The Investigation For The Removal And Transformation Of Roxarsone In The Wastewater

The wide use of roxarsone as feed additive promoted the development ofanimal husbandry and achieved good economic benefits. However, a huge volumeof poultry waste containing roxarsone entered into environment, causing seriousarsenic pollution in the environment. In this study, multi-walled carbon nanotubes(MWCNTs) and iron-modified MWCNTs ware investigated for the removal ofroxarsone from aqueous solution. The degradation and transformation of roxarsoneduring anaerobic digestion were also investigated. The main conclusions wereobtained as follows:(1) Firstly, the removal of roxarsone from aqueous solution by MWCNTs wasinvestigated. The adsorption isotherm was well described with a Freundlich model,while the adsorption kinetics was well described with a pseudo-second-order model.Roxarsone was adsorded very quickly on MWCNTs in the first1h and it was only3h to reach equilibrium. The values of thermodynamic parameters such as standardfree energy change, standard enthalpy change and standard entropy changeindicated that the adsorption of roxarsone on MWCNTs was exothermic andspontaneous. The dynamic adsorption capacity in the column adsorption was a littlelower than that in the static method.(2) Secondly, iron-modified MWCNTs were investigated for the removal ofroxarsone from aqueous solution. The adsorption capacity of iron-modifiedMWCNTs was more than that of MWCNTs. The adsorption isotherm was welldescribed with PMM model. The adsorption kinetics could be well described with apseudo-second-order model. Thermodynamic analysis shows that the adsorption ofroxarsone on iron-modified MWCNTs is an exothermic and spontaneous process.The adsorption of roxarsone decreased as the pH and ionic strength increased.Desorption of roxarsone by MWCNTs and iron-modified MWCNTs had nosignificant desorption hysteresis.(3) Lastly, the effects of roxarsone on anaerobic digestion and the degradationwith sucrose as substrate were investigated. Roxarsone shows obvious inhibitoryeffect on both acidogenesis and methanogenesis, and the inhibitory increased withthe increase of roxarsone concentration. Sulfadiazine showed little influence onacidogenesis, and had a little effect on methanogenesis. During the first3days of anaerobic digestion, the concentration of roxarsone declined rapidly and thendecreased to zero in the solution. Sulfadiazine in the solution was difficult todegrade during the anaerobic digestion. In the long term of anaerobic digestion,nearly half of roxarsone was transformed into inorganic arsenic,70%of which wastrivalent arsenic.