Influence of silicon on the adsorption characteristics of Cadmium in soils
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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.Cite
The effect of added silicon on the adsorption and the thermodynamics of lead(Pb)in paddy soils with two different pH was investi- gated by the adsorption isotherm experiment. The HNO3 was used to neutralize the alkalinity of Na2SiO3, and the different concentration of NO3- and Na+ between treatments were balanced with the addition of NaNO3, thus the effect of pH and accompanying ions by added silicate on Pb adsorption was eliminated. Results showed that Freundlich adsorption models could fit well to simulate the Pb adsorption characteristics at three different temperatures and in two types of soils. Added silicon enhanced Pb adsorption in acid soil, while added silicon decreased Pb adsorption in alkaline soil. The adsorption of Pb in the two types of soils was physically endothermic progress with increasing entropy as indi- cated by the results of thermodynamics function:△G0,△H0,△S0.After addition silicon, △G decreased,and △H and△S increased in acid soil, while in alkaline soil, △G increased, and △H and △S decreased, indicating that the spontaneity of Pb adsorption increased in acid soil but decreased in alkaline soil after addition of silicon.
Alkalinity
Endothermic process
Alkali soil
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Adsorption is a major process responsible for accumulation of heavy metals which in turn leads to an increase in health hazards. Understanding absorption process of these heavy metals is crucial for the assessment of their fate and reclamation in polluted soils. In this study, the sorption mechanism of cadmium metal on lateritic soil was evaluated at two temperatures (298K and 323K) using the batch method. The lateritic soil samples used were collected from Ilaro, Ogun State. The Langmuir and Freundlich isotherms were used to describe the adsorption of the metal ion. Results of thermodynamic parameters revealed that the equilibrium constant (Keq) decreased with increase in temperature. The small but positive value for the free energy (∆G) indicated that the absorption of cadmium ion on the soil was spontaneous. The enthalpy of the reaction (∆H) was negative implying that the aborption of this metal ion is exothermic. The positive value (63.23Jmol -1 ) of the entropy of reaction, ∆S is an indication of disorder in the cadmium sorption process in the soil which leads to an increase in the degree of randomness at the surface of the soil which in turn results to an increase in the adsorption capacity of the ion in the soil. Keywords: Cadmium, Sorption, Freundlich, Langmuir, isotherms and thermodynamic
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This paper presents an evaluation of the environmental behavior of arsenic species in soils (F. F. Dias, Master Thesis, University of Delaware, 1997). The results obtained were used to determine adsorption constants that were incorporated in mathematical models using forward and backward stepwise linear regression to correlate data. The amount of adsorption was significantly different depending on soil properties, such as organic matter, iron oxide content, and surface area. Arsenic speciation on the soil surface was deduced from desorption data, with As(V) being more strongly retained in the soil. As(III) was oxidized on the soil surface and desorbed as As(V); an important factor since As(V) is less toxic. In order to develop an adequate adsorption model, Langmuir and Freundlich isotherms were obtained for each soil without pH alteration. Results indicated that the maximum amount of As(V) adsorbed was greater than the amount of As(III) adsorbed. Adsorption edges for As(III) and As(V), with pH varying from 3 to 10, were obtained at concentrations that ranged from 0.1 to 200 mg L−1. The soils studied exhibited an L-type Langmuir isotherm. Maximum As(III) adsorption occurred around pH 6 to 9, while maximum As(V) adsorption occurred in the 4 to 5 pH range. Experiments to determine arsenic kinetics were carried out and showed that adsorption and desorption equilibrium was reached within 48 hours for both species.
Langmuir adsorption model
Soil test
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Adsorption of heavy metals by soils plays a key role in controlling the availability of heavy metals.The retention of heavy metals depends not only on the nature of soil such as the mineral and organic constituents,the composition of the soil solution and its pH,but also on the external factors such as properties of metals,the added cations and anions,the temperature of reactions and the presence of organic or inorganic matters.The objectives of this study were to understand the adsorption behaviour of Sr2+ in various conditions.In present paper,series of experiments were conducted to study the absorption characteristics of Sr2+ in the surrounding soils of a petrochemical sewage reservoir in Xinjiang through static adsorption tests.The effects of adsorption conditions such as pH,temperature,initial concentration of strontium ions,ionic strength and adsorption time on the adsorption capability were investigated.The results indicated that the adsorption capability increases with the increase of temperature and initial concentration of strontium ions.The pH value has significant influence on the soil adsorption of Sr2+.Besides that,the adsorption increases remarkably with the rise of pH.The adsorption performance was optimum when the adsorption equilibrium time of Sr2+ was 3 hours.The adsorption capacity was found to decrease slightly after 3 hours and gradually stabilize.The adsorption behaviour of Sr2+ in the tested soil gradually declined with the increase of ionic concentration under the condition of pH=5,but the soil adsorption of Sr2+ showed an increasing trend when the concentration was greater than 0.2 mol/L.The results of kinetic experiment showed that the adsorption curves of soils for Sr2+ comply to Henry equation and Freundlich equation.Within 3 hours of kinetic adsorption,the removal of Sr2+ can be well described by first order reaction kinetic equation.These results provide a basic data set and theoretical basis for establishing decision making of environmental protection.
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Abstract The adsorption of Cd and Cu by three different soils was studied in the presence of variable concentrations of calcium chloride. Cadmium adsorption fitted the Freundlich equation while Cu better fitted the Langmuir equation. Increasing ionic strength decreased metal adsorption because of the competition of Ca‐ions for the adsorption sites of the soils. Results could be useful from the standpoint of correct management of sewage sludges and animal slurries applied to agricultural soils because they have an electrolyte concentration greater than that found in normal soil solutions.
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In this work, both batch method and column method were used to investigates the adsorption of Ni and Zn on different kinds of soils found in the Northeastern Thailand including clay, silt, and sand. The batch method was used to examine the adsorption isotherm and the effect of contact time, initial concentration, and temperature on the adsorption. The column method was used to simulate the real condition of wastewater adsorption. The results indicated that the adsorption isotherms were fitted with Freundlich and Langmuir isotherm. All soils adsorbed Ni better than Zn. The adsorption capacity of soils tended to increase when the temperature increased from 30°C to 50°C. Soil surface characteristic was an important factor that affected on the adsorption capacity of soils. For the column test, the adsorption decreased with the increasing amount of metal solution. Leaching of the metal occurred in a fractional volume.
Silt
Langmuir adsorption model
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[Objective] The research aimed to study the effects of adsorption and desorption on the bioavailability of heavy metals in soil. [Method]Batch techniques were used to study the energy characteristics of adsorption and hysteresis effect of desorption of cadmium in 12 typical agricultural soils from China. [Result]Adsorption process of cadmium in soils was spontaneous process. With the increasing temperature, it could promote the adsorption of cadmium in soils. The value of Gibbs free energy( ΔG°) could be used to predict the desorption ability of cadmium in soils. The main mechanism of the cadmium adsorption in soils was the chemical bond force. There was hysteresis effect in the process of cadmium desorption,and with the increase of cadmium concentration in the equilibrium the hysteresis effect was enhanced. When pH and calcium carbonate content were higher,the hysteresis index was greater because of the cadmium forming the inner complexes and cadmium carbonate precipitation in the soils. [Conclusion]The study could provide some reference and gist for prevention of heavy metal polluted soils and for remediation of contaminated soils.
Hysteresis
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Adsorption is the initial process when heavy metals enter soil.Adsorption is important because it directly influences the bioavailability of heavy metals in soil.In this experiment,batch techniques were used to study Cd adsorption by 12 typical agricultural soils from China.The adsorption data were fitted using kinetic equations.The results showed that Cd adsorption could be divided into two steps regardless of temperature:fast reaction and slow reaction.At 25 ℃,Cd adsorption reached 72.69% of the soil's maximum adsorption capacity within the first 5 minutes of the reaction and 83.76% of the soil's maximum adsorption capacity within the first 30 minutes of the reaction.At 40 ℃,Cd adsorption reached 77.50% of the soil's maximum adsorption capacity within the first 5 minutes of the reaction and 87.02% of the soil's maximum adsorption capacity within the first 30 minutes of the reaction.Chemical adsorption was the main adsorption mechanism for the fast reaction,whereas physical adsorption was the main adsorption mechanism for the slow reaction.Furthermore,the whole reaction mainly depended on the fast reaction,which is attributed to chemical adsorption.The Elovich model best described Cd adsorption by these soils.The adsorption rate and amount of Cd adsorption were both greater for alkaline soils than for acid soils.High temperatures also increased the adsorption rate and the amount of Cd adsorption by the soils.The Cd adsorption rate was mainly affected by pH.
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The cadmium adsorption and desorption by simulant acid rain on different clay minerals in soil have been discussed. When the pH value is 7, adsorption time is 1 h, and the medium is 0.05 mol.L -1 KNO 3 , the saturated adsorption capacity of cadmium on montmorillonite, illite, and sepiolite is found to be 2.88,1.08,2.82 mg/g respectively, but on kaolinite nothing is found. A correlation analysis shows that the saturated adsorption capacity of cadmium on silicate clay minerals are related to their physical and chemical natures and the correlation between the saturated adsorption capacity of cadmium and the amount of manganese oxide is extremely significant. Both pH value and ionic strength of simulant acid rain have significant effects on desorption of cadmium. The desorption rate of cadmium increases with the decline of the pH value and the increase of ion strength. The desorption reaction is fast, and in addition the cadmium desorption rate has little change with the delay of desorption time and the increase of desorption times. Montmorillonite, illite and sepiolite have high adsorption rates for cadmium, and the desorption rates are low under the simulant acid rain conditions. The strong adsorption of cadmium on montmorillonite, illite and sepiolite is an important reason for cadmium retention in soil.
Illite
Sepiolite
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Characteristics of Arsenic adsorption in red soil, in Zunyi, Guizhou, affected by pH were studied in this research work. Results show that with increase in Arsenic concentration in the soil solution, Arsenic adsorption of the soil increased, while its adsorption rate declined, high-energy adsorption sites began to gradually replace low-energy adsorption sites in absorbing As, and the absorbing reaction rate reduced. The isotherm adsorption could be well fitted by the Langmuir equation. Soil pH was an important factor that affected adsorption and desorption of Arsenic. Acid environment(pH=4~6.5) facilitated adsorption of Arsenic while alkaline environment(pH=7~8.5) did desorption of Arsenic.
Langmuir equation
Langmuir adsorption model
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