The Environmental Behaviors Of Roxarsone In The Soil

Roxarsone is an additive added to livestock and poultry feed for resisting bacteria, stimulating growth and improving the conversion of feed. It has been used extensively in China. Once introduced into the soil environment, it elevated the concentration of arsenic in the soil. Studying the mechanism of degradation and mobility of roxarone will help us to understand the potential hazards of the organoarsenical roxarsone. In this study, the soil environmental behaviors of roxarsone in three typical Chinese soils (calcareous alluvial soil, brown soil and red soil) were studied through field investigation, adsorption-desorption experiments and leaching experiment. The results were summarized as follows.1. Arsenic pollution in Wuhan's greenhouse vegetable soils was investigated, and the results showed that the mean concentration of arsenic in the greenhouse vegetable soil was 16.39 mg-kg-1, which was higher than the background value of Wuhan soil (15.00 mg-kg-1). According to the distribution of arsenic in the greenhouse vegetable soil, the concentration got higher as the investigation area got closer to the urban district. Different land use types and soil property affected the concentration of arsenic in the soil profile. The soils that had been fertilized with manure for more than 5 years, exhibited greater As concentration in the soil profiles. For instance, As concentrations in soils depth of 60-80cm from Jiangxia and Huangpi were 1.76 times and 2.25 times over the surface soils respectively. For other types of soils that was not much fertilized, the maximum As concentration in depth of 60-80cm was only 78.1% of 0-20cm.2. Potential risk to human health of As pollution was evaluated by the sigle factor contaminat index and Nemerow index. The results showed that, the vegetables in Wuhan had not been polluted by arsenic, and there was no signigicant difference between the vegetables from local and other cities. The results of hierarchical cluster indicated that based on the arsenic bioconcentration factor (BCF), the vegetables could be separated into three groups, i.e., higher arsenic BCFs (leek, rape, Brassica parachinensis bailey and radish), middle arsenic BCFs (red pepper, cucumber, Chinese cabbage, spinach and scallion), and lower arsenic BCFs (sponge, eggplant, balsam pear, cowpea, Chinese wax gourd, bottle gourd, green Chinese cabbage, celery, cabbage and amaranth).3. The adsorption behaviors of roxarsone in the test soils were investigaged in lab batch experiments. The results showed that soils varied in As absorption capacity. red soil would get adsorption equilibrium in roxarsone concentration of 50 mg·kg-1 in 1h, while calcareous alluvial soil and yellow brown soil in roxarsone concentration of 1 mg·kg-1 in 48h. The soil particle size could affect the sorption of roxarsone in the range: <2μm>2-50μm>>50μm; Higher solution pH promoted roxarsone release from the test soils. It would elevate the sorption of roxarsone with removal of organic matter. The roxarsone adsorption evidently decreased with the phosphorus concentration increasing.4. A leaching soil column was set up to study the distribution of roxarsone and degradation production in soils. The results showed that the breakthrough curves were detected only for calcareous alluvial soil and brown soil in the 7 week experiment. The peak concentrations of roxarsone in the two soils were 9.356 mg·L-1 and 1.154 mg·L-1, and was detected at 28h and 43h respectively. Species of As(V) and As(III) were detected at 87h in calcareous alluvial soil, at 81h and 68h in brown soil, and 35h in red soil.The distribution of roxarone was different from the three test soils. The maximum concentration of roxasone was 0.749mg·kg-1 in calcareous alluvial soil at the depth of 25-30cm. Different from calcareous alluvial soil, the concentrations of roxarone deceased in the deeper layers in yellow brown soil and red soil, with the minimum concentration being 0.155 mg·kg-1 and 0 mg·kg-1 respectively. In the profile of the test soils, species of As(V), As(Ⅲ) and DMA as the degradation productions of roxarsone were all detected.Based on the analysis of sequential extraction procedure, roxarsone addition could increase the concentration arsenic bounded to Fe and Mn fraction as well as residual fraction significantly. The increment of exchangeable fraction in brown soil indicated that the addition of roxasone could improve the bioavailability of arsenic.