| Sediment is the basic constitute of aquatic ecosystem, and also an important pollutant source. Under some conditions, the pollutants remained in sediments could be released into water, by desorption, dissolution, biodegradation and so on, thereby pollute the water again. Hence, remediation of polluted sediment is a key part of the pollution abatement of rivers, lakes. The in situ remediation has advantages, such as cheap, operability, free of secondary pollution, and becomes a main approach to solve the sediment pollution. Among the in situ remediation technologies, immobilization of pollutants is an efficient way to prevent the migration of contaminants from sediment to water system. Several advanced techniques, such as ICP-MS, high performance liquid chromatography (HPLC), fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) were used in the present research to investigate the influence of organic matter, solution pH and antibiotic sulfamethoxazole (SMX) on the cadmium (Cd) adsorption and desorption on sediment, compare the adsorption capacity of two biochar for Cd and SMX, and then explore the effect of biochar on the sorption and immobilization of Cd and SMX on sediment. The main conclusions are as follows:1. Batch equilibrium and edge experiments were performed to discuss the isotherm characteristics of Cd, and assess the effects of organic matter, pH and SMX on adsorption and desorption of Cd on sediment. The results indicated that the adsorption of Cd was well fitted with the Freundlich isotherm. Indicated by the Freundlich parameter Kf (457.05>359.13L/kg), the sorption affinity of Cd on NS (natural sediments) was stronger than RS (sediments removed organic matter), and the organic matter prevented desorption of Cd from sediment, which meant that organic matter played an important role in the Cd sorption on sediment. Increasing pH from3to9led to elevated adsorption amounts and decreased desorption quantity of Cd, confirming the importance of pH in the immobilization of Cd by sediment. The presence of SMX significantly inhibited Cd adsorption on sediment and enhanced desorption of Cd, which revealed the increased environmental risks of Cd under combined pollution conditions.2. The simultaneous sorption behavior and characteristics of Cd and SMX on rice straw biochar (RS400) and Thalia dealbata biochar (TD600) were investigated. Isotherms of Cd and SMX conducted at pH6were well modeled by the Langmuir equation (R2>0.93). The calculated maximum adsorption parameter (Q) indicated that the adsorption capacity of RS400for SMX was larger than TD600, and TD600showed stronger adsorption ability for Cd than RS400. The presence of Cd significantly promoted the sorption of SMX on both biochars. Thermodynamic analysis indicated that the adsorption of Cd and SMX on both biochars was spontaneous physical processes. When the pH ranged from3to9, the sorption of Cd on RS400had the characteristics of a parabola pattern with maximum adsorption at pH5, whereas the pH showed no significant influence the sorption of Cd on TD600. The adsorption quantity of SMX on RS400decreased with increasing pH, with maximum adsorption at pH3, and the adsorption amount of SMX on TD600firstly increased and then decreased with the increasing pH. The amount of SMX adsorbed on both biochars were significantly affected by the diameter of biochars, and maximum adsorption occurred with d250biochar (biochar with a diameter of <250(>150) μm) for both biochars. SEM and FTIR showed that the removal of Cd by biochar may be attributed to precipitation and the formation of surface complexes between Cd with carboxyl and hydroxyl groups on biochar surface. The larger adsorption capacity of RS400for SMX than TD600might be due to the result of larger SA, higher O/C ratio and larger number of oxygen-containing functional groups.3. The capability of biochar to adsorb and retain Cd and SMX by sediment through edge and column leaching experiments were studied. The adsorption amount of Cd and SMX on sedimenta added with different biochar mass ratio increased with the increasing ratio until5%, the sediment added with10%biochar showed no preferable adsorption than5%, which may be due to the aggregation of biochar under high concentration of biochar. Sediment added with RS400exihibited stronger capacity to adsorbed Cd than TD600, and the mixture showed efficient ability to adsorb Cd and SMX at low concentrations. The results of column leaching experiment indicated that retention of Cd from sediment was significantly enhanced by RS400, whereas biochar posed no effect on SMX retention. As Cd would be adsorbed on both outer surface sites and inner sites of biochar, and SMX with low solubility could only be retained on outer surface, SMX was more easily removed by elute. This study indicated that RS400could be utilized as an efficient adsorbent for Cd and SMX, and further studies on improving the retention ability of biochar for SMX were needed. |
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