Biosorption Of Heavy Metals From Aqueous Solution By Chlorella Pyrenoidsa And Its Reaction Mechanism

With the rapid development of industry, heavy-metal pollution hasbecome one of the most serious contaminations that threat environmentand human beings. The remains of aquatic algae are widespread andconstitute an important organic fraction in natural freshwaterenvironments. Accordingly, their adsorption of heavy metals mayinfluence or determine the environmental processes and behavior of themetals. In this study, we choosed four heavy metals with different degreesof toxity to investigate their adsorption from water by the freshwater algaChlorella pyrenoidsa. In addition, the adsorption mechanism wasevaluated in the second part. The main results are shown as the followingtwo parts.In the first part, we investigated the adsorption of four toxic heavymetals that are of great concern, Cu²⁺、Pb²⁺、Cd²⁺and Cr³⁺, from water by the freshwater alga Chlorella pyrenoidsa. In addition, the factors thatinfluence the adsorption were evaluated. The results show that the processequilibrium of all four heavy metals reached quickly, and these metalsresisted desorption except for Cd²⁺. The adsorption isotherms obtained forCu²⁺、Pb²⁺、Cd²⁺and Cr³⁺followed a Langmuir shape, with the adsorptioninitially increasing rapidly and then approaching saturation with furtherincreasing metal concentration. By fitting adsorption data to theLangmuir equation, the obtained maximal adsorptions (q_max) were0.612、0.438、0.278、0.495mmol/g for Cu²⁺、Pb²⁺、Cd²⁺and Cr³⁺, respectively,which suggests that the alga Chlorella pyrenoidsa may be applied as aneffective biosorbent. The adsorption of both metals increased as the pHincreased in the range that heavy metals have not precipitated. As theconcentration of NaNO3（ionic strength） increased, the adsorption ofheavy metals decreased. The enhancement of adsorption by the increasingof temperature indicated that the adsorption was endothermic. Dissolvedfulvic acid played a negative role on the adsorption of heavy metals.In the second part, mechanism of heavy metal adsorption on algaChlorella pyrenoidsa was investigated. Taking Cu²⁺as the represent, weattained the complexation stabiliy constants （K） between Cu²⁺and modelcompounds with hydroxyl and carboxyl. According to the complexationstability constants, we found that Cu²⁺was easier to form ancomplexation with carboxyl that hydroxyl when the two kinds of groups existed together. The complexation stability constant （K） of Cu-algae wasbetween K （-COOH） and K （-OH），which indicated that Cu²⁺combinedwith both hydroxyl and carboxyl on the surface of algae cell during theadsorption process. The difference between the complexation stabilityconstants of Cu-COOH and Cu-OH inferred that only Cu-COOHcomplexation exsisted in the very low heavy metal concentration andCu-OH complexation began to form when the heavy metal concentrationcame to a certain degree. This inference was proved by potentionmetrictitration and Fourier transform infra-red （FTIR） studies on adsorptionalgae samples.