Reactions of hydroxyl radicals have been studied up to total pressures of 1 atm and in the presence of strongly quenching gases by using excimer laser photolysis of H 2 0 2 or H N 0 3 for the generation and pulsed dye laser fluorescence for monitoring the concentration of the radicals as a function of reaction time. Results are reported of a study of the effects of molecular oxygen on the reactions of OH with acetylene and with some sulfur containing compounds (e.g. CS2).The high sensitivity of the pulsed dye laser induced fluorescence technique (LIF) allows absolute rate constant measurements of OH reactions under tropospheric conditions, i.e. up to total pressures of 1 atm and in the presence of strongly quenching gases like O2 and H 2 0 . We report on measurements in which excimer laser photolysis is used for the pulsed generation of OH radicals from H 2 0 2 or HN'03 parent gases and where a pulsed, frequency doubled, high energy dye laser serves for monitoring the time decay of the OH concentration in an excess of reactant gas by LIF of the OH(A2Ct t , X2H) transition near 300 nm [l].In order to check the method and the apparatus, test measurements were performed in synthetic air at a total pressure of 1 atm with H2 and some simple alkanes and alkenes. The results (Table 1) are in reasonable agreement with literature data. cm6 s-', Cm3 S -' , cm3 s -' which are in good agreement with recent results of Wahner ut al. [S] obtained with a laser photolysis/laser absorption technique. In the presence of oxygen, the OH decay curves were found to be bi-exponential (Fig. 2). Such behaviour is characteristic for systems in which O H radicals are regencrated by fast secondary reactions. This was verified by n-C4Hlo -The reaction of OH with CzH2 was studied in detail to clarify some discrepancies in the literature data on the rate constant of this reaction [2 -63 in particular concerning the different results for its pressure dependence for He and Ar buffer gases and the contribution of a bimolecular process to the reaction. Fig. 1 shows the results for He, Ar, and N2.Similar pressure dependences are observed for these inert who measured pressure independent rate constants in He buffer gas. Also, the zero pressure intercept, i.e. the pressure independent bimolecular contribution to the rate constant, is found to be about a factor of 10 lower than given in the

A long-term field experiment was carried out with a wheat-maize rotation system to investigate the accumulation and bioavailability of heavy metals in a calcareous soil at different rates of sewage sludge amendment. There are significant linear correlations between the contents of Hg, Zn, Cu, Pb, and Cd in soil and sewage sludge amendment rates. By increasing 1 ton of applied sludge per hectare per year in soil, the contents of Hg, Zn, Cu, Pb, and Cd in soil increased by 6.20, 619, 92.9, 49.2, and 0.500 µg kg-1 , respectively. For Hg, sewage sludge could be safely applied to the soil for 18 years at an application rate of 7.5 t ha-1 before content exceeded the soil environmental quality standards in China (1 mg kg-1). The safe application period for Zn is 51 years and is even longer for other heavy metals (112 years for Cu, 224 years for Cd, and 902 years for Pb) at an application rate of 7.5 t ha-1 sewage sludge. The content of Zn and Ni in wheat grains and of Zn, Cu, and Cr in maize grains increased linearly with increasing sewage sludge amendment rates. The content of Zn, Cr, and Ni in wheat straws and Zn, Cu, and As in maize straws were positively correlated with sewage sludge amendment rates, while the content of Cu in wheat straws and Cr in maize straws showed the opposite trend. The bioconcentration factors of the heavy metals in wheat and maize grains were found to be in the order of Zn>Cu>Cd>Hg>Cr, Ni>Pb>As. Furthermore, the bioconcentration factors of heavy metals in wheat were greater than those in maize, indicating that wheat is more sensitive than maize as an indicator plant. These results will be helpful in developing the critical loads for sewage sludge amendment in calcareous soils.

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Phytotoxicity thresholds for heavy metals are derived from dose–response curves, which show the relationships between exposure dose and toxicity response. However, the results of tests or observations are commonly based on total heavy metal concentration, not the exposure dose that causes phytotoxicity; additionally, the phytotoxicity response differs with plant species. In the present study, the ethylenediaminetetraacetic acid (EDTA)-extractable copper (Cu) concentration was determined in order to evaluate Cu extractability. As two important horticultural food crops in Asia, tomato (Lycopersicon esculentum ‘Meifen No. 1’) and bok choy (Brassica rapa var. chinensis ‘Susheng 28’) were used to investigate Cu phytotoxicity in a wide range of Chinese soils with and without leaching treatment, after which relationships between Cu phytotoxicity thresholds based on EDTA-extractions and soil properties were established. The phytotoxicity thresholds showed that biomass of bok choy was more sensitive to Cu than tomato. Multiple linear regression analyses showed that soil factors, including organic carbon (OC), citrate dithionate extractable manganese (CD-Mn), cation exchange capacity (CEC), and CaCO3 explained over 85% of the variance in Cu phytotoxicity thresholds. The EDTA-extractable Cu dose–response relationships were further improved by incorporating soil properties. The new phytotoxicity predictive models indicated soil properties (soil pH, OC, CEC, and oxalate-extractable Mn) and EDTA-extractable Cu concentration explained more than 90% of the variance in the phytotoxicity response of tomato and bok choy biomass. The new phytotoxicity predictive models could be used to develop a reasonable remediation strategy for contaminated soils.

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