g.The faster heating rate and longer dwell time could promote the porous structure development of adsorbents.The relative elemental contents of C,H and O in the adsorbents decreased as pyrolysis temperature rose,which was due to the release of volatiles with the loss of surface functional group elements(H,O and N)during thermal treatment process.The adsorbents produced at high temperature had greater aromaticity and less polar.The ferric-activated SBA showed higher adsorption capacities for Pb2+ ions,phosphate and tetracycline than non-activated SBA.The adsorption data fitted well with the Pseudo-second order model.The equilibrium data of Pb2+ ions and phosphate fitted the Langmuire model well,and the Freundlich isotherm model best described the tetracycline adsorption process.Thermodynamic tests showed that the phosphate and tetracycline adsorption increased with increasing temperature,suggesting that the adsorption process was spontaneous and endothermic,while the adsorption process of Pb2+ ions was spontaneous and exothermic.The adsorption capacities of ferric-activated SBA for Pb2+ ions,phosphate and tetracycline were almost 0.18,20.4 and 68.6 times higher than those of non-acitvated SBA,respectively.The mechanism of Pb2+ ions removal involved ion exchange,precipitation,and surface complexation.The higher removal of ferric-activated SBA was due to the enhancive surface area that provided a larger number of active sites for adsorbate,the Pb-S precipitates(Pb(SO4)·Pb O),and the formation of the surface complex.The main adsorption mechanism for phosphate might be electrostatic interaction and ligand exchange.The adsorption capcity of ferric-activated SBA increased with the increase of mass ratio(activator/dried sludge),due to the enhacive amout of active sites for phosphate.The adsorption mechanism for tetracycline was probably porefilling effect,hydrogen bonding effect,Van der Waals forces and surface complexation.Ferric-activated SBA showed a higher adsorption capacity for tetracycline than SBA,because the enhanced mesoporous structure favored the diffusion of tetracycline into the pores,the iron oxides and oxygen-containing functional groups in the adsorbents captured tetracycline by surface complexation.Ferric activation could facilitate the capacity of sludge derived adsorbents for removing pollutants from domestic wastewater.In the combined ferric-activated SBA and coagulant process,the optimum sequence was coagulation preceeded by adsorption.By involving the adsorption process,the removals of NH3-N,TP,COD,and SCOD increased by 3.2,1.3,1.7 and 0.6 times,respectively.In the hybird adsorbent and coagulant process,the adsorbents dosed into the treated wastewater could adsorb the pollutants,and also sever as coagulant aids to promote the settleability of flocs.The combination of coagulant and adsorbents could enhance the removal efficiency of pollutants in sewage water,reduce the consumption of coagulants,which will provide a new way of sewage sludge recycling and domestic wastewater pretreatment.