**2.1 Source of LMWOA**

Low-molecular-weight organic acids (LMWOAs) occur widely in soils and primarily originate from root exudation (Gao *et al.*, 2010; Ling *et al.*, 2009; Lu *et al.*, 2007; White *et al.*, 2003). Moreover, microorganisms, animals, and the degradation of organic matters can also produce LMWOA (Jones, 1998). People have noted the role of root exudates in the early 18th century, and then have a more profound understanding of the composition and the role of plant root exudates in latest 30 years. The composition and transportation of root exudates and its function in soil structure formation, soil mineral weathering, soil nutrient activation, promoting the nutrient absorption and rhizospheric soil nutrient movement and poison resistance (Al, acid) have been reported (Oburger *et al.*, 2009; Strom *et al.*, 2001; van Hees *et al.*, 2002; van Hees *et al.*, 2003). Root exudates are lubricant of the root - soil interface and microbial energy source, and they can improve the rhizosphere environment. They are the key materials for plants to adapt to nutritional stress (Strom *et al.*, 2002; Strom *et al.*, 2005) and environmental stress (do Nascimento *et al.*, 2006; Gao *et al.*, 2010; Guo *et al.*, 2007; Jones *et al.*, 2001; Liao *et al.*, 2006; Luo *et al.*, 2006; Toyama *et al.*, 2011; Zhu *et al.*, 2009). There are over 200 kinds of root exudates, including saccharides, organic acids, amino acids and allelochemicals, as well as small amount of fatty acids and steroids and trace growth substances and enzymes (Baudoin *et al.*, 2003; Muratova *et al.*, 2009). Their types and numbers will vary with the plant types and rhizosphere environment. Among them, the LMWOA are the main component of root exudates, which with one to several carboxyl and the most common LMWOAs identied in soils include citric acid, tartaric acid, maleic acid, malic acid, formic acid, acetic acid, oxalic acid, succinic acid, fumaric acid, propionic acid and so on (Baudoin *et al.*, 2003; Kpomblekou-A & Tabatabai, 2003). Most of them are the intermediate products of tricarboxylic acid cycle. In special cases, plant secretes specific organic acids, such as mugineic acid which is secreted by rice and wheat when they lack iron (Inoue *et al.*, 2009; Kobayashi *et al.*, 2010). Molecular structure and charged characteristics of LMWOA can affect many processes in the soil, some stresses of adversity factor can induce plant roots to secrete a large number of organic acids, and this is an adaptive reaction of plant to ecological environment (Inoue *et al.*, 2009; Kobayashi *et al.*, 2010).

The object of this article is to discusses the dynamic release behavior of several organochlorine pesticides like DDT isomer (DDTs) and HCH isomer (HCHs) with LMWOA from variable charge soil (red soil) with self-designed dynamics device, and provide some reference to the migration and fate of these kinds of substances and also the phytoremediation and ecological risk assess ment of organic pollutants in the

Their source include: root exudates, microorganisms, animals, and the degradation of organic matters. The LMWOA, as one of the typical rhizosphere active components presenting in soil and a key molecule in the solubilization of phosphorus-containing crystalline or amorphous minerals or as a desorbing agent for orthophosphate adsorbed on other soil minerals, is sure to have an influence on the existing state of organochlorine pesticides in soil and on the interaction between organochlorine pesticides and soil

Low-molecular-weight organic acids (LMWOAs) occur widely in soils and primarily originate from root exudation (Gao *et al.*, 2010; Ling *et al.*, 2009; Lu *et al.*, 2007; White *et al.*, 2003). Moreover, microorganisms, animals, and the degradation of organic matters can also produce LMWOA (Jones, 1998). People have noted the role of root exudates in the early 18th century, and then have a more profound understanding of the composition and the role of plant root exudates in latest 30 years. The composition and transportation of root exudates and its function in soil structure formation, soil mineral weathering, soil nutrient activation, promoting the nutrient absorption and rhizospheric soil nutrient movement and poison resistance (Al, acid) have been reported (Oburger *et al.*, 2009; Strom *et al.*, 2001; van Hees *et al.*, 2002; van Hees *et al.*, 2003). Root exudates are lubricant of the root - soil interface and microbial energy source, and they can improve the rhizosphere environment. They are the key materials for plants to adapt to nutritional stress (Strom *et al.*, 2002; Strom *et al.*, 2005) and environmental stress (do Nascimento *et al.*, 2006; Gao *et al.*, 2010; Guo *et al.*, 2007; Jones *et al.*, 2001; Liao *et al.*, 2006; Luo *et al.*, 2006; Toyama *et al.*, 2011; Zhu *et al.*, 2009). There are over 200 kinds of root exudates, including saccharides, organic acids, amino acids and allelochemicals, as well as small amount of fatty acids and steroids and trace growth substances and enzymes (Baudoin *et al.*, 2003; Muratova *et al.*, 2009). Their types and numbers will vary with the plant types and rhizosphere environment. Among them, the LMWOA are the main component of root exudates, which with one to several carboxyl and the most common LMWOAs identied in soils include citric acid, tartaric acid, maleic acid, malic acid, formic acid, acetic acid, oxalic acid, succinic acid, fumaric acid, propionic acid and so on (Baudoin *et al.*, 2003; Kpomblekou-A & Tabatabai, 2003). Most of them are the intermediate products of tricarboxylic acid cycle. In special cases, plant secretes specific organic acids, such as mugineic acid which is secreted by rice and wheat when they lack iron (Inoue *et al.*, 2009; Kobayashi *et al.*, 2010). Molecular structure and charged characteristics of LMWOA can affect many processes in the soil, some stresses of adversity factor can induce plant roots to secrete a large number of organic acids, and this is an adaptive reaction of plant to ecological environment (Inoue *et al.*, 2009;

environment .

colloids.

**2. Research advance** 

**2.1 Source of LMWOA** 

Kobayashi *et al.*, 2010).

#### **2.2 Residual characteristic of organochlorine pesticides in soil and rhizosphere**

Organochlorine pesticides (OCPs) could quickly be adsorbed or bound to the soil or soilorganic matter due to their high hydrophobicity and low water solubility after they were introduced into the soil. With time, the diminishes of these OCPs's bioavailability due to an "aging" effect and the formation of "bound-residues", which takes place during processes of decomposition and humication of organic matter (Alexander, 2000). The residual charactaristics of OCPs in rehizosphere and bulk soil maybe related to the properties of OCPs (water solubility, degradability, volatility,etc.), characteristic of soil mineral (the type and content of organic matter, diameter composition of soil mineral particle, content of oxidationreduction materials, moisture content, etc.), properties of plant (root system characteristic, kind and quantity of root exudates, lipoprotein content, specific surface, etc.) (Alexander, 2000; Calvelo Pereira *et al.*, 2006; Chen *et al.*, 2007; Gonzalez *et al.*, 2005; Inui *et al.*, 2008; Mikes *et al.*, 2009; Mo *et al.*, 2008; Skaates *et al.*, 2005; White *et al.*, 2002; Yang *et al.*, 2008; Yao *et al.*, 2007). For the OCPs with high volatility and relative easy degradation, the concentrations in rehizosphere are generally lower than that in bulk soil, and the results generally appear in the indoor simulation experments. For the OCPs with low volatility and relative harddegradation, the concentrations in rehizosphere are generally higher than that in bulk soil, and the results generally appear in the field experments. The results of pot experiment maybe also differ with field experiment for the same compounds. This kind of contradictory results is mainly due to the pollution sources can be repeatedly inputted by irrigation and dry and wet deposition in field, pollutants would be enrich in rehizosphere with water flow; and the pollution source is single input in indoor simulation experiment, plant absorption or biodegradation result in the concentration of OCPs in rhizosphere soil is lower then in the bulk soil.

For example, Chen et al. (2007) showed that the measured DDXs in the rhizosphere soils were significantly higher than those in the bulk soils. p,p'-DDT, p,p'-DDD, and p,p'-DDE in the soil accounted for 38%, 47% and 15% of the total. For total DDXs, approximately one third remained on the outer surface of the roots. The partition of DDXs between rhizosphere soil and root surface depended on contaminant affinity to soil organic matter, soil organic matter content and root specific area (Chen *et al.*, 2007), Calvelo Pereira et al. (2006) reported that the roots of *Avena sativa L., Chenopodium spp., Solanum nigrum L., Cytisus striatus (Hill) Roth, and Vicia sativa L.* tended to reduce levels of the HCH isomers in the rhizosphere (Calvelo Pereira *et al.*, 2006),white et al. (2002) found that the chlordane concentration in the rhizosphere (soil attached to roots) was signicantly less than that in the bulk soil. However, the enantiomeric ratio of the chiral components and overall component ratios had changed little in the rhizosphere relative to the bulk soil. (White *et al.*, 2002).

#### **2.3 Interaction mechanism between LMWOA and pesticides in soil**

LMWOAs have been shown to disrupt the sequestering soil matrix, thereby enhancing the desorption of organic pollutants in soil (White *et al.*, 2003). Consequently, it is expected that LMWOAs, in theory, may affect OCPs availability in soil environment. However, to date, few research has been conducted in this area, and there is limited information on the availability and sorption–desorption behaviour of OCPs from natural soils by organic acids. Gonzalez et al. (2010) showed that sodium citrate and oxalate, at levels usually exuded by plant roots, effectively enhanced desorption of p,p'-DDT, p,p'-DDE and α-cypermethrin, while

no effects were observed for α-endosulfan and endosulfan sulfate, the non-ionic surfactant

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

p,p'-DDT 1.2~5.51) 6.2~6.91

p,p′-DDE 65 5.69~6.96

o,p′-DDT 1.2~5.5 6.76

o,p′-DDE 65 6.94

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

2000)

a teflon filter disc.

**3.1.2 Chromatogram conditions** 

**3.1.3 Dynamic experiment methods** 

Release Kinetic of Organochlorine Pesticides from Red Soil 523

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

DDTs solubility log*K*ow HCHs solubility log*K*ow

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

Institute of National Chromatogram Center in Dalian, P.R.China).

calculation was conducted with external standard method.

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

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 +

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

**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

α

β

γ

δ

*-*HCH 10 3.8




Company, Germany. The characteristics of pesticides studied are listed in table 1.

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

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

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

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