Chemical Behaviour And Biological Response Of Roxarsone In The Soil

Roxarsone （3-nitro-4-hydroxy arsenate） is an organoarsenic feed additives,which is used to improving feed utilization and animal growth stimulation,in the sametime it also have a broad-spectrum bactericidal and anti-ball insect role. Therefore, it iswidely used in livestock breeding industry in China. However, roxarsone is lessabsorption in the animal body, and excreted mainly in the drug prototype. Livestock andpoultry faeces harmless handling mechanism is not perfect in our country yet, it is usedfor agricultural production as organic fertilizer only after simply process or treatment.Consequently roxarsone come into the environment with organic fertilizer or with farmssewage system, which are likely to have adversely affected on soil and water quality,and may also have eco-toxicity on microorganisms in the environment, animals,plantsand even human.This study selected a feed additive-roxarsone as a tested pollutants, Simulatedindoor incubation experiment,regular sampling and laboratory analysis were carried outto study the roxarsone behavior in the soil and its biological response status. Includingroxarsone degradation characteristics in soil; the effect of roxarsone on soil microbialactivity in soil; the adsorption and desorption characteristics in soil in order to providethe reference data and theoretical basis for the further study of roxarsone environmentaleffects, enrich the behavior of organoarsenic compounds in the soil and the scientificregulation of theroxarsone biological effects. The main results are as follows:1. Indoor simulation experiments has carried out on different concentrations ofroxarsone residues, it is showed that the degradation rate of roxarsone in the soildecreases with the increasing of the residual concentration,and slowing down withextension of time, its half-life ranged from42.92d to82.13d, roxarsone are easilydegradable organic compounds reference to the grading standards of pesticide in the soildegradation half-life.2. The quantity change of roxarsone in the soil primarily by three ways: microbial degradation, light degradation and soil particles adsorption,which take up53%,12.99%and34.01%of the total decrement respectively. In the range of moisture andtemperature conditions in this experiment, the degradation of roxarsone is directproportional with soil temperature，so that the increase of temperature can promote thedegradation of roxarsone in soil; the reduction of soil organic matter result indecreasding microbial activity and bring about slowing down the roxarsonedegradation.3.The adsorption and desorption process of roxarsone in soil and componentsremoved soil are reach balance in24h.The max adsorption quantity is116.3mg·kg^-1insoil, which is larger in organic matte removed soil and fewer in free iron oxides andamorphous iron oxides removed soil significantly the contribution rate is^-15.10%、62.79%and34.88%.The desorption process of roxarsone which absorbed on iron oxidesby citric acid exist lag phenomenon.Hysteresis index indicates that the difficultcoefficient is free iron oxides removed soil>amorphous removed soil>contrastsoil>organic matter removed soil, so the least desorption quantity is in free iron oxidesremoved soil.4.The roxarsone adsorption and desorption balance time are2h on hematite、geothite and ferrihydrite. The adsorption capacity of roxarsone is increased with theinitial concentration on three kinds of synthetic iron oxides. The isothermal adsorptionline is affiliated “L” model, means the affinity is stronger to low concentrationroxarsone,but the adsorption ratio growth is lower with the increasing of concentration.5.Langmuir equations can fitting the adsorption of roxarsone on three kinds ofsynthetic iron oxides, calculated the max adsorption quantity were23.81、45.05and94.34mg·kg^-1, which is200⁸00times larger than natural soil. The order ofadsorption quantity is ferrihydrite>geothite>hematite.The parameters of Freundlichequation by the lgkf and1/n also shows that the adsorption capacity of ferrihydrite isbetter than goethite and hematite, and adsorption strength just the opposite.6. Langmuir equations can fitting the desorption of roxarsone best, the maxdesorption amount were14.90、24.81and31.45mg·kg^-1.The desorption process of roxarsone which absorbed on iron oxides by citric acid existing lag phenomenon，average hysteresis index were0.268、0.351and0.714, so roxarsone has a strongdesorption trend on ferrihydrite.7.Adding different concentrations of roxarsone can significantly inhibit soil urease,catalase, and neutral phosphatase activity, and the peak inhibiting ratio appeared at the10th incubation day.After the first10days of incubation,all the processing enzymeactivity gradually began to recover. The toxic effect of the extent and chronologicalorder are neutral phosphatase> catalase> urease, and time of the toxic effect was55¹88.4d,50.0¹54d and21.8⁵1.7d respectively.8.Using the mode y=c/1+bx derived from the Michaelis-Menten kineticsequation calculat the ED50value of urease activity,catalase activity and neutralphosphatase activity:1667mg· kg^-110000mg·kg^-1soil,434.8mg· kg-1¹111.1mg· kg^-1soil169.5mg· kg-1⁵88.2mg· kg^-1soil,respectively,and the ED50valuesof neutral phosphatase activity was minimum, and to a certain extent can characterizethe soil’s pollution degree of the roxarsone.9. Adding different concentrations of roxarsone can significantly inhibit soilrespiration, amination and nitrification. The effect on soil respiration is not significant atthe end of incubation （27d）. To the end of incubation （27d） the effect on soilammoniation and nitrification is not significant by low concentration of roxarsone(20、50、100mg·kg^-1)but it is still significant difference by high density dealing(150、250mg·kg^-1).