Treatment Of Chromium-containing Wastewater On Forward Osmosis With Sewage Sludge Ash Thermo-sensitive Hydrogel As Draw Agent

Incineration is a mature technology for the harmless treatment of sewage sludge（SS）,whose product obtained is sewage sludge ash（SSA）.With the advancement of urbanization and industrialization,the amounts of SSA have greatly increased.Therefore,the utilization of SSA is of great significance to avoid eventual environmental impact.In recent years,temperature-sensitive hydrogels have been extensively studied in many fields due to their unique property of undergoing phase transitions under temperature stimulation.However,because of its poor water absorption capacity,this hydrogel needs to be modified to improve its properties.In order to prepare a temperature-sensitive hydrogel with strong water absorption capacity and good recycling performance,this study used a free radical polymerization method to synthesize a temperature-sensitive hydrogel modified by introducing SSA into the hydrogel and applied to the treatment of chromium-containing wastewater in the forward osmosis process.The research of this study mainly includes the following contents:（1）The effects of the incineration temperature,particle size and dosage of SSA on the properties of the hydrogel were studied by taking the water absorption rate and dewatering rate as the evaluation indicators,and the optimal preparation conditions of the hydrogel were determined.The research results show that the synthesized hydrogel（SSA-TSH）has the best water absorption capacity when the incineration temperature of SSA is 800℃,the particle size is 100-150 mesh,and the dosage is 2.5wt%.The hydrogel prepared in this condition maintains good dewatering properties.（3）The structures and heavy metal contents of SS and SSA were characterized by scanning electron microscopy（SEM）,Fourier transform infrared spectroscopy（FTIR）and X-ray fluorescence spectroscopy（XRF）.The results show that the structure of SS is loose and the volume density decreases after incineration.SS and SSA contain high content of Si O₂,Fe₂O₃and Al₂O₃.SEM,FTIR and differential scanning calorimetry（DSC）were used to characterize the physical and chemical structures of the hydrogels before and after modification.SEM results showed that Si O₂in SSA produced additional cross-linking with the hydrogel matrix and improved the pore structure of the hydrogels.In the FTIR diagram,the Si-O-H characteristic peak in SSA-TSH hydrogel proved that SSA was successfully introduced into the hydrogel.DSC results showed that the minimum critical solution temperature（LCST）of hydrogel increased from 35.58℃to 38.26℃with the addition of SSA,and the temperature sensitivity decreased.（3）The water absorption and dewatering properties of the unmodified hydrogel（TSH）and SSA-TSH were evaluated,and their adsorption and desorption abilities for Cr（VI）,TP,NH₃-N and COD pollutants were investigated.The experimental results show that the equilibrium swelling ratio of the SSA-TSH hydrogel is 65.5 g/g,which is higher than that of TSH（53.0 g/g）.The study of water absorption kinetics shows that the water absorption behavior of both hydrogels is dominated by chemical reaction.The dewatering experiments showed that the dewatering rates of TSH and SSA-TSH were over 95.3%and 94.7%within 5 min,respectively,and the dehydration state could be basically reached within 10 min.The results of adsorption and desorption experiments revealed that the adsorption rate of the SSA-TSH for the four pollutants was higher than that of the TSH,while its desorption rate was lower than that of the TSH,indicating that that the modified SSA-TSH hydrogel has stronger binding ability with Cr（VI）,TP,NH₃-N and COD.（4）TSH and SSA-TSH hydrogels were used as draw agents to treat chromium-containing wastewater in FO system.The effects of simulated wastewater with different Cr（VI）concentrations（0,10,20,40 mg/L）on removal efficiency and water flux of Cr（VI）,TP,NH₃-N and COD in two FO systems were investigated.And the recycling properties of two hydrogels were studied.The research results show that:1)The removal ratios of Cr（VI）,COD and TP by the two FO systems are all over 96%with influence of the concentration of Cr（VI）.Whereas,The removal ratios of NH₃-N was only about 70%,and gradually decreased with the increase of Cr（VI）concentration.In general,the removal ratios of Cr（VI）,COD and NH₃-N by the SSA-TSH-driven FO system were all higher than those of the TSH-driven FO system.2)In the two hydrogel-driven FO systems,the FO membrane can retain most of the pollutants,while some of the pollutants after passing through the membrane can be further removed by the hydrogel.3)In the simulated wastewater with different Cr（VI）concentrations,the water flux of the SSA-TSH FO system in the first 1 h all reached more than 2.8 LMH,which was higher than 2 LMH of the TSH.In addition,the 24 h average water fluxes of both FO systems showed a decreasing trend with Cr（VI）.4)After 3 cycles,the water flux reduction value of the SSA-TSH after reuse is only 3.3%of the initial use,which is lower than 5.7%of the pure TSH hydrogel,indicating that the stability of the SSA modified temperature sensitive hydrogel is better has better stability and recycling ability than that of the pure hydrogel.