**5. Conclusions**

510 Pesticides in the Modern World - Risks and Benefits

multiple liner regression confirmed that IMI persistence was primarily correlated with OC

The DT50 values were further tested to determine the effects of soil type on the DT50 of IMI. Statistically significant differences in soil persistence, were found among the Krk (mean DT50 = 132.09 days) and Istria (mean DT50 = 77.00 days) region (*p* = 0.000002). In addition, results of degradation study within the regions showed a statistically significant difference in DT50 values between the soils Istria I (mean DT50 = 92.53 days) and Istria II (mean DT50 = 61.46 days) (*p* = 0.002), as well as between soils Krk I (mean DT50 = 108.16 days) and Krk II

Examining the soil properties reveals that the most contrasting difference between tested soils, with respect to IMI degradation, is soil OC content, which was in the range from 1.30 to 1.91% for Istrian soils and from 0.42 to 0.55% for Krk soils. Higher OC content in Istrian soils would cause more IMI sorption by the soil based on the concept proposed by (Park et al., 2003). An equilibrium sorption study, conducted to verify this hypothesis showed that IMI equilibrium sorption constants were higher for the Istrian soils than for the Krk soils (2.60 and 3.28 for the Istria I soil and Istria II soil; 1.28 and 1.53 for the Krk II soil and Krk I soil). Higher OC content in Istrian soils might have been accompanied by higher microbial population and activities that promoted biodegradation processes of IMI (Cox et al., 1997; Getenga et al., 2004; Park et al., 2003). In describing degradation of 2,4-D (2,4 dichlorpphenoxyacetic acid) Picton & Farenhorst (2004) hypothesized a mechanism according to which initially readily available chemical resulted in apparent rapid degradation, while subsequent increased binding to soil caused noticeable reduction in degradation rate. When comparing IMI degradation in all the tested soils, we observed that IMI degraded faster in Istrian soils than in Krk soils, although more IMI was sorbed in Istrian soils. Thus, it appears that sorption did not significantly inhibit IMI degradation in the soil. Otherwise, IMI should have degraded faster in the Krk soils than in the Istrian soils. Fitting two-compartment model to the measured data showed that the degradation rate constants in the rapid degradation pool in the Istrian soils were greater than those in the Krk soils at both concentration level (Table 6). Calculations using two compartment model based on the data in Table 6 revealed that readily available IMI amount in the rapid degradation pool initially represented 51-58 and 54-59% of the applied IMI in Krk soils and 62-64 and 65- 71% in Istria soils at the low and high concentration level, respectively. These results suggested that IMI in the rapid degradation pool is not equivalent to the dissolved IMI molecule, as Wolt, (1997) proposed. Although pesticide molecule in soil solution is generally thought to be readily available to microorganisms for biodegradation, there is evidence that

( ) *<sup>2</sup> DT = 72.7581 OC - 4.9850 CEC - 0.4689 cla <sup>50</sup> y + 11.8127 pH + 116.50 R = 0.825* (14)

content, with a regression equation of:

(mean DT50 = 156.02 days) (*p* = 0.00001).

sorption can accelerate pesticide degradation (Park et al., 2003).

Metabolism of IMI was also studied in four Croatian soils at both concentration levels. The amount of 6-CNA, which was detected in all the tested soils as a metabolic product, varied irregularly with the time (Figure 9). The maximum concentration of 6-CNA in the tested soils was in the range from 280 to 720 µg/kg for the 5 mg/kg concentration level, while the corresponding concentration of 6-CNA was from 36.2 to 54.9 µg/kg for the 0.5 mg/kg

**4.2.2 6-CNA formation** 

The sorption-desorption and degradation of IMI was examined to understand the influence of concentration and soil properties on its behavior and fate in soils of Croatian coastal regions. The experimental data revealed that the sorption and desorption isotherms of IMI in the tested soils were nonlinear over the concentration range used, which can be best described by the Freundlich equation. Soil sorption capacity of IMI depended significantly on the soil properties. Especially, the sorption behavior of IMI was largely dependent on the soil OC content, where the soils with higher OC content (Istria soils) showed higher sorption capacity and less potential mobility of IMI. Given the spatial difference between tested soils, statistically significant differences in soil sorption capacity were found among and within soils of Istrian and Krk region. According to calculated *KOC* values, IMI can be categorized as a medium mobility pesticide indicating that rational use of IMI entails little danger of the ground-water contamination. In all soils, a higher sorption capacity was observed at lower IMI concentrations, indicating that the percentage of desorbed amount of pesticide increased with increasing initial solution concentration. Desorption experimental data deviated significantly from the sorption data, indicating that these processes were distinctly different in tested soils. It can be assumed, that the desorption process appeared to be the result of a complex, time dependent interplay of several chemical and physical processes and irreversible binding of IMI to soil surfaces, leading to hysteresis. The negative and low values of the Gibbs free energy of the IMI sorption indicated exotermic characteristics of sorption reaction and corresponded to the physical process, suggesting that partitioning into soil organic matter was the main mechanism of IMI sorption in the soils used. IMI kinetic behavior in all tested soils at the high concentration level can be described by the first-order

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The study results emphasize the need for controlled IMI usage, especially in soils with low humus content (Krk soils), thus avoiding a risk of IMI leeching. Considering the abundant current use of IMI in the Croatian olive growing areas, regular monitoring is needed to evolve a strategy to manage the environmental hazards due to the IMI and its degradation products. Further research, aided also with the actual field data, will be directed to investigate the IMI's metabolism and binding mechanisms in order to better understand degradation pathway and the causes for hysteresis phenomena.

#### **6. Acknowledgment**

This work was supported by the Croatian Ministry of Science, Education and Sport (grant number 062-0621341-0061).

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**28** 

 *Hohai University,* 

*China* 

*Chinese Academy of Sciences,* 

**Effects of Low-Molecular-Weight-Organic-Acids** 

**on the Release Kinetic of Organochlorine** 

*1State key laboratory of hydrology-water resources and hydraulic engineering,* 

*3State Key Laboratory of Soil and Sustainable Agriculture Institute of Soil Science,* 

Organochlorine pesticides (OCPs) had been widely used in the world. Due to their strong persistence (The biological half-life period of DDT can reach 10 years in the soil (Li et al., 1999), the biological half-life period of DDE is longer than DDT((ATSDR), 1994)), high enrichment capability and amplification capability and potential high toxicity. They have been banned since the early 1970s in the global scope in succession, and China also banned these pesticides in 1983. After decades of biodegradation, its residual concentration in soil has been reduced significantly (Generally ranges from several to several hundred ng/g). But through the enrichment amplification of food chain, it still has high detection rate in soil, vegetables and human milk. DDE and DDD are the aerobic and anaerobic degradation products of DDT respectively, their insecticidal broad-spectrum and toxicity are higher than the DDT. As a kind of environmental estrogens, it may cause some damage to the health of human body and reproductive system (Maness *et al.*, 1998; Romieu *et al.*, 2000); hexachloroclohexanes (HCHs) have 8 kinds of isomers (*α-,β-,δ-,ε-,γ-,η-,θ-, and l* -HCH), and *γ-*HCH has obvious insecticidal efficacy (ATSDR), 1994). These two types of organochlorine

The solubility of OCPs in water is low and the transference capability of OCPs in the soil is limited, but dissolved organic matter (DOM) in the environment could obviously change the transference behaviour of OCPs, and the predecessor's research mainly focuses on the influence of macromolecular DOM to the migration behaviour of OCPs, and the mechanism lies mainly on its chelation behaviour to OCPs (Chiou *et al.*, 1987; Chiou *et al.*, 2000; Chiou *et al.*, 1986; Hassett & Anderson, 1982; Landrum *et al.*, 1984), studies about the effect of low molecule weight DOM on the retained behaviour of OCPs have rarely seen. In the rhizosphere microenvironment, the existence of low molecular organic acids (LMWOA) of plant root exudates must have great influence on the migration of OCPs, the experiment conducted by White (2003) showed that the seven kinds of low molecular weight organic acid could significantly increase the desorption of p,p'- DDE, the increment could reach at

pesticides are typically persistent organic pollutants (POPs).

the range of 19%-80% (White & Kottler, 2002; White *et al.*, 2003).

**1. Introduction** 

Zhao Zhenhua1, Xia Liling2, Wang Fang3 and Jiang Xin3

**Pesticides from Red Soil** 

*2Nanjing Institute of Industry Technology,* 

*Agricultural & Food Chemistry*, Vol.49, No.6, (June 2004), pp. 2899-2907, ISSN 0021- 8561

