**3.1 Materials and methods**

#### **3.1.1 Instruments and reagents**

n-Hexane (chromatographically pure, Tedia Company, USA); petroleum ether with boiling range of 60 °C to 90 °C (analytical reagent, Hangzhou Refinery, Zhejiang Province, P.R. China); methylene chloride, acetone and concentrated sulfuric acid (analytical reagent, Nanjing Chemical Reagent Plant, P.R. China); anhydrous sodium sulfate (analytical reagent, it was sieved by boult of 200 mesh, then treated at 225 °C for 4 h and stored in sealed


δ


container before use); Organochlorine pesticides standard (o, p'-DDT, o, p'–DDE, p, p'-DDT, p, p'–DDD, p, p'-DDE, α-, β-, γ- and δ-HCH)were purchased from Dr. Ehrenstorfer Company, Germany. The characteristics of pesticides studied are listed in table 1.

1) Solubility in water at 24~25℃, µg/L

o,p′-DDE 65 6.94

522 Pesticides in the Modern World - Risks and Benefits

Tween 80 behaved similarly to the acids, whereas the anionic sodium dodecyl sulfate enhanced desorption of all pesticides (Gonzalez et al., 2010). Luo et al. (2006) reported that soil organic carbon played a predominant role in the retention of DDT. Oxalate significantly increased the desorption of p,p'-DDT, with the largest increments ranging from 11% to 54% for different soils. Oxalate addition also resulted in the increased release of dissolved organic carbon and inorganic ions from soils. Root exudates had similar effects to those of oxalate and significantly increased DDT desorption from the soils. So, Low molecular weight dissolved organic carbon amendments caused partial dissolution of the soil structure, such as the organo-mineral linkages, resulting in the release of organic carbon and metal ions and thus the

We can put forward some mechanisms about the LMWOA or root exudates with OCPs desorption: (1) Mechanism of minerals dissolution. LMWOA induced the dissolution of soil minerals and resulted in the desorption of OCPs adsorbed by soil minerals; (2) Mechanism of indirect release. Soil inherent organic matter is dissolved and released by LMWOA and resulted in the desorption of OCPs adsorbed by soil inherent organic matter. (3) Mechanism of direct release. The LMWOA directly chelated with OCPs and released it. (4) Mechanism of locking and unlocking. LMWOA interacted with soil inherent organic matter like a key and induced the change of its conformation and properties, resulted in the OCPs were

Effect of LMWOA on the conformation and properties of soil inherent organic

Chelated with OCPs by LMWOA and release it.

matter

**3. Release kinetic of organochlorine pesticides from soil in LMWOA system** 

n-Hexane (chromatographically pure, Tedia Company, USA); petroleum ether with boiling range of 60 °C to 90 °C (analytical reagent, Hangzhou Refinery, Zhejiang Province, P.R. China); methylene chloride, acetone and concentrated sulfuric acid (analytical reagent, Nanjing Chemical Reagent Plant, P.R. China); anhydrous sodium sulfate (analytical reagent, it was sieved by boult of 200 mesh, then treated at 225 °C for 4 h and stored in sealed

**Mechanism of stimulating release on OCPs by LMWOA**

subsequent enhanced desorption of DDT from the soils (Luo *et al.*, 2006).

locked or unlocked by soil inherent organic matter (Figure 1).

Soil minerals is dissolved by

Soil inherent organic matter is dissolved and released by

**3.1 Materials and methods 3.1.1 Instruments and reagents** 

LMWOA

LMWOA

Fig. 1. Mechanism obout the stimulating release of OCPs by LMWOA

 

Table 1. Some properties of the organochlorine pesticides (Chiou et al., 1987; Harald et al., 2000)

Agilent-6890 GC/ECD gas chromatography and HP7683 automatic sampler with HP chemical workstation (Hewlet-Packard, USA) and HP-5 fused capillary column (30 m ×320 μm ×0.25μm) as chromatographic column; Sigma 2-16K high speed freezing centrifuge (Sigma, Germany); water bath rotary vacuum evaporator (Yarong Biochemical Instrument Plant, Shanghai, P.R.China); HS-10360D ultrasonic cleaning machine (Heng'ao Science and Technology Company, Tianjin, P.R.China); BS200S-WE1 electronic balance (1/10000, Sartorius Company, Germany); SPP cartridge and filter disc (Chemical and Physical Institute of National Chromatogram Center in Dalian, P.R.China).

Celite 545 (chromatographic grade, 0.020-0.045 mm, Serva Company) was dried in muffle furnace at 550 °C for 4 h, then treated at 200 °C in oven before addition of 3 % of deionized water to deactivate it, and stored in a sealed container before use; Purified SPE column was prepared by packing with a teflon filter disc + 1g celite 545 + 1g anhydrous sodium sulfate + a teflon filter disc.

#### **3.1.2 Chromatogram conditions**

Initial oven temperature was kept at 60 °C for 1 min. Then the temperature was increased to 140 °C at a speed of 20 °C min-1 and subsequently increased to 280 °C at a speed of 12 °C min-1 and kept at 280 °C for 4 min. The injector temperature was set as 220 °C, while the detector temperature was set as 280 °C. High purity N2 (99.999%) was used as carrier gas at a flow rate of 2 mL min-1. 2 µL of sample was injected in a splitless mode. Quantitative calculation was conducted with external standard method.

#### **3.1.3 Dynamic experiment methods**

**Soil sample** The red soil, Hydragric Acrisols - according to WRB (World Reference Base for soil resources) (ISSS/ISRIC/FAO, 1998), was sampled at depths of 5 cm to 20 cm from the Red Soil Ecologic Experimental Station of Chinese Academy of Science in Yingtan, Jiangxi province, P. R. China (28°12′34.1″N, 116°55′32.3″E), lyophilized and sieved (≤1 mm) for further analysis. The clay minerals of soil sample were mainly composed of kaolinite and hydroxyaluminum vermiculite, and contained a spot of hydromica and a trace amount of gibbsite. The main physical and chemical characteristics of the soil were as follows: pH 5.07, clay content 32.3%, organic matter content 1.14 %, Cation Exchange Capacity (CEC) 10.10 Cmol (+) kg-1.

**Spiked soil** Soil samples were sieved to <2mm and stored at room temperature until spiking procedure. Spiked soil samples were prepared by adding 500 µL of standard mixture of 13 kinds of OCPs (the concentration is 10 ng µL−1 for each compound dissolved

Effects of Low-Molecular-Weight-Organic-Acids on the

o,p'-DDT

0 250 500 750 1000

p,p'-DDT

0 250 500 750 1000

o,p'-DDE

0 250 500 750 1000

p,p'-DDE

0 250 500 750 1000

elution inflection point.

□-:oxalate,-▽-:tartrate,-○-:citrate

Velocity of leaching / ngml-1

Velocity of leaching / ng

 ml-1

Velocity of leaching / ngml-1

Velocity of leaching / ng

 ml-1

Release Kinetic of Organochlorine Pesticides from Red Soil 525

water and organic acids is much higher than that of DDTs, the difference is about 3 times. The release ability of water to DDTs is very low. This kind of pesticides is released by water at a certain concentration which practically is lower than its solubility, concentration, the elution volume does not have evident effects to its release rate. The release pattern of HCHs with high solubility by water follows a quick release at first, and then a slow release after an elution inflection point at which the elution volume is about 300mL when it achieves the

0

0

0

0

Time / min Time / min Fig. 2. Release velocity of organochlorine pesticides from red soil with LMWOA.-\*-:water,-

10

20

30

40

10

20

30

40

10

20

30

40

10

20

30

40

α-HCH

0 250 500 750 1000

β-HCH

0 250 500 750 1000

γ-HCH

0 250 500 750 1000

δ-HCH

0 250 500 750 1000

in n-hexane) to 20 g of soil according to the reference (Tor et al., 2006). This spike level corresponds to 250µg kg−1. Then 20mL of acetone was added and suspension was mixed for 30 min with a mechanical shaker. After the bulk of the solvent was evaporated at room temperature, the samples were stored at 4 ◦C in stoppered glass bottle for six month in the dark. Then the extractions were carried out.

**Preparation of eluent** 3 kinds of low molecular weight organic acid eluent solutions (oxalic acid, tartaric acid and citric acid) were all prepared as 10mmol/L solutions by the analytical reagents, and their pH were adjusted to 5.5 by NaOH or HNO3. The eluent's pH selected was based on the common pH of southern variable charge soil in China.

**The dynamic devices** are as follows: ①Storage Bottles; ②P200Ⅱtype high performance liquid chromatography pump (Scientific Instruments Co., Ltd. YiLite, Dalian, P. R. China); ③dynamic reaction cell made of PTFE to provide reaction space; ④SBS-100 automatic fraction collector (Huxi Analytical Instrument Factory Co., Ltd, Shanghai, P. R. China).

**Dynamics experiment methods** Weigh 7g of spiked soil, put it into the dynamic reaction cell, and seal the cell tightly after wetting the sample with distilled water. The upper and lower ends of dynamic reaction cell, respectively, connected with the automatic fraction collector and high performance liquid chromatography pump. The leaching velocity was set for 1mL/min, the collection time of each glass tube was 10 minutes and continuously collected 100 glass tubes of leacheate. The experimental temperature of the dynamic reaction cell was controlled at 298±0.5K by using thermostatic waterbath. When the samples were determined, two glass tubes were combined as one test sample point.Transferred the collected liquid into separating funnel, and add 10μL of internal standard (Five chlorine nitrobenzene methanol solution, 5ng/μL). After homogeneous mixing, added 10 mL petroleum ether and 0.5g NaCl in the separating funnel, and oscillated for liquid-liquid extraction. After adding 0.5 mL of acetone to eliminate stubborn emulsification phenomenon, transferred organic extraction phase into pear-shaped bottle, and then added 10 mL petroleum ether to repeat the extraction step. These two extract were combined and put it in pear-shaped bottle. The extract was concentrated to about 1mL by rotary evaporators, and translated it into purifying SPE column. The SPE column was eluted with 10mL of 10% dichloromethane / petroleum ether (V:V), and the leacheate was concentrated to about 1mL by rotary evaporators again and blew by nitrogen and metered volume to 1mL by n-hexane. Determined by GC-ECD, and quantified by external standard method. The results showed that, the recovery rate of this liquid-liquid extraction method was 80%~105% to different pesticides, and the relative standard deviation was 3%~8%, which meet the demands of the analysis of trace organic compounds.

#### **3.1.4 Quality control and data analysis**

Laboratory blank values for all the compounds were generally low and posed no problem to the analytical quantification. The overall reproducibility was evaluated using the replicate analyses (n = 3). The coefficient of variation (CV) was between 0.01 and 0.35 for the various compounds, and it was less than 0.3 in 90 % of the cases. Therefore, the reproducibility of the measurements was considered to be satisfactory.

#### **3.2 Results and discussion**

#### **3.2.1 Effects of LMWOA on the release rate of organochlorine pesticides**

The average releasing rate is calculated by the each pesticides quantity contained in leaching solution (20mL) divided by elution volume. From figure 2, the release rate of HCHs by

in n-hexane) to 20 g of soil according to the reference (Tor et al., 2006). This spike level corresponds to 250µg kg−1. Then 20mL of acetone was added and suspension was mixed for 30 min with a mechanical shaker. After the bulk of the solvent was evaporated at room temperature, the samples were stored at 4 ◦C in stoppered glass bottle for six month in the

**Preparation of eluent** 3 kinds of low molecular weight organic acid eluent solutions (oxalic acid, tartaric acid and citric acid) were all prepared as 10mmol/L solutions by the analytical reagents, and their pH were adjusted to 5.5 by NaOH or HNO3. The eluent's pH selected

**The dynamic devices** are as follows: ①Storage Bottles; ②P200Ⅱtype high performance liquid chromatography pump (Scientific Instruments Co., Ltd. YiLite, Dalian, P. R. China); ③dynamic reaction cell made of PTFE to provide reaction space; ④SBS-100 automatic fraction collector (Huxi Analytical Instrument Factory Co., Ltd, Shanghai, P. R. China). **Dynamics experiment methods** Weigh 7g of spiked soil, put it into the dynamic reaction cell, and seal the cell tightly after wetting the sample with distilled water. The upper and lower ends of dynamic reaction cell, respectively, connected with the automatic fraction collector and high performance liquid chromatography pump. The leaching velocity was set for 1mL/min, the collection time of each glass tube was 10 minutes and continuously collected 100 glass tubes of leacheate. The experimental temperature of the dynamic reaction cell was controlled at 298±0.5K by using thermostatic waterbath. When the samples were determined, two glass tubes were combined as one test sample point.Transferred the collected liquid into separating funnel, and add 10μL of internal standard (Five chlorine nitrobenzene methanol solution, 5ng/μL). After homogeneous mixing, added 10 mL petroleum ether and 0.5g NaCl in the separating funnel, and oscillated for liquid-liquid extraction. After adding 0.5 mL of acetone to eliminate stubborn emulsification phenomenon, transferred organic extraction phase into pear-shaped bottle, and then added 10 mL petroleum ether to repeat the extraction step. These two extract were combined and put it in pear-shaped bottle. The extract was concentrated to about 1mL by rotary evaporators, and translated it into purifying SPE column. The SPE column was eluted with 10mL of 10% dichloromethane / petroleum ether (V:V), and the leacheate was concentrated to about 1mL by rotary evaporators again and blew by nitrogen and metered volume to 1mL by n-hexane. Determined by GC-ECD, and quantified by external standard method. The results showed that, the recovery rate of this liquid-liquid extraction method was 80%~105% to different pesticides, and the relative standard deviation was 3%~8%, which

Laboratory blank values for all the compounds were generally low and posed no problem to the analytical quantification. The overall reproducibility was evaluated using the replicate analyses (n = 3). The coefficient of variation (CV) was between 0.01 and 0.35 for the various compounds, and it was less than 0.3 in 90 % of the cases. Therefore, the reproducibility of

The average releasing rate is calculated by the each pesticides quantity contained in leaching solution (20mL) divided by elution volume. From figure 2, the release rate of HCHs by

**3.2.1 Effects of LMWOA on the release rate of organochlorine pesticides** 

was based on the common pH of southern variable charge soil in China.

meet the demands of the analysis of trace organic compounds.

**3.1.4 Quality control and data analysis** 

**3.2 Results and discussion** 

the measurements was considered to be satisfactory.

dark. Then the extractions were carried out.

water and organic acids is much higher than that of DDTs, the difference is about 3 times. The release ability of water to DDTs is very low. This kind of pesticides is released by water at a certain concentration which practically is lower than its solubility, concentration, the elution volume does not have evident effects to its release rate. The release pattern of HCHs with high solubility by water follows a quick release at first, and then a slow release after an elution inflection point at which the elution volume is about 300mL when it achieves the elution inflection point.

Fig. 2. Release velocity of organochlorine pesticides from red soil with LMWOA.-\*-:water,- □-:oxalate,-▽-:tartrate,-○-:citrate

Effects of Low-Molecular-Weight-Organic-Acids on the

o,p'-DDT

<sup>0</sup> <sup>250</sup> <sup>500</sup> <sup>750</sup> <sup>1000</sup> <sup>0</sup>

p,p'-DDT

<sup>0</sup> <sup>250</sup> <sup>500</sup> <sup>750</sup> <sup>1000</sup> <sup>0</sup>

o,p'-DDE

<sup>0</sup> <sup>250</sup> <sup>500</sup> <sup>750</sup> <sup>1000</sup> <sup>0</sup>

p,p'-DDE

<sup>0</sup> <sup>250</sup> <sup>500</sup> <sup>750</sup> <sup>1000</sup> <sup>0</sup>

Accumulating elution amount / ng

 g-1

Accumulating elution amount / ng

 g-1

Accumulating elution amount / ng

 g-1

Accumulating elution amount / ng

 g-1

Release Kinetic of Organochlorine Pesticides from Red Soil 527

α-HCH

<sup>0</sup> <sup>250</sup> <sup>500</sup> <sup>750</sup> <sup>1000</sup> <sup>0</sup>

β-HCH

<sup>0</sup> <sup>250</sup> <sup>500</sup> <sup>750</sup> <sup>1000</sup> <sup>0</sup>

γ-HCH

<sup>0</sup> <sup>250</sup> <sup>500</sup> <sup>750</sup> <sup>1000</sup> <sup>0</sup>

δ-HCH

<sup>0</sup> <sup>250</sup> <sup>500</sup> <sup>750</sup> <sup>1000</sup> <sup>0</sup>

Accumulating elution percentage / %

Accumulating elution percentage / %

Accumulating elution percentage / %

Accumulating elution percentage / %

Fig. 3. Accumulative release kinetic of organochlorine pesticides from red soil with

LMWOA leaching. -\*-:water,-+-:oxalate,-▽-:tartrate,-○-:citrate

Time / min Time / min

Table 2 shows that the kinetic release of organochlorine pesticides in water system is basically accord with the first-order kinetic equation (*R2*: 0.99 - 0.9999, p<0.0001), p, p'-DDE appears to be more consistent with two-constant equation (its *Se* is lower than that of the

The release pattern of DDTs by oxalic acid is single-peak type curve. That means the release rate increases along with the elution volume increase, and it reaches the maximum elution rate when the elution volume is 40mL, after that the rate gradually decreases, and it is not stable until the volume reaches to 120mL. The release pattern of DDTs by oxalic acid is the same to that by water except that the elution inflection point is 120mL.

The release patterns of DDTs and HCHs by tartaric acid belong to bimodal curve. When the volume of leacheate was 40mL, the release rate of the two kinds of pesticides reached maximum, and then the rate slightly decreased. When the volume of leacheate increased to about 100mL, the rate reached another maximum. And it became stable till the volume reached 240mL. The release pattern of DDTs and HCHs by citric acid solution was also single-peak type curve, and their inflection point appeared at about 140mL.

The existence form of organochlorine pesticide in soil includes free form, loose bound form and tight bound form (for example aging residual form). When the leaching solution flowed through the soil, the free form and loose bound form would be released firstly, after loose bound form was eluted completely, the tight bound form OCPs were slowly dissolved out with approximately constant speed. The leaching pattern of OCPs by tartaric acid displayed the bimodal curve, it maybe relate to the comparatively weak elution ability to loose bound form pesticides. The first peak represents the release of free form pesticides, the second peak represents the release of loose bound form pesticides, and then the slow release of the tight bound form. This indicates that the release intensity of tartaric acid to the loose bound form pesticides is smaller than the citric acid. As the citric acid has stronger desorption ability to the free form and loose bound form pesticides, these two forms pesticides will be leached out the soil together and form a single peak.

#### **3.2.2 The cumulative release of organochlorine pesticides from soil by LMWOA**

Figure 2 shows the diagram of the cumulative release of organochlorine pesticides by several LMWOA from red soil. Table 2 lists the fitting results of the dynamic release data of organochlorine pesticides to several common kinetic equation, while *t* is the time, *Qt* is the cumulative release amount of pesticides, *a* and *b* are the parameters of the kinetic equation (with different meaning in different equations), *k* is the apparent speed constant in the firstlevel dynamic equation, *qmax* is the apparent equilibrium desorption amount. The multiple correlation coefficient (*R2*) and standard error (*Se*) can be used to judge the degree of fitting. That is to say the larger *R2* and the smaller *Se* contribute to a better fitting degree.

$$s\_c = \sqrt{\frac{\sum \left(s\_t - \hat{s}\_t\right)^2}{n-2}} \text{, } R^2 = 1 - \frac{\sum \left(s\_t - \hat{s}\_t\right)^2}{\sum \left(s\_t - \overline{s}\_t\right)^2}.$$

Where the *St*、 ˆ *ts* 、 *ts* and *n* are the measured value, the predictive value, the average value and sample number, respectively.

Figure 3 shows that the introduction of LMWOA strengthens the release of organochlorine pesticides to a certain extent (compared with water, it increases by 15~18 percentage for DDTs, while the HCHs increases by 7~25 percentage). The release ability of LMWOA for DDTs is: citric acid (18~26%) > tartaric acid (14~20%) > oxalic acid (6~10%) > water (3~8%). On the other hand, the release ability of LMWOA for HCHs is: tartaric acid (60%) > citric acid (49~55%) > oxalic acid (41~48%) > water (35~41%). The results match the experiment results conducted by White who used batch method and pot experiment to study the effect of 7 kinds of LMWOA to p, p' – DDE (White *et al.*, 2003).

The release pattern of DDTs by oxalic acid is single-peak type curve. That means the release rate increases along with the elution volume increase, and it reaches the maximum elution rate when the elution volume is 40mL, after that the rate gradually decreases, and it is not stable until the volume reaches to 120mL. The release pattern of DDTs by oxalic acid is the

The release patterns of DDTs and HCHs by tartaric acid belong to bimodal curve. When the volume of leacheate was 40mL, the release rate of the two kinds of pesticides reached maximum, and then the rate slightly decreased. When the volume of leacheate increased to about 100mL, the rate reached another maximum. And it became stable till the volume reached 240mL. The release pattern of DDTs and HCHs by citric acid solution was also

The existence form of organochlorine pesticide in soil includes free form, loose bound form and tight bound form (for example aging residual form). When the leaching solution flowed through the soil, the free form and loose bound form would be released firstly, after loose bound form was eluted completely, the tight bound form OCPs were slowly dissolved out with approximately constant speed. The leaching pattern of OCPs by tartaric acid displayed the bimodal curve, it maybe relate to the comparatively weak elution ability to loose bound form pesticides. The first peak represents the release of free form pesticides, the second peak represents the release of loose bound form pesticides, and then the slow release of the tight bound form. This indicates that the release intensity of tartaric acid to the loose bound form pesticides is smaller than the citric acid. As the citric acid has stronger desorption ability to the free form and loose bound form pesticides, these two forms pesticides will be leached

**3.2.2 The cumulative release of organochlorine pesticides from soil by LMWOA**  Figure 2 shows the diagram of the cumulative release of organochlorine pesticides by several LMWOA from red soil. Table 2 lists the fitting results of the dynamic release data of organochlorine pesticides to several common kinetic equation, while *t* is the time, *Qt* is the cumulative release amount of pesticides, *a* and *b* are the parameters of the kinetic equation (with different meaning in different equations), *k* is the apparent speed constant in the firstlevel dynamic equation, *qmax* is the apparent equilibrium desorption amount. The multiple correlation coefficient (*R2*) and standard error (*Se*) can be used to judge the degree of fitting.

That is to say the larger *R2* and the smaller *Se* contribute to a better fitting degree.

<sup>2</sup> ( ) ˆ 2 *t t s s*

2

*R*

Figure 3 shows that the introduction of LMWOA strengthens the release of organochlorine pesticides to a certain extent (compared with water, it increases by 15~18 percentage for DDTs, while the HCHs increases by 7~25 percentage). The release ability of LMWOA for DDTs is: citric acid (18~26%) > tartaric acid (14~20%) > oxalic acid (6~10%) > water (3~8%). On the other hand, the release ability of LMWOA for HCHs is: tartaric acid (60%) > citric acid (49~55%) > oxalic acid (41~48%) > water (35~41%). The results match the experiment results conducted by White who used batch method and pot experiment to study the effect

2

( ) <sup>ˆ</sup> <sup>1</sup> ( ) *t t t t s s*

<sup>−</sup> = − <sup>−</sup> ∑ ∑

*ts* 、 *ts* and *n* are the measured value, the predictive value, the average value

*s s*

2

*<sup>e</sup> <sup>n</sup> <sup>s</sup>* <sup>−</sup> − <sup>∑</sup> <sup>=</sup> ,

of 7 kinds of LMWOA to p, p' – DDE (White *et al.*, 2003).

same to that by water except that the elution inflection point is 120mL.

single-peak type curve, and their inflection point appeared at about 140mL.

out the soil together and form a single peak.

Where the *St*、 ˆ

and sample number, respectively.

Fig. 3. Accumulative release kinetic of organochlorine pesticides from red soil with LMWOA leaching. -\*-:water,-+-:oxalate,-▽-:tartrate,-○-:citrate

Table 2 shows that the kinetic release of organochlorine pesticides in water system is basically accord with the first-order kinetic equation (*R2*: 0.99 - 0.9999, p<0.0001), p, p'-DDE appears to be more consistent with two-constant equation (its *Se* is lower than that of the


Table 2. Fit values of parameters for different kinetic equations

Effects of Low-Molecular-Weight-Organic-Acids on the

the HCHs, which own a larger solubility in water.

consistent with Elovich equation .

Release Kinetic of Organochlorine Pesticides from Red Soil 529

first order equation), and it implies that the release kinetics of the organochlorine pesticides studied in water is still a surface diffusion on soil particles. The kinetic release of o, p' and p, p'-DDE, o, p'-DDT and HCHs by oxalic acid seems to be more consistent with the parabolic diffusion equation, and it indicates that the release is controlled by a number of diffusion mechanism, the outward diffusion process of the pesticides from soil particle interior is the limit step of the whole release process; but p, p' - DDT more conforms to the double constant equation, it may be related to the dissolution and the heterogeneity of energy of soil particles surface induced by oxalic acid (activation and inactivation function of granular surface). In tartaric acid system and citric acid system, the parabola diffusion equation (tartaric acid system) and the double constant equation (citric acid system) may be better to describe the kinetic release behaviours of DDE and DDT; Besides α-HCH conforms to the double constant equation, the kinetic release of β-, γ- and δ-HCH seem to be more consistent with the Elovich equation. And it tells us that in the tartaric acid leaching system. So the release of DDTs is mainly characterized by several diffusion mechanisms in the tartaric acid leaching system and characterized by the release mechanism of different energy position in citric acid system. It may involve some more complex release mechanisms for the release of

Overall, the release of organochlorine pesticides in water system is consistent with the firstorder kinetic equation which is good at describing a simple diffusion mechanism. The release of DDTs by the oxalic acid and citric acid system can be well described by doubleconstant equation which is good at describing a uniform energy distribution; that in tartaric acid system can be describe by parabolic diffusion equation which is controlled by a number of diffusion mechanism. For HCHs, their release behaviour in oxalic acid system conforms to parabolic diffusion equation, and that in tartaric acid and citric acid system are more

**3.2.3 Discussion on the release mechanism of organochlorine pesticides by LMWOA**  The difference of organochlorine pesticides release pattern in different LMWOA systems may be related to the differences of the pesticides' three-dimensional structure and different action mechanism of LMWOA on different bound pesticides on the soil surface. Hydrophobic pesticides are adsorbed mainly through hydrophobic force, van der Waals force, hydrogen bonding and other in soil internal space systems, inorganic mineral surface (surface physical adsorption), amorphous organic matter (soft carbon, fast linear distribution) and aggregate organic matter (hard carbon, slow linear adsorption) four regions, especially in the latter two regions that the inherent soil organic matter (SOM) contributed the most, and the soft carbon-bound pesticide has not solute competition and hysteresis of sorption and desorption, which explains the fast and slow release process (Chiou *et al.*, 1986). LMWOA with Carboxyl and hydroxyl can affect the migration of pesticides by competitive adsorption, structural changes (such as the aggregate decentralized by chelating the metal ions that acting as cross-linking agent in the SOM) of soil and SOM, the release (Chiou *et al.*, 2000) and mineral dissolution (Landrum *et al.*, 1984), their release ability to pesticides related to their ability of the dissolution to soil minerals and multi-coordination ability (Yang et al., 2001). Oxalic acid with smaller molecule volume has two activity carboxyl functional groups, citric acid with larger molecule has three carboxyl and one hydroxyl, and tartaric acid have two carboxyl and two hydroxyl groups (Figure. 4). The effect of LMWOA on pesticides bound to soil organic matter includes unlocking action

*Se* o,p′-DDE 64.3 0.0005 0.9998 0.11 -7.1 0.97 0.9834 0.93 -31.06 7.47 0.8635 2.67 0.06 0.126 0.9986 0.27 p,p′-DDE 116.6 0.0003 0.9999 0.09 -8.6 1.12 0.9753 1.33 36.06 8.63 0.8449 3.32 0.06 0.132 0.9999 0.07 o,p′-DDT 123.9 0.0006 0.9995 0.36 -15.3 2.13 0.9851 1.95 -68.5 16.56 0.8686 5.79 0.14 0.123 0.9977 0.77 water p,p′-DDT 189.6 0.0005 0.9997 0.39 -22.6 3.10 0.9851 2.83 -99.79 24.06 0.8680 8.43 0.2 0.124 0.9982 0.98 o,p′-DDE 54.9 0.0028 0.8852 4.14 3.1 1.63 0.9791 1.77 -39.4 12.95 0.9078 3.71 2.29 0.066 0.9769 1.86 p,p′-DDE 61.84 0.0025 0.8978 4.36 1.5 1.82 0.9776 2.04 45.36 14.36 0.8943 4.43 1.99 0.070 0.9765 2.09 o,p′-DDT 59.19 0.0032 0.8802 4.56 5.1 1.76 0.9813 1.80 -41.33 14.08 0.9220 3.68 3.04 0.062 0.9792 1.90 oxalate p,p′-DDT 141.9 0.0013 0.9684 4.79 -13.1 3.59 0.9767 4.12 -102.61 27.86 0.8631 9.97 1.03 0.096 0.9894 2.77 o,p′-DDE 124.4 0.0028 0.9583 6.23 -2.8 4.06 0.9783 4.49 -114.44 33.29 0.9615 5.98 3.86 0.072 0.9774 4.58 p,p′-DDE 130.8 0.0026 0.9447 7.33 -2.8 4.16 0.9804 4.36 115.31 33.79 0.9465 7.22 3.77 0.073 0.9799 4.42 o,p′-DDT 137.4 0.0027 0.9587 6.80 -4.2 4.46 0.9811 4.60 -126.16 36.44 0.9573 6.91 3.91 0.074 0.9798 4.76 tartrate p,p′-DDT 172.1 0.0020 0.9705 6.83 -14.8 5.33 0.9903 3.91 -156.29 42.81 0.9360 10.06 2.69 0.084 0.9887 4.23 o,p′-DDE 218.1 0.0025 0.9555 10.74 -3.1 6.83 0.9919 4.60 -186.14 55.19 0.9477 11.7 6.34 0.073 0.9916 4.66 p,p′-DDE 299.7 0.0017 0.9655 12.2 -24.0 8.54 0.9862 7.96 243.14 67.34 0.8988 20.8 3.69 0.087 0.9908 6.48 o,p′-DDT 172.6 0.0033 0.9330 10.43 9.9 5.38 0.9829 5.27 -137.71 44.06 0.9641 7.64 8.33 0.063 0.9856 4.83 citrate p,p′-DDT 182 0.0027 0.9464 9.84 1.4 5.70 0.9898 4.29 -152.14 46.16 0.9510 9.41 6.21 0.069 0.9900 4.25 α-HCH 346.1 0.006 0.9908 7.38 77.5 9.8 0.9026 24 -219.6 85.1 0.9874 8.63 39.0 0.329 0.9408 18.7 β-HCH 414.9 0.005 0.9905 9.45 56.5 12.7 0.9445 22.9 -313.3 247.6 0.9906 9.40 32.4 0.381 0.9641 18.4 γ-HCH 404.6 0.006 0.9901 9.37 114.4 10.8 0.8777 29.9 -218.1 94.5 0.9818 11.6 56.4 0.299 0.9286 22. 9 water δ-HCH 399.7 0.004 0.9907 9.15 40.2 12.5 0.9598 19.0 -317.7 241.4 0.9877 10.5 26.3 0.404 0.9729 15.6 α-HCH 392 0.004 0.86 31.4 59.7 11.3 0.9954 5.66 -236.4 90.0 0.9305 22.1 28.6 0.386 0.9909 7.99 β-HCH 438.6 0.005 0.8536 34.9 92.8 12.2 0.9923 8.01 -238.3 228.9 0.9578 18.7 43.3 0.346 0.994 7.05 γ-HCH 427.4 0.004 0.8261 37.1 83.3 11.9 0.9938 7.02 -231.1 95.5 0.9315 23.3 38.3 0.358 0.988 9.76 oxalate δ-HCH 425 0.005 0.827 36.1 99.4 11.6 0.9914 8.03 -214.4 217.1 0.9536 18.7 45.9 0.333 0.9916 7.94

α-HCH 541.4 0.004 0.9733 21.6 48.3 17.3 0.9441 31.3 -456.7 146.9 0.9912 12.4 34.8 0.409 0.958 27.1

β-HCH 575.1 0.005 0.9834 17.9 86.3 17.7 0.8956 45.0 -451.9 354.1 0.9855 16.8 49.9 0.368 0.9247 38.2

γ-HCH 726.6 0.005 0.9783 20.4 116.5 22.2 0.8984 45.9 -555.7 192.3 0.9863 16.9 65.3 0.363 0.9283 38.6 tartrate

δ-HCH 466.3 0.005 0.9737 18.4 67.4 14.5 0.897 36.4 -370.3 288.4 0.9833 14. 7 39.5 0.371 0.9242 31.2

α-HCH 419 0.005 0.8814 30.9 88.9 11.9 0.9697 15.7 -246.4 98.9 0.9779 13.4 42.8 0.343 0.9825 11.9

β-HCH 506.9 0.008 0.8968 31.7 179.6 12.6 0.8978 31.5 -201.4 251.0 0.9855 11.9 88.9 0.267 0.9464 22.8

γ-HCH 426.9 0.007 0.8717 30.6 133.5 11.1 0.9341 21.9 -191.9 94.4 0.9868 9.8 64.7 0.288 0.966 15.8 citrate

δ-HCH 447.9 0.007 0.8614 33.4 140.7 11.7 0.931 23.6 -200.3 227.9 0.9826 11.8 68.2 0.288 0.9621 17.5

Table 2. Fit values of parameters for different kinetic equations

First-order kinetic: *ln(1-qt/qmax)=-kt* Parabolic diffusion: *Q*

pesticides *qmax k R2*

 *=a+bt t* *Se a b R2 Se a b R2*

*1/2*

Elovich equation: *Q*

 *=a+blnt*

*Se A b R2*

*t*

Double constant equation: *Q=atb*

first order equation), and it implies that the release kinetics of the organochlorine pesticides studied in water is still a surface diffusion on soil particles. The kinetic release of o, p' and p, p'-DDE, o, p'-DDT and HCHs by oxalic acid seems to be more consistent with the parabolic diffusion equation, and it indicates that the release is controlled by a number of diffusion mechanism, the outward diffusion process of the pesticides from soil particle interior is the limit step of the whole release process; but p, p' - DDT more conforms to the double constant equation, it may be related to the dissolution and the heterogeneity of energy of soil particles surface induced by oxalic acid (activation and inactivation function of granular surface). In tartaric acid system and citric acid system, the parabola diffusion equation (tartaric acid system) and the double constant equation (citric acid system) may be better to describe the kinetic release behaviours of DDE and DDT; Besides α-HCH conforms to the double constant equation, the kinetic release of β-, γ- and δ-HCH seem to be more consistent with the Elovich equation. And it tells us that in the tartaric acid leaching system. So the release of DDTs is mainly characterized by several diffusion mechanisms in the tartaric acid leaching system and characterized by the release mechanism of different energy position in citric acid system. It may involve some more complex release mechanisms for the release of the HCHs, which own a larger solubility in water.

Overall, the release of organochlorine pesticides in water system is consistent with the firstorder kinetic equation which is good at describing a simple diffusion mechanism. The release of DDTs by the oxalic acid and citric acid system can be well described by doubleconstant equation which is good at describing a uniform energy distribution; that in tartaric acid system can be describe by parabolic diffusion equation which is controlled by a number of diffusion mechanism. For HCHs, their release behaviour in oxalic acid system conforms to parabolic diffusion equation, and that in tartaric acid and citric acid system are more consistent with Elovich equation .

#### **3.2.3 Discussion on the release mechanism of organochlorine pesticides by LMWOA**

The difference of organochlorine pesticides release pattern in different LMWOA systems may be related to the differences of the pesticides' three-dimensional structure and different action mechanism of LMWOA on different bound pesticides on the soil surface. Hydrophobic pesticides are adsorbed mainly through hydrophobic force, van der Waals force, hydrogen bonding and other in soil internal space systems, inorganic mineral surface (surface physical adsorption), amorphous organic matter (soft carbon, fast linear distribution) and aggregate organic matter (hard carbon, slow linear adsorption) four regions, especially in the latter two regions that the inherent soil organic matter (SOM) contributed the most, and the soft carbon-bound pesticide has not solute competition and hysteresis of sorption and desorption, which explains the fast and slow release process (Chiou *et al.*, 1986). LMWOA with Carboxyl and hydroxyl can affect the migration of pesticides by competitive adsorption, structural changes (such as the aggregate decentralized by chelating the metal ions that acting as cross-linking agent in the SOM) of soil and SOM, the release (Chiou *et al.*, 2000) and mineral dissolution (Landrum *et al.*, 1984), their release ability to pesticides related to their ability of the dissolution to soil minerals and multi-coordination ability (Yang et al., 2001). Oxalic acid with smaller molecule volume has two activity carboxyl functional groups, citric acid with larger molecule has three carboxyl and one hydroxyl, and tartaric acid have two carboxyl and two hydroxyl groups (Figure. 4). The effect of LMWOA on pesticides bound to soil organic matter includes unlocking action

Effects of Low-Molecular-Weight-Organic-Acids on the

**5. Acknowledgment** 

**6. References** 

the Central Universities (2009B17014).

pp. 1254-1258.

114-123.

Release Kinetic of Organochlorine Pesticides from Red Soil 531

The release velocity of HCHs was far higher than that of DDTs by water and LMWOA. Their difference was nearly 3 times. The variation amplitude of the release velocity and the influence of elution volume on release velocity for DDTs by water were all small and not obvious. The release velocity curves of OCPs from soil by LMWOA were all peak-type

The authors would like to acknowledge National Science and Technology major program (2009ZX 07317-007), National Natural Science Foundation of China (50839002), Ministry of Education Key Laboratory of Integrated Regulation and Resource Development on Shallow Lakes foundation, Hohai University (2007KJ003) and the Fundamental Research Funds for

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curve, and it included 2 stages which are rapid release and low release.

and locking action, if the activated pesticides do not leave the reaction system, they may soon be locked and bound with SOM, and resulted in the pesticide level in the system to reduce. Locking and unlocking mechanism to pesticides is the dominant mechanism for oxalic acid with smaller molecule, and the action mechanism of tartaric acid and citric acid with larger molecules to pesticides dominated by the chelation is the same as to macromolecules DOM (Yang *et al.*, 2001) and unlocking mechanism, it can avoid free pesticides being locked again, Therefore, the release amount of pesticides by tartaric acid and citric acid is much higher than that by oxalic acid and water.

Fig. 4. Sketch about the interaction among pesticides, LMWOA, humic acids and soil mineral.
