| Natural aquatic biofilms are widely found in natural water environments.They are complex and open biological systems composed of a variety of microorganisms and inorganic minerals.They have strong enrichment and adsorption capabilities for heavy metals in water.Sulfhydryl is a highly active group in organisms,which can form stable complexes with heavy metals to achieve biological purification and detoxification of itself,and also plays an important role in the restoration of heavy metal pollution in water.In this study,the effect of sulfhydryl in biofilms on metal adsorption characteristics was explored by masking sulfhydryl groups in biofilms,which is helpful to fully understand the selfpurification mechanism of natural water and the migration behavior of heavy metals.In order to understand the adsorption characteristics of sulfhydryl groups on Pb,Cu,and Cd in natural water biofilms,a substance that can specifically bind to sulfhydryl groups—monobromide(trimethylammonium)dibromide(qBBr)as a sulfhydryl masking agent.On the basis of determining the reaction time and concentration of sulfhydryl groups in biofilms fully masked by qBBr,the effect of qBBr on the physiological activity of biofilms was studied,and the content of sulfhydryl groups in biofilms was quantitatively determined to determine the appropriate conditions for the sulfhydryl group masking experiments.Differences in the adsorption characteristics of Pb,Cu,and Cd by unmasked/qBBr-masked sulfhydryl biofilms at different p H,and the binding constants of the three metals and sulfhydryl groups were calculated to determine the contribution and role of sulfhydryl groups in the adsorption process of the three metals.The experimental results showed that the ratio of qBBr to biofilm(dry weight)was 8μmol/g,and the sulfhydryl groups in the biofilm could be completely masked after 10 h of reaction.Under different qBBr concentrations,the content of chlorophyll a in the biofilm was basically unchanged,and the contents of superoxide dismutase and malondialdehyde increased slightly with the increase of qBBr concentration.affect the adsorption of metals to biofilms.The fitting calculation results of the potentiometric titration data showed that the sulfhydryl sites in the biofilm accounted for about 5.7% of the total adsorption site concentration,and the non-sulfhydryl groups such as carboxyl,phosphate,and amino groups accounted for most of the biofilm adsorption sites.The adsorption kinetics and thermodynamic characteristics of the three metals by the masked/unmasked sulfhydryl biofilms can be fitted by the first-order kinetic equation and the Langmuir equation,respectively.In the p H range of this study,the maximum metal adsorption capacity of biofilms increased with the increase of p H,and the adsorption capacity showed a trend of Pb>Cu>Cd.With the increase of p H,the gap in the amount of biofilm adsorption of masked/unmasked sulfhydryl groups gradually increased,indicating that the contribution of sulfhydryl groups in the adsorption of the three metals also increased.Compared with other non-sulfhydryl functional groups,the three metals have significantly stronger binding stability to sulfhydryl groups,and their sulfhydryl binding constants are1.8 ～ 2.7 orders of magnitude higher than those of non-sulfhydryl groups.The binding ability of sulfhydryl groups to the three metals in biofilms is Pb> Cu>Cd.When the metal loading is relatively low,the metal will preferentially bind to the sulfhydryl site,and the binding amount accounts for the majority of the total adsorption.The lower the metal loading,the higher the proportion of the sulfhydryl-bound metal in the total adsorption.Due to the low proportion of sulfhydryl sites in the total adsorption sites in biofilms,with the increase of metal loading,non-sulfhydryl sites will gradually dominate the adsorption of metals after the combination of metal and sulfhydryl groups reaches saturation. |
