Bioremediation Mechanism Of Cadmium By Immobilized Biochar-based Microalgae

The rapid development of the mining,metallurgy,chemical industries have accelerated the emissions of heavy metal wastewater,serious heavy metal pollution in the surface water has attracted more and more attention.The traditional methods of heavy metal removal have such shortcomings like inefficient,high cost,and easy to produce secondary pollution.The microalgae are a kind of single cell microbe,they grow fast and has strong resistance for the harsh environment.It can not only remove heavy metals through biosorption and bioaccumulation,but also realize the transformation of biomass energy by photosynthesis.However,the use of live algae in heavy metal adsorption is limited to a number of conditions:nutritional conditions,heavy metal toxicity and biomass recycling.Biochar is a common material and has been widely used in the field of heavy metal adsorption.Because of its low density,high hydrophilicity,rich functional groups on the surface of microorganisms,strong adsorption capacity and high stability in the environment,biochar has been considered as a good microbial carrier.Therefore,this project proposes a new method for bioremediation of cadmium by microalgae-biochar immobilized complex,which achieves the goal of improving the removal rate of cadmium in sewage and improving the biomass recovery of microalgae.The specific research contents and results are as follows:1)As suspended biological adsorbent,suspended microalgae biomass and water hyacinth leaf biochar powder can fully contact with the heavy metal ion in water.In order to obtain more economical and efficient free combined adsorbent,this paper investigate the effects of pH,adsorbent addition and the mass ratio,of biochar to Chlorella sp.on Cd2+ adsorption and surface properties(including surface functional group,surface electric properties and surface morphology)of the water hyacinth leaf biochar,microalgae Chlorella sp.and microalgae-biochar complex with different mass combination ratios.The surface potential of Chlorella sp.and biochar changed with pH and adsorbent dosage and reached its minimum surface potential(-29.3 mV)with the combination ratio of 2:3,which is advantageous to the adsorbe the positively charged Cd2+ ions.The surface potential changed with the adsorbent concentration and the composite ratio,and carboxyl,asymmetric bending vibration groups and the-CH2 symmetric stretching vibration groups were additionally involved in the heavy metal binding process.2)Influence of cadmium ion concentration,mass combination ratio and contact time were further investigated after the optimum pH and adsorbent dosage of 6 and 1 g 1-1 was defined.Isotherm absorption model and kinetic adsorption model were established to study the biosorption mechanism and process.It can be concluded from the best suited Sips isotherm model that the maximum absorption of the microalgae-biochar complex with microalgae to biochar mass combination ratio of 2:3 and 4:1 were 217.41 mg g-1 and 186.03 mg g-1,which were higher than the maximum absorption of Chlorella sp.and biochar(169.79 mg g-1 and 95.1 mg g-1 respectively).But there are profound difference between the biosorption processes under the two ratio.The biosorption process of microalgae-biochar complex with microalgae to biochar mass combination ratio of 2:3 fits pseudo kinetic adsorption model with a characteristic of slow bioaccumulation while particle diffusion model can better describe the biosorption process of microalgae-biochar complex with microalgae to biochar mass combination ratio of 4:1.3)Microbes can attach on the surface of the filler to increase the capability of solidliquid separation of the microbe and the raw material.In order to reduce the production cost of biofilm filler and the cost of microalgae harvesting,waste water hyacinth biomass(water hyacinth leaves,roots and biochar derived from water hyacinth leaves and roots)were made into stacked filler on which the microalgae can form a biofilm thus to co-adsorb low Cd2+ concentration wastewater as a whole.From the surface properties test of the four kinds of fillers,it can be found out that filler made out of the water hyacinth leaves derived biochar presents good hydrophiticity,abundant pore structure and surface functional groups,which enabled a highest flocculation rate of Chlorella sp.of 81.52%.Light and the microalgae cultivation cycle is vital for microalgae flocculation rate and the yield and composition of the biofilm extracellular polymers(EPS).The L-EPS content of the adhesive EPS was roughly negatively correlated with flocculation rate;meanwhile,flocculation rate and the mass ratio of protein and polysaccharide in soluble EPS were positively correlated with the amount of heavy metal cadmium removal.4)In order to further improve the surface attaching area and hydrophilicity of the filler,?-cyclodextrin polymer biochar filler was prepared through controlling the parameters in the inverse suspension polymerization to realize the rapid flocculation of microalgae.The flocculation efficiency can reach 73.33%.The maximum adsorption capacity of the filler before flocculation and the filler with biofilm attached on it after flocculation was 10.02 and 15,489 mg g-1,respectively.The pseudo kinetic adsorption model can describe both the biosorption process of filler before and after biofilm attachment.