Cadmium Distribution and Migration as Influenced by Polymer Modifiers in a Loam Soil
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This paper studies the influence of four modifiers (organic-inorganic composite modifiers, inorganic polymer compound modifiers, polyacrylate compound modifiers, organic polymer compound modifiers) on soil pH, cation exchange capacity (CEC), Cd concentration and their distribution and migration in the soil profile under high Cd concentration (40 mg/kg) during continuous remediation tub experiment. The results showed that: (1) Four modifiers significantly increased the pH and CEC in each soil layer, and inorganic polymer compound modifiers providing the best results in the 0 �20 cm soil layer. (2) There is an extremely significant negative correlation between the carbonate-bound Cd and exchangeable Cd, with considerable effect in the 0 �20 cm and 20 �40 cm soil layers. The inorganic polymer compound modifiers had the best effect on the soil exchangeable Cd. (3) Both pH and CEC in all soil layers were negatively correlated with exchangeable Cd and positively correlated with other forms. (4) Fourier transform infrared spectrometry (FTIR) analysis showed that the modifiers immobilize soil Cd mainly through chemical precipitation, complexation and adsorption so as to reduce the bioavailability of Cd. Thus, we concluded that four modifiers are suitable for Cd mediation/Cd stabilization purposes.Keywords:
Cation-exchange capacity
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Adsorption isotherm experiments were conducted to investigate the effect of silicon on the adsorption of cadmium in two paddy soils. The pH value of media was adjusted to neutral to eliminate the effect of silicon addition on soil pH. The results showed that when adding the same amount silicon, the adsorption amount of cadmium in soil with low pH was lower than the soil with high pH value. While cadmium added at 0～50 mg·L-1, the adsorption of cadmium in two soils conformed to the Freundlich adsorption isotherm best. With increasing of the additive silicon at the range of 0～120 mg·L-1,the adsorption quantity of cadmium had different degrees of decease. Hence, the idea that additive silicon decreases the effect of cadmium in soil and relieves the poison for plant is not the adsorption of cadmium in soil, and other mechanisms may exist, which need to be developed.
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Cation-exchange capacity
Soil test
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Research has been done on processing methods of cadmium in wastewater,then obtaining optimum adsorption conditions by using of slag cement as adsorbent,which adsorbs cadmium in uranium-containing wastewater.Results show that the slag cement can adsorb more than 98.5% of cadmium to top point in 10 min when pH is in the intervals from 3 to 12,the concentration of cadmium is 10 mg / L in solution,the dosage of adsorbent is 0.1% of the solution.The order of adsorption is Cu2 + UO 2 2 + Fe3 + Zn2 + Cd2 +when the solution contains Fe3 +,UO 2 2 +,Cu2 +,Zn2 +.Those coexisting ions have a great influence on the adsorption rate of cadmium.Segmented adsorption can be used to improve the adsorption rate of cadmium.
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The excess sludge driven from corn processing wastewater treatment station was applied to absorbing lead and cadmium.The factors affecting the adsorption including adsorption time,temperature,pH value,excess sludge mass and the coexisting of lead and cadmium were investigated and analyzed.The result shows the adsorption of lead and cadmium onto the sludge is a quick process and the temperature has a less effect on the adsorption of lead and cadmium onto the sludge.The optimal adsorption pH range of the system for lead or cadmium is from 4 to 8.The adsorption capacity of lead and cadmium onto the sludge increases with the sludge mass increasing;the optimal sludge mass for lead adsorption is 0.5 g/L,the optimal sludge mass for cadmium adsorption is different for the different initial concentration of cadmium and the value is between 0.5 g/L and 2 g/L.In the coexisting adsorption system of lead and cadmium,lead coexisted has an inhibitory effect on the adsorption of cadmium and cadmium coexisted has little effect on the adsorption of lead.
Lead (geology)
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Two experiments were conducted to investigate and quantify the weak and strong adsorption of an indicatorbacterium (Escherichia coli) in soilwater systems composed of Tangi silt loam (14% clay) or Commerce clay loam (35%clay). Percent adsorption of E. coli was significantly higher in Commerce clay loam than in Tangi silt loam for both weakand strong adsorption. The distribution coefficient of weak adsorption of E. coli in the Commerce soilwater system was foundto be significantly higher than that of strong adsorption. However, the distribution coefficient of weak adsorption of E. coliin the Tangi soilwater system was found to be significantly lower than that of strong adsorption. For strong adsorption,together with literature data, a high correlation (R 2 = 0.89) was found between percent adsorption and clay content.Significant correlation (R 2 = 0.67) was found between distribution coefficient and clay content. The relationships developedmay be used for modeling purposes.
Silt
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Cadmium adsorption features by montmorillonite and illite in alkaline conditions and the influence of inorganic salts were studied by experiment. The cadmium adsorption equilibrium time was 6 hours when the pH values in initiative solutions were 9, and the adsorption cadmium capacity by montmorillonite and illite was 6.910 mg.g -1 and 6.642 mg.g -1 . When there are inorganic salts in the solutions of which the pH values in initiative solutions were 9, cadmium adsorption capacity by montmorillonite and illite changed pronouncedly. Taking chloride for example, the orders of cations influencing on cadmium adsorption effect were NH 4 + >Ca 2+ >Na + >K + >Mg 2+ ; Taking sodium salt for example, the influence of anions in alkaline conditions were complicated: Cl - , NO 3 - decreased cadmium adsorption capacity while CO 3 2- , H 2 PO 4 - , HSO 4 2- increased adsorption cadmium capacity by precipitation adsorption.
Illite
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Under different environmental conditions,based on the distribution coefficient(Kd),the adsorption properties in loam of 238U was studied by batch method,in order to provide some basis for the screening waste repository backfilling materials.The results showed that: adsorption properties of loam in the 238U is good and loam is a good barrier material;The influence of different 238U concentrations and different solid-liquid ratio on the adsorption properties of soil is relatively large,followed by the size.With the solid-liquid ratio and particle size decreased,the adsorption ratio showed increasing trend,while with the initial concentration of 238U increased,the adsorption ratio decreased.When the 238U concentration is 6.681mg/L and solid-liquid ratio is 1:5,the loam adsorption of 238U is the maximum,which is 3.3×104ml/g.
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Summary The mechanisms governing the retention and release of Cd in two soils, a loam and a loamy sand, pretreated with anaerobically digested sewage sludges or with chemical fertilizers, were studied using batch equilibration in 0.05 m Ca(NO 3 ) 2 solution containing up to 6 μ g Cd/ml. Adsorption rather than precipitation as Cd 3 (PO 4 ) 2 limited solution Cd 2+ concentration. With the addition of 50 μ g Cd/g, however, precipitation as CdCO 3 was likely at pH 7.6. Cadmium adsorption increased with increasing soil pH. The differences in Cd adsorption between different soil treatments were attributed mainly to the soil pH (6.9 to 7.9) induced by sludge application. About 82 to 92 per cent of adsorbed Cd was retained by cation exchange and complexing sites. Soils treated with sludge increased the amount of exchangeable Cd but reduced the amount of complexed Cd compared with the fertilized soil. Cadmium retention by cation exchange became more dominant as the amount of Cd in the soil was increased.
Sewage sludge
Cation-exchange capacity
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Most riverine sediments have the high capability to adsorb and store heavy metal ions. In the present study, the adsorption capacity of the bed sediments collected from the Karaj River (Iran) were experimentally studied for cadmium ion adsorption. Multi-stage batch adsorption experiments were carried out for a constant sediment concentration of 20 g/L and different initial cadmium concentrations of 0.2, 0.5, 1, 10, 20, and 50 mg/L. The cadmium solutions with known concentrations were added to the bed sediment with the mean diameter of 0.53 mm in five stages to characterize the capacity of the sediments to adsorb cadmium. The batch adsorption experiments were conducted as both kinetics and equilibrium. The results showed that by adding cadmium ions to the sediment at each stage, the adsorption capacity was less than the previous stage, and for a cadmium concentration of 0.2 mg/L, the adsorption percent and the amount of adsorbed cadmium was reduced from 88 to 70% and 9 to 6.8 mg/kg, respectively. These changes decreased with increasing initial Cd concentrations. This process is useful for seasonal rivers in which a certain concentration of heavy metal pollution may occasionally flow over the bed.
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