| Cadmium (Cd), which is toxic and can be intook by human through the food chain, can be bioaccumulated by Agaricus blazei Murrill (AbM). So, it would be a potential danger to the AbM safety. Additionally, the toxicity, biological activity and bioacessibility of trace elements not only depend on their total concentrations, but also depend on chemical speciation. Hence, speciation analysis of Cd is of important significance to AbM safety research. The research of bioaccumulation for Cd on AbM is mainly concentrated in the Cd content, Cd pollution control, the analysis of the components related to the bioaccumulation of Cd, etc. No research which from adsorption kinetics and thermodynamics perspective can be found for adsorption properties and mechanism for Cd onto AbM. To research the adsorption characteristic and mechanism not only can provide some theoretical basis for quality and security control of AbM, also has certain reference value for exploring macro fungus as environment bioremediation agent. This paper mainly do some research on the following several aspects:speciation of Cd in AbM, bioavailability of Cd and adsorption properties and mechanism for Cd onto AbM.1. Speciation analysis of Cd in AbMThe fresh AbM, used as research raw material, was dried, then the speciation of Cd in fruiting body was analysed through sequence-progression fractionation method and the bioavailability of Cd in fresh AbM was assessed by in vitro biomimetic digestion system and the relationship between them.The content of Cd in five ingredients is different because of difference of the main components in five ingredients. The concentration of Cd in water extracted ingredient is low, but the content of Cd in water extracted ingredient has the largest percentage compared with its total content due to the dry matter of water extracted ingredient is the heaviest. The process of extraction of water mainly extracted Cd on cell wall, and the content of Cd in water extracted ingredient has the largest percentage, which indicated that Cd is accumulated in cell wall of fruiting body. It is generally acknowledged that the water extracted ingredient of fruiting body is mainly soluble protein and carbohydrate substances, which are the substances can be intook by person. Therefore, Cd in water extracted ingredient has the strongest migration on fruiting body and the greatest harm for person.After the saliva and gastric juice digestive progresses, compared with the initial total Cd content in fresh AbM, the bioaccessibilities of Cd in fresh AbM was77.8%, which is higher than Cd content of water extracted ingredient. This is probably the acid was add to gastric juice (pH=2～3), a portion of acid-soluble substance was extract. Then, after the duodenal digestive progress, the bioaccessibilities of Cd was69.4%, which is close to percent of Cd content in water extracted ingredient. These fully shows that Cd in AbM is mainly solvable. The migration ability of Cd in water extraction ingredient is strong, and the toxicity effect is significant. Therefore, the dissolved Cd after duodenal digestive progress can migrate to human and intook by human. When AbM was digested by these two sequential biomimetic digestive juices, the bioaccessibility of Cd showed some differences. It is likely that these two phases had different pHs (pH of the gastric juice was very low, whereas the pH of gastrointestinal juice was near to neutral), when AbM was digested by these two sequential biomimetic digestive juices, the dissolved Cd coordinated complexes formed in gastric juice could be reformulated by Cd-coordinating substances in gastrointestinal juice.2. The research of adsorption mechanism for Cd2+onto AbMThe AbM, which is used as a biosorbent, was tested for removing of Cd ions from aqueous solution. The pH, biosorbent dosage, contact time, and initial metal ion concentration were found to have an effect on the biosorption efficiency of Cd. The adsorption mechanism was analysed by three adsorption isotherm models (Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) isotherm models), four kinetic models (pseudo-first-order model, pseudo-second-order model, intra-particle diffusion model and liquid film diffusion model) and thermodynamic parameters.The desired pH value for biosorption was found to be6.0, and the adsorption capacity was affected by the change of pH value. The desired biosorbent dosage for Cd sorption was2g/L. Higher biosorbent dosages resulted in higher percent removal of Cd, but the adsorption capacity, the amount adsorbed per unit mass of adsorbent, decreased with the increase of biosorbent dosages. Percent removal of Cd increased with the increase of contact time, then it would slow down at90min. Lastly, Cd removal attained equilibrium after approximately180min. An increase in Cd concentration resulted in a decrease in the percent Cd removal, and an increase in adsorption capacity.The equilibrium adsorption data were tested with three isotherm models and were best fitted with the Langmuir isotherm (R2>0.98). The kinetics of Cd adsorption was very well described by the pseudo second-order kinetic model with R2values exceeding0.99. Liquid film diffusion process was the rate-limiting step occurring during sorption of Cd onto AbM compare with intra-particle diffusion. The calculated thermodynamic parameters showed the exothermic and spontaneous nature of the biosorption of Cd onto AbM. |
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