Sorption and desorption of 2,4-dichlorophenol in soil environment

Zhu Kun ¹, Jiang Mei², Zhan Huiying³
(¹Department of Environmental Eng., Lanzhou Jiaotong University, Lanzhou, 730070, China; ²Guangzhou Chemical Technology College, Guangzhou, Guangdong 510370, China; ³Department of Chemistry, Gansu Lianhe University, Lanzhou, 730000, China )

Received on Jun.29, 2005; Supported by the National Natural Science Foundation of China (No.29977015) and  Science Foundation of Gansu Province (No.GH2002-14).

Abstract The sorption-desorption processes and mechanisms of 2,4-dichlorophenol (DCP) on the soil surfaces were studied by using column tests. The results proved that the isothermal sorption curves of DCP were nonlinear, and could be explained with Freundlich Equation. The sorption rates of DCP in the soils increased with the increase of the initial concentrations, and were positively related to the calculated Freundlich constant (K_F) and the adsorption capacity of soil (C_smax). Moreover, the desorption behaviors of DCP in the soil environment showed the significant retention that was affected by pH and the existence of the metal cations in the environment. Sorption of DCP could be increased when pH decreased from 10.8 to 6.7, indicating that the sorption of neutral DCP was higher than its ionic form in the soil matrix as DCP is an ionizable compound (pK_a=7.7). At the high pH values, most of the neutral molecules could be ionized to be free ions or ion pairs that have the stronger hydrophilicity and easier enter aqueous phase. The apparent sorption retention of DCP on the soils could be observed if the soils contain two-valence metal cations. It was found that DCP retention was enhanced by cations following the sequence as: Cd²⁺ > Mn²⁺ > Zn²⁺ > Ca²⁺ > Mg²⁺. Sorption and desorption of DCP in the soil matrix can be described by an extended Dual-Mode Model.
Keywords 2,4-dichlorophenol; Sorption, Loess soil, Retention，Column test

1.  INTRODUCTION
Chlorophenols are a group of chemicals in which chlorines (between one and five) have been added to phenol. There are five basic types of chlorophenols in which 2,4-dichlorophenol (2,4-DCP) as a di[two]chlorophenol, has a wide environmental concern because it is commonly used as mothproofing and miticide. The largest uses for 2,4-DCP have also been found as an intermediate, especially in the productions of herbicides and pesticides. Man who is exposed to 2,4-DCP, could through breathing cause risk of cancers, and through the skin developed acne cause mild injure to his livers ^[1]. 2,4-DCP can be present in drinking water when chlorine is used to disinfect it, therefore, the most likely source from which children could be exposed to 2,4-DCP, is water that has been disinfected by chlorine.
    Children may also be harmed in areas where has been sprayed as herbicides and pesticides. Volatilization may be the major dispersal mechanism of DCP into the atmosphere, as a result, it has often been detected in atmospheric emissions from the combustion of municipal solid waste, hazardous waste, coal, wood, and 2,4-DCP-based herbicides^[2]. It was reported that 2,4-DCP was also found in the leachate from an industrial landfill ^[3]. Moreover, some chlorophenols, such as 2,4,5-T that is often used as herbicide on food crops, can break down to form 2,4-DCP. Therefore it has often been detected both in surface water and groundwater in addition to various aqueous systems and terrestrial systems.
    However, the majority (85%) of known environmental releases of 2,4-DCP were to surface water through effluents discharged from industries that manufacture iron and steel, electrical components, photographic processing, pharmaceuticals, organic chemicals and from paper & pulp mills^[4,5]. Due to the great solubility and high mobility and wide distribution in the environment, 2,4-DCP has been taken as one of the priority pollutants in effluent by USEPA^[6] that regulates a wastewater discharge limitation 112mg/L for one day，and monthly average maximum concentration should not exceed 39mg/L under the best available treatment technology while 0.3 mg/L for the drinking water quality standard and 3.08mg/L for both ambient water quality criteria and fishery water. The environmental fate and transport of 2,4-DCP are controlled by its physical and chemical properties and environmental conditions. Volatilization of 2,4-DCP from water is expected to be slow and, therefore, not a major removal process from surface waters. The biological treatment of waste containing 2,4-DCP has showed that none of the chemicals is removed by stripping^[7]. Volatilization from topsoil is also not expected to be a significant removal process. Thus, the environmental fate and transport of 2,4-DCP has the tendency to partition into sediments and lipids and to bioconcentrate.
    As an anthropogenic organic compound, 2,4-DCP is usually regarded to be a polar or hydrophilic chemical due to the H-bonding ability of the hydroxyl group although it has the significant hydrophobic surface areas. Moreover, the hydroxyl group permits orientational interactions with the sorbent. According to the chemical properties, DCP can be prevented from the environmental contamination in transport in the soil-water matrix, particularly regarding its sorption capacity that is the most important mechanism for the attenuation of organic pollutants in the natural loess soil.
    Sorption onto the soil surfaces and aquifer media is the key process that controls mobility, transformation and biological degradation of organic contaminants in the water-soil environment. Publications have reported on sorption of HOCs to soil and sediments in aqueous systems, indicating that sorption isotherms follow both linear and nonlinear equations^[8]. Linear isotherms are primarily considered to be resulted from partitioning (dissolution) of HOCs into the matrix of soil-organic matter. On the other hand, the sorption isotherms could become non-linear because of non-ideal behavior, such as competition among solutes for sorption sites and sorption-desorption hysteresis^[9]. There are two schools of thoughts dealing with the cause of isotherm nonlinearity: (1) retention by heterogeneous soil organic matter that contains both "rubbery" (or soft) and "glassy" (or hard) polymer-like sorption domains^[10], and (2) the presence of small quantities of high-surface-area carbonaceous materials such as soot^[11]. Sorption capacity and/or nonlinearity have been shown to be a function of certain polarity indices of organic matter, i.e. elemental ratios (O/C) and aromatic or aliphatic characters. Interpreting the shape of isotherms is a commonly used macroscopic approach to assess the role of soil organic matter in the sorption of HOCs. Xing and colleagues investigated the sorption of 2,4-DCP on two soils by using a batch equilibration method, and concluded that the sorption conformed the Freundlich isotherm, but the relevant constant (K_F) and the exponent (n) were altered in the different soils ^[9]. It was found that the sorption of 2,4-DCP could not completely be resulted from partitioning (dissolution) of solute into the matrix of soil-organic matter, and might be in association with hole-filling process of the sorbate at specific sites within SOMs^[12]. Sorption behavior is further complicated due to the physical complexity and heterogeneity of soil materials, but the mobility capability can, in general, be characterized by the retardation factor (R) that is usually determined by the column tests and breackthrough curves.
    The objectives of this study were to examine the sorption-desorption characteristics of 2,4-DCP and the transport behaviors in the natural loess soil with a set of open flow-through columns that could closely simulate the corresponding field conditions in an attempt to define its sorption mechanisms. Furthermore, it allowed to access the reversibility of reaction mechanisms without the artificial disturbance in soil phase. The results are useful to ascertain the transport and trasformation of 2,4-DCP and other chlorophenols in loess soil, and can also be utilized in the remediation techniques of chlorophenols-contaminated soils.

2. MATERIALS AND METHODS
2.1 Materials
A loess soil sample used in the experiments, was taken and sampled from the land subsurface 50-60cm deep in an uncontaminated area in Lanzhou City, China.
    The soil sample was textured, and the overall characteristics are: sand 18.75%, silt 53.85% and clay 27.4%; pH 8.1; organic carbon 0.101%, CEC 3.21cmol/Kg. The organic carbon content (OC) was measured using K₂Cr₂O₇-H₂SO₄ oxidation at 180-185^oC in an oil bath according to the methodology reported by Li et al^[13]. The Cation Exchange Capacity (CEC) of the natural loess soil was measured by atomic absorption spectrometry with Ca²⁺ as an index cation and 0.1 M BaCl₂ solution as the extracting reagent.
    Chemicals 2,4-dichlorophenol was used as the sorbate. The compound is usually regarded as a polar hydrophobic organic compound (HOC), and its solubility is defined as 4000mg/L with logK_ow 2.75 and pK_a7.7. CaCl₂ 0.01mol/L was added as a mineral constituent to adjust ion strength of the solutions while hydrochloric acid and sodium hydroxide were used to adjust the pH values of solutions. A number of inorganic salts, such as magnesium chloride, calcium chloride, zinc chloride, manganese chloride and cadmium chloride were respectively used to investigate the influence of divalent metal cations on the retention of 2,4-DCP in the loess soil. All the chemicals used in this study were analytical grade, and the solutions were prepared by using distilled water.
2.2 Sorption procedures
Batch tests were conducted at the pH range from 6.7 to 10.8. A weight of 5 g of the loess soil was mixed with 50 ml of the respective organic compound solutions with concentrations ranging from 2 to 200 mg/L in 50-ml Erlenmeyer flasks with glass caps. To obtain sorption equilibrium, the soil-solution mixtures were initially stirred with a vortex agitator and then placed on a shaker for continuous mixing at 200 rpm for about 24 h at 20^oC in the dark. After equilibration, the samples were centrifuged at 3200×g for 20 min to separate aqueous and solid phases. The DCP concentrations in aqueous phases were determined by a 7500 UV spectrophotometer (Shanghai, China). Sorbed amounts of DCP were calculated as the difference between the initial and final concentrations in aqueous phase. Blank sample without sorbate was also prepared to determine the potential losses of solutes to flask walls and from processes other than sorption.
2.3 Column tests
Column apparatus  A set of thick-walled plexiglass columns 2.5cm inside diameter and 50cm in length were prepared. Supported by a rack, a sieve-plate on which a piece of Teflon filter screen was placed, was set at the bottom of each column. The installation of column is illustrated in Figure 1. The air-dried loess soil of some 100g was slowly packed into each column and continuously compacted with a wooden rod until the soil height approximately reached 12.5 cm. The bulk density was calculated as 1.63 close to that under the natural condition. The solution was distributed onto the top of column with a syringe pump, and a relatively steady flow rate through each column was desired under a stable hydraulic load and pressure.

Fig 1. Illustration of installation for the column tests

    Column tests The soil packed in columns was initially wetted by the slow converse flow from the bottom with a solution of 0.01M CaCl₂ until the system was completely saturated, meanwhile pH was adjusted until the effluents attained a constant pH value desired. Then, DCP solutions with the different concentrations were respectively introduced into the three columns with a steady flow rate of 10 mL·h^-1. The effluents were continuously collected from the outlet at each column bottom for the determination of DCP concentrations and pH values. The column was then prepared for the experimental displacement by passing a minimum of 50 pore volumes of the experimental mobile phase through the column profile. Breakthrough curves were then conducted by introducing a pulse of the solute containing mobile phase into the column and monitoring column effluent until the solute effluent concentration C (mmol^-1) approached the influent concentration C₀ (mmol^-1), i.e. C/C₀ = 1. Column effluent fractions collected from the H₂O BTCs, was used to characterize column hydrodynamics. Each column test was run in triplicate. The experiments were conducted at the room temperature (20±1^oC).
2.4 Analytical Methods
The effluent samples were centrifuged at 8000 rpm for 20 min to separate aqueous and solid phase. DCP concentrations in the aqueous phase were determined by using a 754 UV spectrophotometer (Shanghai, China) at the wave-length of 283nm where the interference of background absorbance caused by eluted soluble humic materials could be omitted by the comparison with the blank sample.

3. RESULTS AND DISCUSSIONS
3.1 Breakthrough curves of DCP on the soil column
Experiments were run with the three different concentrations of DCP solution 0.81, 0.55 and 0.33 mM at pH8.1 flowing respectively through the columns soaked initially by CaCl₂ solution of 0.01 mol/L. The effluent had the constant pH8.1 from the beginning. Experimental data were calculated by the equation as:
C_w = f (v)                                     (1)
    Where，C_w is the DCP concentrations measured in effluent at different time, and v is the effluent volume.
    The variety of DCP concentrations in the effluent was positively related with that in the influent until the adsorption tended to be saturated. When flushing with the fresh water, a significant retardation of desorption was observed due to the curve shape with a long tail, which was one of the characteristics for non-linear adsorption, and was reported by other batch experiments too^[12].
    As the columns of length 50 cm were packed with the 12.5 cm soil of porosity 0.46, each column was pumped with water before the feed was switched to a container containing DCP solution at the beginning. Assuming that chemical dispersion was small, water front traveled much faster than the chemical due to its low adsorption to the soil. The chemical front was sharp line as in plug flow, actually, DPC concentration dropped off over a long distance because of the influence of dispersion and mass transfer. If dispersion were negligible, the sorption would be linear and reversible in the study. The definition of retardation factor could be expressed as the ratio of relative transport velocity of water to contaminant in the soil matrix. When the sorption achieves equilibrium under the dynamic state, R value can be calculated by the following equation:
                                            (2)
    Where: u and u_c are porous velocities of water and contaminant, respectively; M_s - total mass of solid media; V_w - Total volume of liquid in the soil metrix;- Bulk density for porous medium; q - Effective soil porosity.
    When steady state is reached, R can also be defined as ^[14]：
                                             (3)
   Where, m_s is the soil mass packed in the column, C₀ is the initial concentration of DCP in the injected solution, Vp is the total hole volume of packed soil, and C_{tot, ads} is the total sorbed concentration in loess soil. It can be seen that R is affected by the property of contaminant, so it is also related to ratio of water velocity (u) to the contaminant velocity (u_c).
    On the basis of experimental data, the results were shown as the breakthrough curves that are shown in Figure 2. It can be seen that the sorption and desorption curves consist of two parts: the first part is sharp and the second part is more diffuse and spreading. It was also observed that the retardation of the sorption fronts depended on the initial concentration, but the desorption curves was independent of the input concentrations in a coordinate system when C_w reached 0.33mM and tended to be decreased. At the same time, the observed adsorption-desorption breakthrough curves appeared to be asymmetrical with the long-tails, which typically proved the DCP nonlinear sorption and desorption hystereses^[14]. According to the Extended Dual-Mode Model, soil organic matter (SOM) has two states: the rubbery state and the glassy state. The rubbery state looks like a partition sorbent and the sorption isotherm is linear, however, it has many fixed or flexible holes and serves as the adsorption sites if the polymer is glassy. Therefore, sorption of DCP on soil is a combination of portioning and hole-filling adsorption. Sorption and desorption follow the same path when the polymer is in the rubbery state. Inelastic expansion of holes or creation of new holes by penetrating the enhanced expansion of soil surfaces at the glassy state may lead to stronger affinity for adsorbate in the desorption process, and thus desorption hystereses occur.

Fig 2. Breakthrough curves of DCP with three different initial concentrations
(I=0.1M CaCl₂ and pH=8.1).

3.2 Sorption isotherms
As the exact sorption equilibrium might be assumed at the time scale in the experiments, the theory of sorption equilibrium could be used to interpret the results. Freundlich and Langmur isotherms that are commonly used models to describe retention of organic compounds onto heterogeneous sorbent, were developed to analyze the sorption data^[15]. Here, Langmuir model has the form as:
                                (4)
    Where, C_s is the sorbed concentration of DCP (mmol/kg); C_w (mmol/L) is the aqueous concentration; K_L is Langmuir constant (mL/mol), and C_smax(mmol/kg) is the adsorption capacity.
    Finally, Freundlich model can be expressed as:
C_s= K_FC_w^1/n                                     (5)
   Where, K_F is Freundlich constant that is an index to the sorption capacity of sorbent, and 1/n is the parameter indicating the nonlinearity of isotherm.
    On the basis of the experimental data, the best-fitting curves were drawn and regressed by nonlinear analysis using the least-squares method ^[16]. The results obtained by both Freundlich and Langmuir equations are shown in Figure 3, and the relevant parameters of the two equations are listed in Table 1. It could be seen that the experimental data had a better fit for Freundlich Equation than for Langmuir Equation, especially at the high initial concentrations. DCP sorption was well described by the Freundlich-type isotherm, indicating the sorption equilibrium under the concrete experimental conditions. As indicated by the values of K_F and C_smax, the sorption of DCP became stronger with the increase of initial concentrations.

● experimental data; —,Freundich fit line; ------,Langmuir fit line.
Fig 3. The isotherms for 2,4-dichlorophenol (DCP) sorption on the loess soil with the different initial concentrations (Curve 1 - 0.88mM; 2 - 0.55 mM; 3 - 0.33mM.)

Table 1 Freudlich-type and Langmuir-type parameters calculated with the different initial concentrations in the column tests

    The relationship between the Freundlich exponent (n) and the solution concentration was investigated by Xia and Pignatello in detail through a series of experiments with apolar or polar HOCs including DCP ^[17]. In most cases, n presents the near unity at a low concentration declining to a minimum value (n_min). Thus it was concluded that nonlinearity, as defined by the Freundlich exponent n, was markedly sensitive to the concentration range. The values of n for DCP adsorptive isotherms were defined from the near unity to 0.5 under the different DCP concentrations. In our experiments, Freundlich exponent (n) that decreased with the increase of initial concentrations, was matched with the altering tendency regulation observed by Xia and colleague^[17]. However, the values of n measured as 0.29-0.38 in our experiments, were lower than that they did. The reason could be attributed to the relatively lower level of organic carbon in the natural loess soil.
3.3 Influence of pH
The effect of pH on the retention of DCP in the soil environment was studied at an alternative pH range from 7.2 to 9.8 that is the typical pH range of the natural loess soils in northwest China. Figure 4 shows the comparison of DCP breakthrough curves using the different solutions at pH 7.2, 8.1 and 9.8 respectively under the identical conditions. It was proved that the adsorption approached equilibrium more quickly at the higher pH values. Thus, sorption of DCP tended to decrease with the increase of pH values. For instance, the sorption equilibrium occurred at pH9.8 when the effluent volume of 100ml was collected, but at the same situation it was found at pH 7.2 only when the effluent of 150ml was collected. Similarly, the retention appeared more significant with the increase of pH values when the fresh water flushed the column soil. As DCP is an ionizable compound (pK_a=7.7), most of the neutral molecules could be ionized to be the free ions or ion pairs that have the stronger hydrophilicity in the soil environment and easier enter aqueous phase^[18,19].

Fig .4 Effect of pH on breakthrough of DCP (DCP concentration 0.55 mM and I=0.1M CaCl₂)

3.4 Influence of divalent metal cations
To affix the ion strength and species of cation in the experimental soil, CaCl₂ solution of 0.01mol/L was used to saturate the column soil. Previous studies indicated that metal cations on soil surfaces, especially intermediate( transition) metal cations, could affect the sorption and desorption behavior of organic compounds in the soil matrix or natural solids ^[18]. DCP of 0.55 mol/L was used as a sorbate in the experiment while the soil was saturated with 0.1mol/L Mg²⁺, Mn²⁺, Zn²⁺ and Cd²⁺, respectively to replace Ca²⁺. Then the experiments of sorption and desorption were conducted under the condition of pH8.1, and the results are shown in Figure 5. It can be found that the appropriate sorption retention of DCP on the soil was observed in the presence of Zn²⁺, Mg²⁺ and Ca²⁺ although alkaline metal cation Ca²⁺ and Mg²⁺ almost had the similar effects on the behaviors of DCP. It was evident that DCP retention was enhanced by cations following the sequence as: Cd²⁺ > Mn²⁺ > Zn²⁺ > Ca²⁺ > Mg²⁺.The reason could be explained as follows: the transition metal cations that occupied the adsorptive location, had higher chelating capacity and were to form ligand bound with DCP, as a result, the desorption capacity decreased.

Fig. 5 Effect of metal cations on breakthrough curves of DCP
(DCP concentration 0.55 mM, pH=8.1, metal concentration = 0.1M)

    The total sorbed concentration of DCP was calculated by
C_s = (Q_total - Q_out)/m_s                               (6)
    Where, m_s is the mass of loess soil sample, and Q_out as the amount of eluted DCP, was calculated by numerical integration of the areas under the breakthrough curves when plotting C_w versus accumulated outflow V ^[20]:
    Q_out = (7)
    The losses of compound by photochemical decomposition, volatilization and the quantity adhered to column wall were neglected. Mass equilibriums in the sorption and desorption reactions must be related with the influence of pH and metal cations in the loess soil, and the results are shown in Tables 2 and 3, respectively. Both the sorbed and released amount verified that DCP sorbed onto the loess soil surfaces was quantitatively related to the eluting ion, and the sorption phenomena involved were fully reversible. On the basis of experimental data, retardation factor (R), which expresses the delay of DCP mobility in the soil, was calculated by Equation 2.

Table 2 Mass balances and retardation factors affected by pH values in column tests

Table 3 Mass balances and retardation factors affected by metal cations in column tests

    According to the theory on mass transfer through a natural material, DCP displayed a poor affinity onto the natural loess soil, so we could conclude that the presence of polar HOCs in the vadose zone, such as DCP, had a potential risk for groundwater contamination. However, the decrease of pH and the presence of small amount of metals could enhance the adsorption of DCP in the loess soil and prevent its mobility from topsoil into groundwater aquifer.

4. CONCLUSIONS
Through a series of the column tests in the laboratory, the natural loess soil was used as a sorbent to study the sorption-desorption behaviors of DCP in the soil matrix. The tests were prepared to be closer to the exact field conditions. The typical nonlinear of sorption and desorption was hysteresed for DCP in the soil, and the results well fitted the extended Dual-Mode Model when the sorption data of DCP better obeyed to Freundlich isotherm relationship than Langmuir isotherm. The retention of DCP in the soil was affected by pH and the presence of metal divalent cations. With a low retardation factor in loess soil, DCP displayed a poor affinity on the loess soil surfaces. It is proposed that the disposal of DCP into the vadose zone consisting of loess soil might have a potential threat to groundwater quality. In general, these results provide the evidence and information for an environmental impact assessment and the further remediation in Northwest China and the Central Asia where vast and thick loess soils are expended in topography.

REFERENCES
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2,4-二氯酚在土壤环境中的吸附和解吸
朱琨¹，姜梅²，展慧英³
(¹ 兰州交通大学环境与市政工程学院, 兰州 730070 中国; ²广州化工技术学院，广州 510370 中国; ³甘肃联合大学化学系，兰州 730000 中国)
2005年6月29日收稿；国家自然科学基金项目（29977015）及甘肃省环保局科研基金项目（GH2002-14）资助。
摘要  利用土柱试验对2,4-二氯酚在土壤表面的吸附和解吸过程和机理进行了研究。试验结果发现2,4-二氯酚在土壤表面的吸附等温线为非线性，可以用Freundlich等温式来描述。并且2,4-二氯酚在土壤中的吸附率随其初始浓度的增大而增强，与计算的佛德里希常数(K_F)和土壤的吸附能力(C_smax)成正比。同时2,4-二氯酚在土壤环境中的解吸行为有明显的滞后现象，并受土壤pH值和共存的金属离子的影响。当pH从10.8降低到6.7时， 土壤对2,4-二氯酚的吸附会增大，由于2,4-二氯酚是可离子化的化合物(pK_a=7.7)，在土壤介质中中性分子形态的2,4-二氯酚的吸附大于它的离子形态。而在较高pH时，大部分2,4-二氯酚的中性分子可被离子化而成为亲水性的自由离子或离子对，极易进入水相。若土壤中共存二价金属离子时，2,4-二氯酚会表现出明显的解吸滞后性。阳离子对解吸滞后的影响作用可按如下顺序：Cd²⁺ > Mn²⁺ > Zn²⁺ > Ca²⁺ > Mg^{2 +}排列。2,4-二氯酚在土壤中的吸附与解吸的规律可用扩展的双模式模型来描述。根据在天然介质中的传质理论，2,4-二氯酚对土壤表现出微弱的亲和力，因此象2,4-二氯酚这类极性碳氢化合物在包气带土壤中会构成对地下水的潜在污染，但是只要降低土壤的pH值和掺加极少量的金属离子就可增强土壤对2,4-二氯酚的吸附性，从而防止对地下水的污染。
关键词 2,4-二氯酚，吸附，黄土，滞后现象，土柱实验

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