The Fabrication And Composite Modification Of Titanium Coagulants Through Gelation With Acetylacetone

Inorganic titanium(Ti)salts are a type of promissing coagulant,because of the two merits:excellent coagulation performance and the reusability of the coagulation sludge.However,Ti salt coagulants also have some drawbacks in water treatment,including low effluent pH and uncontrollable formation of hydrolysates.To overcome these shortcomings,prehydrolysis has been attempted to improve the Ti-based coagulants.However,the application of prehydrolyzed polytitanium chloride(PTC)was still hampered by a narrow solution pH and coagulant dose range.Further modification of Ti-based coagulants is still needed.This study adopted a sol-gel method to modify TiCl4(TC)with acetylacetone(AcAc)as a hydrolysis inhibitor.The role of AcAc was to control the hydrolysis-condensation process of the titanic precursors.One of the merits of the sol-gel process is its easy tunability in components.The resultant titanium xerogel(TXC)showed a high-efficient coagulation performance.The physicochemical structure,coagulation behavior,and the underlying mechanism on the improved coagulation performance were investigated in this study.The application potential of TXC was evaluated in several simulated and real wastewaters.The TXC was further improved by co-polymerization with an organic flocculant.The main research contents and conclusions are listed as follows:(1)TXC was successfully prepared with the sol-gel method by choosing ethanol as the solvent and TiCl4 as the precursor.The molar ratio of AcAc/TiCl4 and H2O/TiCl4 was adjusted in the range of 1/32?3/8 and 1?8,respectively.The resultant TXC had an excellent water-solubility and could hydrolyze quickly once dosed into wastewater.The mass fraction of effective Ti was in the range of 20?25%.AcAc was embedded in the Ti-O skeleton and facilitated the formation of a net-work structure in the hydrolysates.As compared with PTC and TC,TXC had a larger surface charge density and a higher isoelectric point.(2)Simulated waters composed of Si microspheres,humic acid(HA),HA-kaolin,N/P,and colloidal chromium(Cr),were utilized to evaluate the resultant TXC.Experimental results indicate that the superiority of TXC over PTC and TC was mainly shown in the following aspects:higher coagulation efficiency,lower residual Ti concentration,wider applicable dose/solution pH ranges/pollutant concentration,and mild decrease in effluent solution pH.For waters of low turbidity and low pH(initial turbidity was 5 NTU,and the solution pH was 5.8),the coagulation efficiency for turbidity removal could reach 90%even at a low dose(1.5 mg Ti/L),and re-stabilization did not appear even at a high dose(12.5 mg Ti/L).The removal efficiency for Cr and N in actual tanning wastewater and soy protein wastewater by TXC was much higher than that of the commercial polymeric ferric sulfate(PFS).In the treatment of Si microspheres and HA-kaolin simulated waters,the flocs formed by TXC were the largest and fastest among TXC,TC,and PTC.The TXC flocs had a stronger resistance to shear forces than the floc formed from TC and PTC,but a lower recoverability after broken.The gelation method was superior to the prepolymerization approach,due to the following reasons:(1)the release of acidic gas weakened the dramatical pH reduction after coagulation;(2)the hydrolysate formation regulated by the added AcAc was favorable to charge neutralization and sweep flocculation;(3)the embedded AcAc in TXC enhanced the removal of organic matter and metal ions by chelation.(3)The practical application potential of TXC was evaluated with a cyanobacteria blooming water,which contains Microcystis aeruginosa.The coagulation efficiency,applicable conditions,effluent toxicity,and the release of toxic microcystins(MCs)during floc storage were investigated.Under acidic to neutral conditions,TXC showed a higher efficiency of approximately 99%for cyanobacteria and a lower residual Ti concentration than the widely-used commercial polyferric sulfate(PFS)and polyaluminium chloride(PAC).The floc size generated by TXC could reach 1300?m within 1 min.Compared with the blank control,coagulation process led to a severe damage to the cells.The MCs in the PAC and PFS systems rapidly increased to 34.5?g/L within 4 days and then stabilized at 40.4 ?g/L,which was similar to that observed in the blank control.However,in the TXC system,the extracellular MCs reached the maximum level(30.2 ?g/L)on the 6th day and rapidly decreased to 7.4 ?g/L on the 16th day.This was attributed to the potent photo-activity of AcAc,which was released from the hydrolysis of TXC.The results demonstrate that TXC is a promising dualeffect coagulant for treatment of cyanobacteria-laden water.(4)The sludge conditioning performance of TXC,PAC,PFS,and polyacrylamide(PAM)were systematically compared in terms of specific resistance to filtration(SRF),the content of protein and polysaccharide in extracellular polymeric substance(EPS),and floc morphology.The sludge conditioning ability of the three inorganic coagulants was in the order of PAC>TXC>PFS.The solid content after conditioning increased from 4.4%to 13.3%,12.4%and 16.4%by TXC,PFS,and PAC(250 mg/(g VSS)),respectively.The conditioning ability was positively related to their surface charge density(PAC(76 mmol/g)>TXC(22 mmol/g)>PFS(12 mmol/g)).The combining capacity between TXC/PFS and proteins was stronger than that of PAC,while opposite for polysaccharide.The content of polysaccharide in loosely bounded EPS was the key factor affecting the sludge dewatering performance but not the coagulated floc size.The charge neutralization ability and the chelating ability with organic matter co-determined the sludge conditioning performance of inorganic coagulants.(5)A series of composite coagulants were fabricated by co-polymerization with poly(diallyl-dimethyl-ammonium chloride)(PDADMAC)in the sol-gel process.The addition of PDADMAC had no significant effect on the Ti content(25.0-26.8%)and the hydrolysate structure.With the increase of PDADMAC,the pHicp values and surface charge density were increased.As a typical representative of turbidity-organic matter coexistence water,Escherichia coli(E.coli)solution was employed to evaluate the coagulation performance of the resultant composites.With the removal of E.coli and the residual turbidity as evaluation index,the composition of PDADMAC to TXC was proved to result in an enhanced resistance to organic matter load and pH variation.The improvements could be attributed to the three effects of PDADMAC:strengthened charge neutralization and adsorption bridging,and weakened complexation between organic matter and titanium.This study demonstrates that gelation with AcAc was a better approach for the modification of titanium salt coagulant than prepolymerization.This approach expanded the application range of Ti-salt coagulant in water pollution control.In addition,the gelation method shed a light on a novel modification way for other water treatment chemicals.