| Heavy metal pollution is one of the most serious water pollution h azards. Adsorption has become the main method to remove heavy metals from wastewater with its lower cost and better effects. Studies were shown that biomass based materials that are rich in active groups which have the ideal adsorption to the heavy metal cations. As a great agricultural country, agricultural waste hugely yields every year, while the rice straw has tremendous proportion in many agricultural wastes. Follow the principle that "treatment of wastes with processes of wastes against one another", therefore, the utilization of rice straw biomass adsorbent preparation of heavy metal wastewater treatment to realize comprehensive utilization of resources has application value and practical significance.This paper prepared biosorbent by citric acid modified rice straw, the effects of environmental factors on biosorption of lead and cadmium were studied, including different adsorption time, initial pH solution, biosorbent dosage, concentration of solution and temperature conditions, the different isothermal adsorption models, adsorption kinetics equations and thermodynamic parameters were analyzed for isothermal adsorption balance, adsorption dynamics and thermodynamics characteristics in the adsorption process of lead, cadmium on modified rice straw; With the aid of scanning electron microscope (SEM), X-ray energy dispersive spectrometer, specific surface area and pore size analyzer, electric potential analyzer, Fourier infrared spectrometer and X-ray diffractometer, the physical and chemical properties and morphology were characterized and analyzed on rice straw before and after the modification to illuminate the adsorption mechanism of lead, cadmium on citric acid modified rice straw. The main conclusions were obtained as follows:(1) The Pseudo-second-order model could fittingly describes adsorption kinetics for both of metals on modified rice straw, adsorption process was in two major process:membrane diffusion and spread diffusion.(2) Adsorption isothermal model fitting results indicated that the Langmuir adsorption isotherm model could better fittingly express the isothermal adsorption process of lead and cadmium. The parameters RL related with Langmuir adsorption isothermal model was showed that the process was indicated a favourable affinity and adsorption potential for lead and cadmium ions; Fitting average free energy of D-R adsorption isotherm model showed that the adsorption mechanism of lead and cadmium was ion exchange process and physical adsorption process respectively on modified rice straw.(3) During the concentration range from25to200mg/L, adsorption capacity of lead and cadmium increased significantly on modified rice straw compare with rice straw. The best initial pH value scope of lead, cadmium adsorption, respectively, for4-6and3-7. Modified rice straw for lead and cadmium adsorption were less affected by temperature, the adsorption process is spontaneous and entropy increase, meanwhile, lead and cadmium were endothermic and exothermic process respectively(4) The analysis of SEM indicated that the material surface was significantly damaged after esterification of citric acid; therefore, it exposed internal structure fully and produced cracks and apertures. The specific surface area and porosity of sorbent was increases. The analysis of porosity and pore size showed that specific surface area and pore volume increased to some extent.(5) The analysis of zeta showed that the electronegativity of adsorbent surface was enhanced; this may be related to the strong polarity of the functional group which was introduced into materials. In the FTIR analysis, the negative functions were confirmed that electric groups of carboxyl were introduced onto the surface; these functional groups played a important role in the adsorption for heavy metal ions.(6) X-ray diffraction spectrum showed that crystallinity of straw cellulose was decreased under the effect of the esterification. X-ray energy dispersive spectrum showed that the Si/O of modified straw was increase, and the spectrums proved the adsorption effect of lead and cadmium. |
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