Efficiency Of Removing Phytotoxic Substances In Nutrient Solution By TiO₂-photocatalytic System

Closed soilless culture system has become an inevitable trend internationally. It has the advantagesof making full use of water and nutrition and avoiding environmental pollution. The nutrient solutionmanagement is the core technology of soilless cultivation in protected horticulture. However,continuous cropping obstacle, which is due to the accumulation of phytotoxic substances in nutrientsolution, often occurs in closed soilless culture system and results in plant growth inhibition and yielddecline. Therefore, it is necessary to remove the phytotoxic substances accumulated in nutrient solutionof closed soilless culture system. In the present study, nano-TiO₂photocatalytic method was used toremove the phytotoxic substances in nutrient solution. First, the accumulation rule of root exudates inreused nutrient solution and their harmful effects on the growth of hydroponic lettuce were investigated;Also the effects of nitrogen forms and levels on hydroponic lettuce growth, root exudates accumulationin nutrient solution were investigated. And then, photocatalytic degradation of six typical phytotoxicsubstances by nano-TiO₂photocatalytic method was studied. Next, in order to find the bestphotocatalytic parameters and to establish nano-TiO₂photocatalytic system, the influences of UVwavelengths, nano-TiO₂particle sizes, immobilized amounts and ventilation on nano-TiO₂photocatalytic efficiency were studied. Finally, the practical effect of the established photocatalyticsystem in hydroponic lettuce system was verified. The main results and conclusions are as follows:1. Root exudates were accumulated in the reused nutrient solution, and the growth of lettuce wasinhibited. Under nutrient solution reused condition, the TOC （Total organic carbon, which was use toreflect the root exudate and phytotoxic substance concentrations in nutrient solution） concentration innutrient solution and the average root secretion of lettuce increased significantly. The TOCconcentrations in reused nutrient solution of hydroponic lettuce （Lactuca sativa L） and purple leaflettuce were increased25.78and16.14%, respectively. Under nutrient solution reused condition, theaverage root secretion of lettuce （Lactuca sativa L） and purple leaf lettuce increased2.14and1.40mg,separately; The shoot and root fresh weight of lettuce （Lactuca sativa L） reduced by38.26and38.35%,respectively, and those of the purple leaf lettuce reduced by35.96and15.36%, respectively; Thechlorophyll, flavonoids and vitamin C content of lettuce （Lactuca sativa L） were significantly decreased,but the total phenols and soluble sugar content did not change significantly; For purple leaf lettuce, thechlorophyll and total phenol content did not change significantly, but the flavonoids was significantlydecreased and the soluble sugar was greatly increased.2. A pot experiment in greenhouse was carried out to examine the effects of two nitrogen forms and twonitrogen levels, including10mmol·L^-1NO₃^--N（N10）,5mmol·L^-1NO₃^--N+5mmol·L^-1NH₄⁺-N（N5+A5）,5mmol·L^-1NO₃^--N（N5）,2.5mmol·L^-1NO₃^--N+2.5mmol·L^-1NH₄⁺-N（N2.5+A2.5）,on lettuce growth, leafy photosynthetic pigments contents and root exudates accumulation in nutrientsolution. The results showed that lettuce shoot and root biomass with N10and N5treatments werehigher than those with N5+A5and N2.5+A2.5treatments, and no difference was found betweentreatments of same nitrogen form at different nitrogen levels. On the contrary, the content of leafychlorophyll a, chlorophyll b, carotenoid contents, and TOC concentration and TOC accumulationcontent in nutrient solution of N5+A5and N2.5+A2.5treatments were higher than N10and N5treatments. For the content of leafy chlorophyll a, carotenoid contents, TOC accumulation content, no difference were found between treatments of same nitrogen form at different nitrogen levels. For thecontent of leafy chlorophyll b, TOC concentration, no difference was found between treatments of N10and N5, but the difference was significant between N5+A5and N2.5+A2.5treatments. These resultsindicated that NH+4-N could be conducive to the growth of lettuce, and lead to root exudatesaccumulation in nutrient solution, but it could improve photosynthetic pigments in leaves. However, insame nitrogen form, nitrogen levels had small effect on lettuce growth, photosynthetic pigments contentin leaf and accumulation of root exudates in nutrient solution in this expeirment.3. Six typical phytotoxic substances could be effectively degraded by nano-TiO₂photocatalysis. Aftersix-hour photocatalytic treatment, the degradation rates of benzoic, salicylic, ferulic, gallic, acetic andtannic acid were up to31.2,51.8,18.0,43.0,71.4and52.5%, respectively, compared to control. Thedegradation rates of the six phytotoxic substances were different by nano-TiO₂photocatalysis, whichmight be related to their structures and the pH of their water solution.4. In order to meet the needs of nutrient solution management in closed soilless culture, box-type andtubular photocatalytic systems were designed. Meanwhile, for the purpose of optimizing the parametersof nano-TiO₂photocatalysis, the effects of the wavelengths of UV light （365nm,254nm）, nano-TiO₂particle size （10nm,50nm）, amounts of nano-TiO₂coated on tiles (6g·m^-2,11g·m^-2,16g·m^-2,21g·m^-2) and aeration on nano-TiO₂photocatalysis were researched. The results showed that when254nmUV light as the excitation source, the nano-TiO₂photocatalytic effect was better than when365nm UVlight as the excitation source. The photocatalytic effect of10nm TiO₂was better than50nm. With theincrease of the amount of nano-TiO₂coated on tiles, nano-TiO₂photocatalytic efficiency increased, butwhen the immobilized amount was over16g·m^-2, the photocatalytic effect improved very small.Comprehensive consideration, the optimal immobilized amount was16g·m^-2. Aeration couldsignificantly improve the efficiency of the nano-TiO₂photocatalysis. The test of waste hydroponicnutrient solution treated with nano-TiO₂photocatalytic system was conducted. The result showed thatroot exudates accumulated in nutrient solution could be effectively removed. When the nutrient solutionwas in flowing state, the degradation efficiency of nano-TiO₂photocatalysis was significantly higherthan that in the stationary state.5. The efficiency of nano-TiO₂photocatalysis on removal of phytotoxic substances in reused nutrientsolution, and the influences of nano-TiO₂photocatalysis on contents of effective Fe, Mn, Zn element inreused nutrient solution and growth of hydroponic lettuce were studied. The results showed thatnano-TiO₂photocatalysis could effectively remove the phytotoxic substances in nutrient solution. Aftertwelve-hour photocatalytic treatment, the degradation rate of TOC in nutrient solution reached74.39%.The effective Fe, Mn, Zn element contents were also reduced by nano-TiO₂photocatalytic treatment,and their degradation rates were up to100,13.01and45.44%, respectively. Under nano-TiO₂photocatalytic treatment, the average lettuce fresh weight increased1.44g, and the total production oflettuce increased13.28%.The contents of chlorophyll, Vc and soluble sugar in lettuce leaves increasedsignificantly with nano-TiO₂photocatalytic treatment.