| Based on the wide application prospect of titanium based coagulants, the coagulation efficiency, coagulation behavior and coagulation mechanisms of titanium tetrachaloride (TiCl4) were systematically investigated. Additionally, floc characteristics of TiCl4were investigated in terms of floc growth rate, floc size, strength, recoverability and floc structure through on-line monitoring of kinetic coagulation process. New and stable polytitanium chloride (PTC) coagulants were also synthesized in this study and the preparation process and conditions of PTC were investigated. The coagulation performance, coagulation behavior and floc characterisitcs were also evaluated for different water treatment. Finally, sludge produced after coagulation process was calcined to produce titanium dioxide (TiO1). Physicochemical characteristics and photo catalytic activity of TiO2were investigated. The main research contents and conclusions are as follows:(1) Coagulation efficiency of TiCl4were investigated in terms of residual turbidity, UV254or UV278removal and DOC removal etc. for the treatment of humic acid-kaolin sydnthetic water, fuvic acid-kaolin synthetic water, reservoir real warer and Xiaoqinghe real water. The effect of coagulant dose and initial solution pH on coagulation performance was investigated. Results showed that TiCl4is an efficient coagulant for water treatment, which can effectively remove the particulates and organic matter in water. The organic matter removal efficiency of TiCl4was obviously higher than that of the conventional aluminium (Al) and iron (Fe) coagulants. At low TiCl4doses, the main coagulation mechanism is adsorption and charge neutralization, while at high TiCl4doses, the main coagulation mechanism changes to sweep flocculation. The organic matter removal efficiency of TiCl4was more stable under various initial solution pH conditions than the conventional coagulants. Compared to the conventional coagulants, TiCl4is superior in terms of turbididy removal under high pH conditions and has wider pH application range.(2) Flocs formed by TiCl4were characterized in terms of floc growth rate, size, strength factor and recovery factor through on-line monitoring of kinetic coagulation process using laser light scattering technology Mastersizer2000(Malvern, UK). The influences of coagulant dose, initial solution pH, shear force and shear period on floc characterisitics were also inverstigated. Results showed that, compared with the conventional coagulants, TiCl4reacted more quickly with pollutants, resuting in larger floc size with higher floc growth rate. In case of humic acid-kaolin synthetic water treatment, the flocs formed by Al based coagulants had more enhanved strength, as refelected by higer strength factors, than the Fe based coagulant and TiCl4. The flocs formed by TiCl4showed the weakest recoverability after breakage, as reflected by the lowest recovery factor. In case of fulvic acid-kaolin synthetic water treatment, the floc recovery factor decreased with increasing TiCl4dose, while floc strength factor was barely affected by coagulant dose. In case of reservoir real water treatment, TiCl4achieved the largest floc size of560.4μm at initial solution pH of8. Floc strength factor follows the order of pH5<pH6<pH8<pH10, while floc recovery factor decreased with increasing pH. That is, the flocs formed in acidic conditions had strong recoverability. In case of Xiaoqinghe real water treatment, both floc growth rate and floc size increased with inceasing coagulant dose and the floc recovery factors under high coagulant dose conditions were lower than those under low coagulant dose conditions. Regardless of water properties and coagulant types, the increase in shear force resulted in the decrease in floc strength and increase in floc recoverability. The extension in breakage period decreased floc strength and recoverability.(3) A two-stage TiCl4dosing strategy is applied for investigating the coagulation effect of TiCl4on particulate and organic matter removal. Coagulation performance of one-shot dosing TiCl4was investigated for comparison. This study also investigated the coagulation mechanism of TiCl4during two-stage TiCl4dosing process. Additionally, this study evaluated the effect of water hardness and ionic strength on coagulation performance and floc characteristics of TiCl4. Resutls showed that a small second TiCl4dose essentially decreased the residual turbidity and improved the organic matter removal. Also, flocks with larger size and better recoverability result and having more compact structure than those obtained without the second TiCl4dose. The main coagulation mechanisms duting second coagulation process are charge neutralization, adsorption and coprecipitation. At the same total TiCl4coagulant concentrations, the two-stage dosing mode reveals significant advantage over the single coagulant dosing in terms of turbidity removal, flock size and recoverability although organic matter removal and flock compactness degree are more or less deteriorated during the two-stage dosing process. Based on the results of the influence of total hardness and ionic strength on coagulation performance, appropriate total hardness and ionic strength of raw water could effectively enhanced particulate and organic matter removal. Besides, the floc characteristics can be improved.(4) This study separately assessed the effect of coagulant aid cationic polyacrylamide (PAM), polydimethyldiallylammonium chloride (PDMDAAC), compound bioflocculant (CBF) and sodium alginate (SA) on coagulation performance, coagulation mechanism and floc characteristics of TiCl4. Results showed that the coagulation efficiency of TiCl4can be significantly improved by coagulant aid, especially under low TiCl4dose conditions. Dosing sequence of TiCl4and coagulant aid and type of coagulant aid are two main factors influencing coagulation efficiency and floc characteristics. For PAM as coagulant aid, the removal of fulvic acid followed the order of TiCl4-PAM> PAM-TiCl4and the floc growth rate and size followed the order of TiCl4-PAM> TiCl4PAM-TiCl4.Additionally, degree of floc compactness decreased due to PAM addition. For PDMDAAC as coagulant aid, the removal of humic acid followed the order of PDMDAAC-TiCl4>TiCl4-PDMDAAC. Both floc recoverability and compactness were significantly improved by PDMDAAC addition.(5) Novel and stable polytitanium chloride (PTC) with series of basicities were successfully synthesized using slow alkaline titration method. Electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) was first utilized to identify various Ti species in both TiCl4and PTC solutions. Jar tests were conducted to assess coagulation performance of PTC using both synthetic and real raw water samples and the floc characteristics were monitored online using a laser diffraction particle size analyzer. Results showed that, compared to TiCl4, higher or comparable turbidity and organic matter removal efficiency could be achieved by PTC with improved floc characteristics in terms of size, growth rate and structure. Besides, the water pH after PTC coagulation was significantly improved towards neutral pH. This study indicates that PTC is an effective and promising coagulant for water purification.(6) Sludge produced after coagulation process was calcined to produce titanium dioxide (TiO2), which was characterized in terms of physicochemical properties and photocatalytic activity. Results showed that, the TiCl4and PTC flocculated sludge were able to recycle and produce functional TiO2photocatalyst. As-prepared TiO2exhibited only anatase structure, whereas P-25TiO2showed both anatase and rutile structures. Besides, the photocatalytic efficiency of as-prepared TiO2was comparable to P-25TiO2. |
PDF Full Text Request