| Adsorption features of cadmium by soil compositions,including clay minerals, iron and manganese oxides and organic matter (Humic acid),were studied by experiment. And effects of inorganic salts species and concentration on adsorption and desorption press were also studied,trying to reveal the effect of inorganic salt on soil cadmium geochemical behavior and mechanism. The main results are as follows:1. The cadmium adsorption equilibrium time was6hours when the pH values in initiative solutions were9, and the adsorption cadmium capacity by montmorillonite was6.910mg.g-1while the adsorption capacity of cadmium by illite was6.642mg.g-1. when the quantity ratio between adsorbent and cadmium in initiative reaction solutions was15X103, the adsorption effect was best. The cadmium adsorption capacity increased with the increase of pH values from7to10. When there are inorganic salts in the solutions of which the pH values in initiative solutions were9, cadmium adsorption capacity by montmorillonite and illite changed pronouncedly. Taking chloride for example, the influence of cations on adsorption cadmium strengthened with inorganic salts concentrations increased in the solutions; when the cations concentrations were same, and the orders of cations influencing on adsorption effect were Ca2+>K+>Na+; Taking sodium salt for example, the influence of anions in alkaline conditions were complicated:Cl-, NO3-decreased cadmium adsorption capacity while CO32-, H2PO4-, HSO42-increased adsorption cadmium capacity by precipitation adsorption.2. The adsorption quantity of cadmium by Goethite and Manganite increased with the increase of pH from acidity to neuter and reached the maximum in Alkaline condition. After the absorption of Goethite and Manganite to cadmium happened, The pH values of the solutions were changed, and the critical pH values were8for Goethite and6.5for Manganite. The absorption balance time of Goethite and Manganite to cadmium was6hours.The absorption isotherms were consonant with Freundlich isothermal equation. The absorption effect of Goethite and Manganite to cadmium was good under the room temperature. The coexisting chlorides reduced the adsorption capacity of Fe2O3and MnO2. The influence intensities of different cations follow the order of CaCl2>>Cl>NaCl.However, the influence of sodium salts on the capacities of Fe2O3and MnO2to adsorb cadmium appeared more complicated:the relatively low concentrations of sodium salts could reduce the adsorption capacity; with increasing concentrations of sodium salts, e.g. NaCl and NaNO3. the adsorption capacity decreased continually. Moreover, due to the competition adsorption and precipitation effects, the adsorption capabilities of Na2CO3, NatH2PO4and Na2HSO4could also be reduced and cadmium concentrations in the solution were reduced as well.3. Humic acid has the big ability of the adsorption of cadmium, Langmuir adsorption mode andTemkin adsorption models are well fitted their adsorption process (R is0.9809**and0.9816**);stage of rapid response within60mins.and60mins to6hours in the slow reaction stage, the adsorptioncapacity of humic acid were2.895mg/g and3.342mg/g,6hours before thebalance of adsorption and reaction kinetics of the process canbe well Elovich equationfitting (R=0.9285**); as the pH increases the adsorption rate showed a gradual increasing trend,and at pH4.5and8.5as the boundary, showing both ends of theincrease in speed, slow increase in the middle of regularity; in the morethe ionic strength of the low concentration, increasing ionic strength to promote humic acid adsorption of cadmium; in highionic strength, showing the opposite regularity; under the same conditions, at different ionic strength affect the size of humic acid adsorption of cadmium:calcium chloride, magnesium chloride>potassium chloride> sodium chloride. |
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