| Wangia profunda SM-A87 exopolysaccharide (SM-A87 EPS) is a safe, high efficiency and biodegradable biosorbent. It was proved to be a potentive biosorbent for heavy metal wastewater. In this study, in order to improve the separation efficiency and regeneration capacity of SM-A87 EPS from aqueous solution, the preparation of immobilized SM-A87 EPS (SM-A87 IEPS) was optimized by orthogonal array design experiments. The eco-materials of polyvinylalcohol (PVA) or sodium alginate (SA) were used as supporters for entrapment of SM-A87 EPS. The influence factors of Pb(II) adsorption onto SM-A87 IEPS were investigated through batch experiments; The thermodynamic and kinetic characteristics of Pb(II) adsorption were analyzed for SM-A87 IEPS; The dynamic adsorption behavior of SM-A87 IEPS was studied through fixed-bed column experiments.In orthogonal array design experiments, four main factors were mass fraction of polyvinyl alcohol (PVA), sodium alginate (SA), EPS, and the hardening time. The major relevant indicator was Pb(II) removal rate, while three subordinate indicators were the spherical shape, extraction rate of polysaccharide and acid resistance of the immobilized EPS grains. The results showed that the optimal ratio of SM-A87 IEPS was obtained asω(PVA):ω(SA):ω(EPS):ω(H2O)= 0:2:1:100, and hardening time was of 16 h. SEM micrographs showed high porosity in the internal of SM-A87 IEPS beads, while narrow crevices and small holes irregular distributed on the rough surface, and the surface become relatively denser after adsorption. The polysaccharide extraction rate of SM-A87 IEPS beads was 2.17%.Effects of SM-A87 IEPS dosage, pH, and adsorption time had been studied by batch experiments. The results showed that at condition of initial Pb(II) concentration of 10 mg/L, pH= 4.0,20℃and adsorbent dosage of 0.2 g/L, the maximum adsorption capacity was abserved to be 56.25 mg/g. There was three adsorption stages including the fast adsorption stage (0-120 min), the gradual adsorption stage (120-360 min) and the equilibrium adsorption stage (360-720 min) in the Pb(II) adsorption process onto SM-A87 IEPS, and the adsorption gradually reached equilibrium after 360 min. The adsorption capacity increased as the increase in the initial Pb(II) concentration. Thermodynamic isotherm of Pb(II) adsorption on SM-A87 IEPS could be better described by Langmuir model, with regression coefficient of 0.984 and the theoretical maximum adsorption capacity of 306.7 mg/g. Pseudo-second-order model well fitted the kinetic process of Pb(II) adsorption by SM-A87 IEPS. The intrapartical diffusion analysis showed that the Pb(II) adsorption onto SM-A87 IEPS was controlled by two or more adsorption stages. The maximum desorption yield (98.12%) of SM-A87 IEPS was obtained by using 0.1 M HC1. After five consecutive adsorption-desorption cycles, slight decrease of desorption ratio and adsorption rate was observed, indicating that SM-A87 IEPS was reusable.For the continuous fixed-bed column system, the effect of bed height and flow rate on the Pb(II) adsorption was studied. The results showed that the larger bed height (2.5 cm) and the lower flow rate (2.0 mL/min) were favor to delaying of the breakthrough time. The Thomas model could well fitted the kinetic process of Pb(II) adsorption in SM-A87 IEPS fixed-bed column. The effluence of bed height on breakthrough time could be well described by BDST model (R2> 0.87). In twice adsorption-desorption cycles, the breakthrough time of cycle 2 was increased while the exhaustion time decreased compared to cycle 1. The desorption rate was fast in the beginning of the twice desorption processes, but becoming slower as time increasing. The adsorption-desorption cycle 1 lasted a long period of time, and satisfied adsorption performance (tb=144.78 h) and high desorption ratio (> 80%) of cycle 2 was observed after more than 450 h of adsorption-desorption time in cycle 1. |
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