Effect Of Titanium Dioxide Nanoparticles On Cadmium Adsorption By Biofilms

During production,use and disposal,nanoparticles inevitably cause environmental problems.Due to their excellent photocatalytic and chemical reactivity,nanoparticles are prone to interact with heavy metals.Biofilms combine with heavy metal ions through biosorption and surface precipitation,which affect the migration and transformation of heavy metal ions in the water environment system.Using flow cells,this study investigated the mechanism of the effect of TiO₂ NPs on the adsorption of heavy metal Cd on biofilms by measuring the amount of Cd adsorption,biofilm composition and diversity in biofilms.The specific research contents include:exploring the effect of TiO₂NPs on Cd adsorption capacity and reaction rate control steps through adsorption kinetics and thermodynamics;comparative analysis of the effect on the structure and function of adsorbed Cd biofilm before and after adding TiO₂ nanoparticles;Liye spectroscopy and X-ray photoelectron spectroscopy were used to represent the adsorption mechanism.The main findings show that:（1）Under different TiO₂ NPs concentrations,the amount of Cd adsorbed by biofilm increased with time,and reached equilibrium at 240min.Compared with the control group,the equilibrium adsorption of Cd by biofilms increased by 23.5%and 35.8%when 0.1and 1mg/L TiO₂ NPs were respectively added to 0.5mg/L Cd.The equilibrium adsorption of Cd by biofilms only increased by 1.8%and 1.9%at the concentration of 1 and 10mg/L TiO₂ NPs which added to 5mg/L Cd.This indicates that the addition of TiO₂ NPs has little effect on the adsorption capacity of biofilms under the condition of high concentration of Cd.The initial p H is an important factor affecting the adsorption of Cd by biofilms:the closer to the optimal adsorption p H（0.5mg/L and 5mg/L Cd correspond to p H=5 and p H=6,respectively）,the higher the adsorption capacity of biofilms to Cd.The adsorption of Cd after the addition of TiO₂ NPs was promoted under different p H conditions（p H=2-9）,but the promotion effect of the addition of 10 mg/L TiO₂ NPs did not improve.The initial p H is an important factor affecting the adsorption of Cd by biofilms.The closer to the optimal adsorption p H（p H=5 and p H=6 for 0.5mg/L and 5mg/L Cd）,the stronger the adsorption capacity of biofilms to Cd.The adsorption of Cd after the addition of TiO₂NPs was promoted under different p H conditions（p H=2-9）,but the promotion effect of the addition of 10mg/L TiO₂ NPs did not improve.The above results indicated that the high concentration of TiO₂ NPs（10mg/L）could inhibit the adsorption capacity of biofilms to Cd.In addition,the pseudo-second-order kinetic model describes the adsorption kinetics well,and the correlation constant reached 0.999,indicating that the adsorption of Cd by the biofilm is controlled by chemical steps.The Freundlich model in the adsorption isotherm has a higher correlation constant,indicating the heterogeneity of adsorption sites on the biofilm surface.（2）The protein（PRO）content in the extracellular polymer was much higher than that of the polysaccharide（PS）.The Cd concentration was 0.5 mg/L,and after adding 0-1 mg/L TiO₂ NPs,the PRO content increased by 3.64%,10.94%and 22.73%,and the content of PS increased by 10.53%,12.50%and 32.52%,compared with the control group.While the content of PRO and PS decreased after adding TiO₂ NPs when the concentration of Cd was 5 mg/L.In addition,the presence of 0.5 and 5mg/L Cd respectively decreased the cell viability of biofilms by 3.1%and 12%,and the addition of low concentrations of Cd and the corresponding two lower concentrations of TiO₂ NPs increased the percentage of necrotic cells in biofilms（4.1%and 7.2%）which is smaller than the increase in the percentage of necrotic cells（13.5%and 15.8%）under the condition of adding high concentrations of Cd and the corresponding two higher concentrations of TiO₂ NPs.The changes in enzyme activities were similar to cell viability.The above results indicated that the conditions of high concentration of Cd and high concentration of TiO₂ NPs would seriously degrade the overall metabolic activity of microorganisms in biofilms.The SEM results showed that the extracellular polymer played a role in protecting the bacteria and algae inside the biofilm during the adsorption of Cd by biofilm,and the addition of TiO₂ NPs resulted in the accelerated shedding of the extracellular polymer and the exposure of the bacteria and algae.And the addition of Cd and TiO₂ NPs caused fluctuations in the relative abundance of microbial species in the biofilm.（3）XPS and FTIR results showed that the carbonyl and hydroxyl groups on the surface of the extracellular polymer were involved in the adsorption of Cd on the biofilm.And the addition of TiO₂ NPs could promote the movement of Cd to the direction of high binding energy,indicating that TiO₂ NPs could promote the adsorption of Cd by biofilms.The peak areas of functional groups representing proteins and polysaccharides in the FTIR were reduced and disappeared which indicated that both proteins and polysaccharides were involved in the adsorption process.In this paper,the toxic effects of heavy metal Cd and the combined action of heavy metal Cd and TiO₂ nanoparticles on biofilms,and the migration and transformation behavior of heavy metal Cd were studied.The interaction of pollutants on their migration and transformation behavior in the environment and the assessment of impacts and resulting environmental risks are of great importance.