Study On The Removal Of Microcystis Aeruginosa By HTCC Combined River Sand And Its Combination With Allelochemicals

Water eutrophication is still one of the major environmental problems faced by rivers and lakes,where water bloom will occur under suitable conditions.Microcystis aeruginosa blooms generate unpleasant odors and release toxins into water,which is very harmful to aquatic life and human health.Therefore,it is urgent to find safe,efficient,environmentally friendly method of algae removal.According to previous researches,chitosan modified river sand has a good effect on the removal of microcystis aeruginosa with ecological security.However,it is not very convenient for chitosan to be employed during practical application due to its only solubility in dilute acid and insolubility in water.Moreover,chitosan is an organic macromolecular flocculant carrying only weak charge,and the flocculation performance is limited,also hindering its practical use.Therefore,in this paper,quaternary ammonium was introduced into chitosan to improve the water solubility and positive charge,which will enhance the practical application of chitosan.The quaternary ammonium salt of chitosan(HTCC)has been synthesized in the isopropyl alcohol system,by the reaction of chitosan(CTS)with 2,3-epoxypropyltrimethyl ammonium chloride(GTA).It is proved that substitution reaction mainly occurs on the –NH2 group of CTS from FTIR spectrometer.By changing the mass ratio of GTA to CTS,HTCC with three different degree of substitution(DS)were prepared.And the water solubility and flocculation effect of HTCC improved with the increase of DS.The results showed that HTCC modified river sand with DS of 94.6% has a good flocculation effect,and the algae removal rate can reach more than 99% after one hour under optimum dosage of HTCC modified river sand(7.5mg·L-1 HTCC and 0.5 g·L-1 river sand)when the algal density was 69.5×10 cell·mL-1 at room temperature.In order to control the growth of new algae after removing algae,allelochemicals were added to the suspensions of HTCC modified river sand where allelopathy inhibition of allelochemicals could control new algae growth.There kinds of phenolic acid allelochemicals of p-hydroxybenzoic acid,catechol and gallic acid were studied systematically,which all have good allelopathy inhibition on Microcystis aeruginosa when applied alone.And the 72hEC50 of the p-hydroxy benzoic acid,gallic acid and catechol were 49.66,15.50 and 2.73 mg·L-1,respectively.The combined effects of these allelochemicals were also estimated via the semi-lethal dose index(SLDI)method,and both the mixtures of catechol with gallic acidand catechol with p-hydroxybenzoic acid showed synergistic effect while antagonism effect was found in the mixture of gallic acid with p-hydroxybenzoic acid.As a demonstration,the combination of catechol and gallic acid were used in follow-up experiments.The results showed that the inhibition rate of new algae reached the highest level after 6 days of removal algae,namely 76.3% when catechol of 1.36 mg·L-1 and gallic acid of 7.75 mg·L-1 added into HTCC modified river sand comparing the growth situation of algae with and without the two allelochemicals.Further researches showed that the dosage of HTCC together with the density and growth period of algae had an obvious effect on the removal rate of algae.It was suggested that HTCC modified river sand should be applied in the logarithmic phase of algae where the algae density was 69.5×10 cell·mL-1,and the best dosage of HTCC was 7.5 mg·L-1and river sand was 0.5 g·L-1.Notably,HTCC can be used in a wide pH range compared with CTS,which has a well flocculation effect during pH from 7 to 12,and temperature will not also influence the algae removal much.Furthermore,the dosage and particle size of river sand also has little influence on the algae removal efficiency of HTCC,so the original sand after simple removal of larger particles can be adopted in practice.Finally,the effect of electrolytes on the algae removal efficiency of HTCC modified river sand and the zeta potential of algae cell surface before and after algae removal were analyzed to study the algae removal mechanism of HTCC modified river sand.Possible mechanism lies in adsorption bridging and electrical neutralization.The water solubility and flocculation properties of chitosan were improved after modified by quaternary ammonium,which has a good effect on the removal of Microcystis aeruginosa.Suitable allelochemicals added to the suspensions of HTCC modified river sand can control new algae growth after removal algae.Therefore,HTCC modified river sand combination with allelochemicals will have a wide potential application in emergency management of blooms.