| In recent years,the wide use of organic arsenic drugs lead to an increasing residue in environments,causing potential ecological risk.Organic arsenic drugs have been extensively used as feed additives beasts and birds cultivation for the improvement of feed efficiency and growth rate,and for the controlling of coccidian intestinal parasite.However,the adsorption of organic arsenic drugs is rare and scarce,with the majority of organic arsenic drugs intake being passed in animal faeces.Organic arsenic drugs are amphoteric molecule that the mobility in soil environments is greatly influenced by environment condition.To understand the fate of organic arsenic drugs in environment,roxarsone and arsnilic acid were chosen as two typical organic arsenic drugs,and iron humate(IH),humic acid modified goethite(HA-?-FeOOH)and ferric oxide(?-Fe2O3)were used as three typical iron oxide adsorbents,to make a research about the adsorption behavior between adsorbents and organic arsenic drugs.Characteristics of adsorbent were studied by means of X-ray diffraction(XRD),nitrogen adsorption,scan electronic microscope(SEM),and Fourier transform infrared absorbance(FTIR).Static experiment was employed to study the adsorption kinetics and thermodynamics behavior.In addition,the influence of pH,ionic strength,competing ions,heavy metal ions and humic acid(HA)on the adsorption behavior of iron oxide to organic arsenic drugs was also investigated.The results can serve as theoretic and practical foundation for the controlling of organic arsenic contaminated water.Results were as follows:1.The adsorbent which had better adsorption capacity to organic arsenic drugs has been synthesized.The structure characterization showed that the introduction of iron ion significantly improved the structure and surface properties of HA.The maximum adsorption capacity of IH to adsorb ROX and p-ASA were 77.76 and 188.68 mg g'1 respectively at the temperature of 308 K,and the adsorption of p-ASA process conform to pseudo-second-order kinetics and Langmuir adsorption isotherms.Various environmental factors showed different effects to the organic arsenic drugs adsorption on the three adsorbents.The adsorption capacity of organic arsenic drugs was improved with the rise of temperature;while,it reduced with the increase of pH and ionic strength.The presence of PO43-inhibited the sorption performance of arsonic acid which carried negative charge,because both anions competed for the adsorption sites.And this could lead to desorption of organic arsenic drugs.Cu2+could improve the adsorption of organic arsenic drugs on IH by chelation effect.Humic acid would reduce the adsorption due to electrostatic repulsion and larger steric hindrance effect.The main mechanism of adsorption of organic arsenic drugs were electrostatic attraction,hydrophobic interaction,ionic exchange and surface complexation.2.Modified by HA,the appearance of the HA-a-FeOOH had great changes.And the changed HA-?-FeOOH had larger specific surface area and better adsorptive property than ever before.The maximum adsorption capacity of HA-?-FeOOH were 80.71 and 59.88 mg·g-1 to adsorb ROX and p-ASA respectively at the temperature of 308 K.The adsorption process followed pseudo-second-order kinetics and the Langmuir adsorption model.At the same time,under acidic condition,the dispersiveness of modified goethite was better,and the exposed active sites became more than before,which could help adsorb organic arsenic drugs molecules rapidly.The adsorption capacity of organic arsenic drugs increased with the rise of temperature,however,it decreased as the pH and ionic strength increased.The presence of PO43-inhibited the sorption performance of arsonic acid which carried negative charge,because both anions competed for the adsorption sites.The augmented adsorption capacity of organic arsenic drugs in the presence of Cu2+ was probably due to the formation of ternary complexes.Cu2+ was regarded as a bridge between HA-a-FeOOH and organic arsenic drugs.The adsorption of organic arsenic drugs was weaken because the complexation between HA-a-FeOOH and organic arsenic drugs.The main mechanism of adsorption of organic arsenic drugs were electrostatic attraction,the bridge of cationic effect,ionic exchange and surface complexation.3.The adsorption process conforms to pseudo-second-order kinetics the secondary dynamics and Langmuir isotherm adsorption model of organic arsenic drugs on?-Fe2O3.Reactions happened between As-0 of organic rasenic and ?Fe-OH.The maximum capacity for ?-Fe2O3 to adsorb ROX and ASA reached 46.53 mg·g-1 and 38.62 mg·g-1 at the temperature of 308 K.The adsorption capacity increased with the temperature increased,on the contrary,decreased with the increase of pH and ionic strength.It was similar with IH and HA-a-FeOOH that the existence of P043-competed with organic arsenic drugs for the limited adsorption sites.And the existence of Cu2+ helped organic arsenic drugs get more positive charge adsorption sites.The electrostatic adsorption happens between HA(which carried negative charge)and ?-Fe2O3(which carried positive charge),so that HA occupied the limited adsorption sites on the surface of ?-Fe2O3,which influenced the adsorption of organic arsine.The main mechanism of adsorption of organic arsenic drugs were electrostatic attraction,coordination effect,ionic exchange and surface complexation respectively.In conclusion,the order of adsorption ability of organic arsenic drugs for three adsorbents:IH>HA-?-FeOOH>?-Fe2O3.The main mechanism of adsorption of organic arsenic drugs on IH and HA-?-FeOOH were surface complexation and hydrophobic interaction.The main mechanism of adsorption of organic arsenic drugs on ?-Fe2O3 were surface complexation and electrostatic attraction. |
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