Adsorption Kinetics of NH4+by Purple Soils with Different pH Values

Dinan ZHANG, Xianjun JIANG

College of Resources and Environment, Southwest University, Beibei 400716, China

The nitrogen fertilizer applied into soil is mainly as NH4+-N[1-2].Part of the NH4+is consumed and retained by ammonia volatilization,biological uptake and soil adsorption; but a considerable part of the NH4+is nitrified to NO3-. The NO3-can be leached easily into deep soil, resulting in groundwater contamination[3],and is also likely to enter surface water, resulting in eutrophication[4-5],thereby affecting the quality of water environment[6]. The adsorption of NH4+by soil can suppress nitrogen loss. So discussing on the adsorption and desorption properties of soil for NH4+is an important foundation for understanding the migration, transformation and other processes of nitrogen in the soil,and is also a theoretical basis for reducing nitrogen loss and preventing groundwater contamination. Purple soil is mainly formed in the Sichuan Basin in Southwest China. It is named as regosol by the Food and Agriculture Organization (FAO) of the United Nations,and Entisol by the United States Department of Agriculture (USDA)[7].Purple soil has soft parent material,poor corrosion resistance, shallow soil layer and serious soil erosion, and is easy to be disintegrated by weathering and to lead to non-point source pollution[8].Although the adsorption of NH4+has been well studied, there are rare reports on adsorption of NH4+by purple soil. Xie et al.[9]ever studied the adsorption of NH4+by purple soil,but they did not investigate the effect of pH on adsorption and desorption properties of purple soil for NH4+.In this study,the purple soils with different pH values were collected from Beibei,Chongqing, and their adsorption and desorption properties for NH4+were discussed so as to quantitatively study the adsorption and desorption laws.The study results were further fitted with three kinds of traditional isothermal adsorption equations to determine the adsorption kinetics of NH4+by purple soils with different pH values.

Materials and Methods

Soil sampling

The soil was collected from Beibei District, Chongqing City (106°25′45″E,29°49′18″N),and it was calcareous purple soil developed from Jurassic mudstone. The top 0-20 cm soil was sampled. After dried, the soil was passed through a 1-mm sieve. The basic properties of the soil were as follows: pH 8.0, organic matter content 0.69%,total nitrogen content 0.1%,total phosphorus content 0.13%, total potassium content of 2.9% , rapidly available phosphorus 6.5 mg/kg and rapidly potassium content of 110.4 mg/kg.

The pH value of the purple soil was adjusted to 6.0 and 7.2 respectively by adding HCl.A certain amount(100 g) of dried soil that had been passed through a 1-mm sieve was dissolved in 250 ml of distilled water.The mixture was stirred vigorously with a glass rod for 1-2 min, and then stood for 30 min.Subsequently,the pH value of the soil suspension was measured with a pH electrode. During the measurement,0.1 mol/L of HCl was slowly added,along with the stirring,to adjust the pH value to 6.0 or 7.2.After the adjustment, the soil samples were dried at(40±1)℃,ground,and then passed through a 1-mm sieve.Finally,the accurate pH values of the soil samples were measured.

Ammonium adsorption

Certain amounts (1.5 g) of airdried purple soil sample were placed in centrifuge tubes. Subsequently, certain volumes(30 ml)of NH4Cl solutions with concentrations of 0, 50, 100, 200,400, 700 and 1 000 mg/L respectively(prepared with 0.01 mol/L of NaCl solution) were added to the centrifuge tubes.After shaken at(25±1)℃for 24 h, the tubes were centrifuged at 4 000 r/min for 5 min. The NH4+concentrations in the supernatants were determined. The NH4+adsorption am-ounts were calculated with the difference assay. Based on the calculation results,the adsorption curves of NH4+by purple soils with different pH values were drawn.

Ammonium desorption

The precipitates in the centrifuge tubes obtained above were washed with 30 ml of distilled water two times.A certain volume (30 ml) of 2 mol/L KCl solution was added to each of the centrifuge tubes. The tubes were shaken at (25±1)℃for 2 h and then centrifuged at 4 000 r/min for 5 min.The obtained supernatants were desorption solutions. The desorption amounts of NH4+were calculated.Based on the calculation results, the isothermal desorption curves of NH4+were drawn.

Chemical analysis

The NH4+concentration was determined with the diffusion method[10].

Calculations and kinetic models

Calculations The calculation formula was as follows:

Wherein, Q indicates adsorption amount/desorption amount; C0indicates the original NH4+concentration,mg/L; Ceindicates the NH4+concentration at the adsorption/desorption equilibrium, mg/L; V indicates the volume of added NH4Cl solution,ml;m indicates the mass of weighed soil sample,g.

Kinetic models The kinetic models used in this study included Langmuir isothermal adsorption equation, Freundlich isothermal adsorption equation and Temkin isothermal adsorption equation.

Data analysis

The graphing and the isothermal adsorption equation simulation were performed using Excel.

Results and Analysis

Effects of purple soils with different pH values on isothermal adsorption of NH4+

The isothermal adsorption curves of NH4+by purple soils with different pH values were shown in Fig.1. The adsorption amounts of NH4+by purple soils with different pH values were all increased with the increase of NH4+concentration, and they ranged from 0.659 to 13.5 mg/g. The adsorption amounts of NH4+were also increased with the increase of pH value. Among the purple soils with three different pH values,the purple soil with the pH value of 8.0 showed the strongest adsorption capacity for NH4+with the largest adsorption amount of 13.5 mg/g, followed by the purple soil with the pH value of 7.2 with the largest adsorption amount of 12.8 mg/g, and the purple soil with the pH value of 6.0 showed the weakest adsorption capacity with the largest adsorption amount of 10.3 mg/g.

Effects of purple soils with different pH values on isothermal desorption of NH4+

The isothermal desorption curves of NH4+by purple soils with different pH values were shown in Fig.2. The desorption amounts of NH4+by purple soils with different pH values were also trended to be increased with the increase of NH4+concentration,and they ranged from 0.115 to 7.96 mg/g. With the increase of pH value, the desorption amounts of NH4+by purple soils were reduced.The desorption capacity of purple soil with the pH value of 8.0 for NH4+was weakest with the largest desorption amount of 2.23 mg/g, followed by that with the pH value of 7.2 with the largest desorption amount of 4.62 mg/g, and the desorption capacity of purple soil with the pH value of 6.0 was strongest with the largest desorption amount of 7.96 mg/g.

Isothermal adsorption equation simulation for purple soils with different pH values

The isothermal adsorption equa-tion simulation related parameters for different purple soils were shown in Table 1. As shown in Table 1, the adsorption kinetics of NH4+by purple soils showed better correlation with Freundlich equation (R ＞0.95), showed relatively good correlation with Temkin equation(0.72≤R≤0.89),but showed poor correlation with Langmuir equation(R＜0.74).The correlation between adsorption curve of NH4+by purple soil with pH value of 7.2 and Langmuir equation was only 0.043. Therefore,in this study, the Freundlich equation was selected to describe the adsorption characteristics of NH4+by purple soils with different pH values.

Table 1 Isothermal equation simulation for purple soils with different pH values

Discussion

Effects of purple soils with different pH vales on isothermal adsorption and desorption of NH4+

Fig.1 showed that the adsorption amounts of NH4+by purple soils with three different pH values ranged from 0.659 and 13.5 mg/g,and they were all increased with the increase of NH4+concentration.According to the chemical equilibrium theory, if the other conditions are remained unchanged,the larger the substrate concentration is, the easier the positive reaction is.The equilibrium will be distorted to the direction of production generation.Therefore, the larger the NH4+concentration is, the easier the adsorption is.Moreover, under the condition of high NH4+concentration, the probability of collision between NH4+and adsorbent surface will be increased[11].In addition,adsorbents all have a certain adsorption capacity for water. With the increase in original concentration, more ions enter into the interior of adsorbents until the adsorption saturation[12].Therefore, the adsorption amounts of NH4+by purple soils with three different pH values were all increased with the increase of NH4+concentration.

The largest adsorption amounts of NH4+by purple soils at pH 6.0,7.2 and 8.0 were 10.3, 12.8 and 13.5 mg/g,respectively. So it could be concluded that the adsorption amounts of NH4+by purple soils were also increased with the increase of pH value. This might be because that NH4+and H+have the same adsorption site. The changes in pH value will inevitably lead to the competition for adsorption sites between NH4+and H+. Under the condition of low pH,H+concentration is very high, so the anionic groups on adsorbents surface are prone to select H+.The lower the pH is, the more intense the competition is. So the soil with higher pH value can weaken the competition for adsorption site between NH4+and H+,increasing the adsorption amount of NH4+[13-16]. In addition, the changes in electrostatic potential and charge on adsorbent surface can also influence the effect of pH value on NH4+adsorption. The electrostatic potential on variable charge surface is reduced with the increase of pH value,increasing the negative charge amount on surface[17]. Therefore, the increased pH is conducive to the NH4+adsorption by soils.

As shown in Fig.2, the desorption amounts of NH4+by purple soils with three different pH values ranged from 0.115 and 7.96 mg/g, and they were basically increased with the increase of NH4+concentration, of which the fundamental principle was the same with that of increased adsorption amount. The largest desorption amounts of NH4+by purple soils at pH 6.0, 7.2 and 8.0 were 7.96, 4.62 and 2.23 mg/g, respectively. So it could be concluded that with the increase of pH value, the desorption amounts of NH4+by purple soils were reduced. This is because the adsorption and desorption of NH4+are essentially the adsorption of charges, of which the adsorption and desorption processes belong to diffusion process[18].The adsorption of NH4+by soil is influenced by anions on soil colloids surface. When the pH value of desorbing solution is too low,the H+, instead of NH4+, will bind with anions, resulting in increased NH4+concentration in low-pH solution[12].Thus, the lower the pH value is, the larger the desorption amount is.

Isothermal adsorption equation simulation for purple soils with different pH values

In the researches on adsorption characteristics of NH4+by soil,isothermal adsorption curve is often used,and it is a thermodynamic method.Isothermal adsorption curve refers to the relationship curve between the equilibrium concentration or activity of adsorbent in solution and amount of adsorbate on solid particle surface under the conditions of a constant temperature. Equilibrium adsorption isotherm equation can be used to quantitatively describe the distribution of ions in the solid phase[19]. The isotherm equations include Langmuir,Freundlich and Temkin.

As shown in Table 1, the isothermal adsorption of NH4+by purple soils showed a very good correlation with Freundlich equation (R ＞0.95). So in this study, the Freundlich equation was selected to describe the adsorption characteristics of NH4+. The adsorption constant K reflects the level of soil adsorption solutes to some extent.When the K value is positive,the reaction can proceed spontaneously at room temperature. The size of the K value reflects the spontaneous degree of adsorption reaction. The larger the K value is, the larger the spontaneous degree is;the larger the K value is,the more stable the products are; the larger the K value is, the stronger the adsorption capacity for solutes is. However, the larger the K value is, theweaker the solutes providing capacity is[20]. The K value of the purple soil at pH 7.2 was largest (0.057 9), followed by that at pH 6.0 (0.037 0),and the K value of the purple soil at pH 8.0 was smallest (0.029 7).So it could be concluded that the adsorption of NH4+by purple soils with three different pH values all could proceed spontaneously at room temperature. The spontaneous degree of purple soil at pH 7.2 was highest, followed by that at pH 6.0, and the spontaneous degree of purple soil at pH 8.0 was lowest.The n in Freundlich equation is used to indicate the size of nonlinearity of adsorption isotherms. When n= 1, the isotherms are liner[21].The closer the n value to 1 is, the better the linearity is[22].Generally, when the value of 1/n ranges from 0.1 and 0.5, the adsorption is considered to be easy, and when the value of 1/n ＞2, the adsorption is considered to be difficult[23-24]. In this study, the 1/n value of purple soil at pH 8.0 was largest(1.013),followed by that at pH 6.0 (0.914), and the 1/n value of purple soil at pH 7.2 was smallest(0.895).Although the 1/n values of purple soils with three different pH values were all not in the range of 0.1-0.5,they were all less than 2,indicating that the adsorption of NH4+by purple soils was all relatively easy.The 1/n value of purple soil at pH 8.0 was closest to 1, indicating the linearity of adsorption isotherm of purple soil at pH 8.0 was best.The 1/n value of purple soil at pH 6.0 ranked second. But the 1/n value of purple soil at pH 7.2 was most distant from 1,indicating the linearity of adsorption isotherm of purple soil at pH 7.2 was poorest.

Conclusions

In this study, the effects of different pH values on adsorption and desorption capacities of purple soil collected from Beibei District,Chongqing City are investigated. The results indicate that the adsorption amounts of NH4+by purple soils with three different pH values are all increased with the increase of NH4+concentration, and they are all increased with the increase of pH value;the desorption amounts of NH4+by purple soils with three different pH values are all increased with the increase of NH4+concentration, but they are all decreased with the increase of pH value;the correlation between the isothermal adsorption of NH4+by purple soils and the Freundlich equation(R ＞0.95) is highest, followed by that between the isothermal adsorption of NH4+by purple soils and the Temkin equation, and the correlation between the isothermal adsorption of NH4+by purple soils and the Langmuir equation was lowest.

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