Preparation And Properties Of The Citric Acid Sustained-release Membrane And Water Purification Bacteria-embedding Capsules Based On Texture Profile Analysis

The problem of water resource contamination becomes increasingly serious in China. Especially, water pollution control is extremely significant in Zhejiang Province due to its relatively high proportion of surface water resources in China. To be specific, eutrophication caused by nitrogen contamination, particularly ammoniacal nitrogen thereof, is hazardous to aquaculture, industrial development and sanitary use of water, and is especially difficult to control. Therefore, this research aims to reduce the content of ammoniacal nitrogen in contaminated water, hence achieving the goal of water pollution restoration. To fulfill this objective, the technique of immobilized microbial remediation was applied by using the combination of sodium alginate, polyvinyl alcohol(PVA) as the wall material of bacteria-embedding systems. The optimal immobilization system was determined by texture profile analysis(TPA). Also, membranes composed of chitosan and PVA were fabricated for the controlled-release of citric acid, and the resulted purification effectiveness was analyzed.Milestones of this research include:(1)Construction of detection system for citric acidA detection system for citric acid was constructed based on the mechanism of Fe3+ reacting with citrate ions to form purple complexesin acidic conditions under ultraviolet light, which could be detected by spectrophotometry. The optimal detection system was determined as a hybrid solution of 200 μl containing two portions- 50 μl of Fe3+(0.5 M) /nitric acid(0.5 M) mixture and 150 μl of citric acid/dd H2 O mixture under the light of 440 nm. This detection system showed high stability and effectiveness in the range of 0.5×10-3 mol/L~0.5×10-1 mol/L, as comparing to the results of HPLC.(2)Investigation into chitosan-citric acid-crosslink systemAn optimal chitosan-citric acid-crosslink system was determined by analyzing encapsulation efficiency and citric acid-loading capacity. This system consists of 40 ml of citric acid(0.1 M) and 50 ml of chitosan(20 mg/m L). Different volumes of 10% PVA were used to coat the cross-linked system, which was then dried into a sustained-release membrane for citric acid. The release experiment in water demonstrated that the citric acid-release rate increased as the applied amount of PVA decreased, although the release rate of all parallel groups reached 90% within the range of 16 h-3 d. So this system was yet to be improved though it made a little difference in controlled-release of citric acid.(3)Fabrication of water purification bacteria-embedding capsulesTexture profile analysis(TPA) was introduced to investigate the major factors influencing mechanical properties and release efficiency of capsules, including wall material, amount of accessories, concentration of curing liquid and curing duration. Finally, an optimal microbial immunization system was determined as follows: V(4%sodium alginate):V(10% PVA)=1:9 for the wall material of microcapsules; mixture of 4% Ca Cl2 and 4% H3BO3 for curing liquid; 20%~60%, 20%~30% and 20%~60%(w/v) for bamboo charcoal, diatomite and zeolite power, respectively, as accessories. The optimal curing duration was 16 h-30 h. The SEM image indicated the existence of internal microspores at both nano-scale and micron-sized in the cross-linked capsules. Simulation experiments proved that part of the bacteria embedded in the capsules had been released in aquatic environment, indicating that the microcapsule system performed well.(4)Simulation of water purification at laboratory scaleWater purification experiments in shaking flasks showed that the concentration of NH4+-N dropped from 39.3 mg/L~44.7 mg/L at 1 h to below 0.1 mg/L at 20 h, and the removal rate of ammoniacal nitrogen achieved 99%, indicating that the bacteria-embedding microcapsules were capable of removing ammoniacal nitrogen effectively. Moreover, the total nitrogen concentration of each parallel group decreased after 24 h. The water purification experiment in a simulated river channel using membrane of the microcapsule-system demonstrated that concentration of ammoniacal nitrogen declined as the hydraulic retention time(HRT)increased in each parallel group. Also,the concentration of ammoniacal nitrogen changed from 6.2 mg/L~6.4 mg/L to 1.2 mg/L~1.7 mg/L, and the total nitrogen concentration decreased from 11.4 mg/L to 2.1 mg/L ~ 2.6 mg/L. Addition of sustained-release membrane for citric acid raised the purification effectiveness slightly. Furthermore, a proper dosage of carbon source was preferred to a higher performance of water restoration by bacteria.