Preparation And Forward Osmosis Performance Of Thermo-responsive Sludge Ash Based Hydrogel Draw Agent

With the development of urbanization throughout the country,the excess sludge in sewage treatment plants is also increasing,so the treatment of excess sludge has always been the focus of attention.At this stage,the main method of sludge disposal in our country is incineration,and the remaining sludge after incineration can achieve the greatest stabilization and reduction.In order to improve the utilization value of the remaining sludge ash（SSA）after incineration,this study proposes to use SSA as a modifier to modify the thermo-responsive NIPAM hydrogel to prepare the SSA modified NIPAM hydrogel.Scanning electron microscope（SEM）,fourier infrared spectroscopy（FTIR）,x-ray diffraction（XRD）and inductively coupled plasma mass spectrometry（ICP-MS）are used to characterized the structure and composition of raw sludge,SSA and the modified NIPAM hydrogel.The properties of modified NIPAM hydrogels with different SSA dosages were discussed through swelling,deswelling experiments and kinetic models.Finally,a series of hydrogels were used as draw agent in forward osmosis（FO）process,and the variation of water flux was studied by changing different experimental conditions and through the mathematical model to analyze the difference between it and salt solution draw agent,in order to explore its practical application ability.The research results include the following aspects:（1）Using SSA（incineration at 800℃）as modifier,N-isopropylacrylamide（NIPAM）and acrylic acid（AA）as monomers,semi-interpenetrating network thermo-responsive hydrogels with different SSA content（0 wt.%,0.5 wt.%,1.5 wt.%,2.5 wt.%,3.5 wt.%,4.5 wt.%）were prepared by free radical polymerization method,which were recorded as SAMH0,SAMH0.5,SAMH1.5,SAMH2.5,SAMH3.5 and SAMH4.5,respectively.Firstly,it can be observed from SEM that the hydrogel added with SSA has an uneven porous structure.In addition,the results of XRD analysis showed that the element with the largest proportion in SSA was silicon and the hydrogels with the addition amount of SSA at 2.5 wt.%or above obviously had one more Si O₂peak,so the modification behavior of SSA was highly likely to be dominated by silica.In addition,the characteristic functional groups of NIPAM,AA and SSA could be found in the FTIR analysis of hydrogels,indicating that SSA had been successfully incorporated into the network of hydrogels.Finally,ICP-MS showed that the heavy metal content in the dehydration of the SSA modified NIPAM hydrogel prepared in this study was even lower than the standard for drinking water,indicating that it could be applied safely.（2）The swelling and deswelling properties of NIPAM hydrogels with different SSA dosages were measured by mass method.For the swelling condition,the swelling ratio of SAMH0 was 57.90 g/g.However,with the addition of SSA increasing from 0 wt.%to 4.5wt.%,the swelling ratio of the modified hydrogels showed a trend of increasing firstly and then decreasing.When the addition of SSA was 2.5 wt.%（SAMH2.5）,the swelling ratio reached a maximum of 66.55 g/g,which was 14.94%higher than that of the SAMH0.In addition,in the deswelling process,all the hydrogels can reach a deswelling ratio of more than 90%within 5 minutes under a water bath of 60℃.The swelling and deswelling performance tests showed that the addition of SSA could not only enhance the water absorption and swelling ability of the hydrogel,but also maintain its excellent thermo-responsive dehydration performance.（3）First-order kinetics showed that both SAMH2.5 and SAMH3.5 had high water absorption constant,and their water absorption capacity was better than that of other modified hydrogels.The theoretical swelling ratio of each hydrogel was calculated by the pseudo-second-order swelling kinetic model,and the results showed that the swelling ratio of SAMH2.5 was the highest,and the experimental swelling data of hydrogels was in agreement with the theoretical swelling data.The diffusion model fitting shows that the water molecule diffusion mechanism of each hydrogel in this study is the super case-Ⅱdiffusion mechanism.（4）Modified NIPAM hydrogels with different dosage of SSA were applied as draw agent in FO process.By changing feed solution conditions（such as pure water,Na Cl feed solutions concentration）,the practical application ability of the SSA modified NIPAM hydrogel was explored.This experiment found that the FO water flux was corresponding to the swelling ratio of hydrogels.With the increase of SSA dosage,the FO water flux gradually increased from 1.18 LMH of SAMH0,reached the maximum at 2.33 LMH of SAMH2.5,and then the water flux gradually decreased to 1.41 LMH of SAMH4.5.Similarly,in different concentrations of Na Cl feed solution（2000 ppm,5000 ppm,8000 ppm）consistent phenomenon also could be observed,the water flux is also with the increase of SSA dosing quantity,show a downward trend after rising first,especially SAMH2.5 showed the highest water flux in all conditions.Finally,in the draw agent reuse test,the water flux of SAMH2.5could reach more than 94%of that of the initial application,even though SAMH2.5 had been repeatedly used for four times in FO process.（5）According to the hydrogel-FO filter model,the theoretical osmotic pressure of SAMH2.5 can reach 589.71 atm,while that of pure NIPAM hydrogel is only 213.88 atm.Moreover,it can be found from the performance rate of FO that the osmotic pressure utilization rate of the hydrogel draw agent can reach more than 47%in the Na Cl feed solution of 8000 ppm,which is much higher than that of the traditional salt solution draw agent.