Investigation On The Adsorption Behavior Of Ionic Herbicide Glufosinate On Goethite Surface And The Joint Toxicity Effect With Heavy Metals

The environmental risk of pesticides is always a public concern.Some new types of pesticides have been developed to improve the effectiveness and reduce their environmental risks.Recently,glufosinate-ammonium(GLU),an alternative of a common pesticide glyphosate(GLY)has drawn increasing attention due to its high reactivity,broad pesticidal spectrum,low toxicity,and good environmental compatibility.GLU is a typical ionic pesticide,and its molecular structure has three active groups(Amino,carboxyl and phosphonic acid groups).Thus,it can present in different molecular forms as the pH changes(GLU+—GLU2-)and it mainly stays as anion in a common pH range.Ionized GLU has been observed to have properties such as strong polarity,high water-solubility and variable morphology.However,information on GLU's environmental behaviors is quite limited to date.In this paper,potentiometric titration method was used to determine the complex constant between GLU and different heavy metals.The behavior of GLU adsorption on the surface of goethite and the effects of environmental factors(pH,ionic strength,coexisting ions,soil organic matter,etc.)on this behavior were systematically studied.The adsorption mechanism was further illustrated by Attenuated total reflectance-Fourier transform infrared spectroscopy(ATR-FTIR)analysis and quantum chemical calculation.Luminescent bacteria and daphnia magna were used as biological models to investigate the joint toxicity effect of GLU and GLY with heavy metal ions.The results provided a foundation for the environmental risk assessment of GLU.The main results are as follows:1.The study of chelate ability between glufosinate-ammonium and heavy metals(1)GLU molecule is amphiprotic and it presents in different forms at different pH.The dissociation constants,pKa2 and pKa3,of GLU determined by potentiometric titration method were 2.67 and 9.57,respectively;(2)Potentiometric titration results showed that GLU has a strong complexation with Cu2+,but less obvious with Zn2+,Pb2+ and Cd2+.The complexing constant of GLU with Cu2+ was found to be 2 to 3 orders of magnitude higher than that of other metals;(3)The metal complexing ability of GLY was stronger than GLU;probably because the P on GLU molecules has one more methyl group than GLY resulting in a weaker affinity with metal than GLY.2.The study of the adsorption behavior of glufosinate-ammonium on goethite surface and its mechanism(1)Due to the difference in molecular structure and morphology,GLU adsorption capacity of GLU was much lower than that of GLY.The adsorption of GLU on goethite surface dropped with the increase of pH,but it was not affected by ionic strength.This may be due to the inner-sphere adsorption of GLU.(2)The background electrolyte cations(Li+,K+,Na+,Ca2+,Mg2+)almost had no effect on GLU adsorption,while inorganic anion ligand(SO42-and PO43-)and a high concentration of fulvic acid(FA)can suppress the GLU adsorption by competitive effect;(3)Cu2+ could form "goethite-Cu-GLU" surface ternary complex to significantly enhance the adsorption of GLU,which achieved the maximum adsorption quantity at pH 5.3,while other divalent metal ions(Pb2+ Zn2+ and Cd2+)almost had no effect on the adsorption of GLU,possibly because of the higher binding ability between GLU and Cu2+ than others;(4)Combining the results from batch adsorption experiments,spectral data and quantum chemical calculation,we hypothesized that GLU mainly form monodentate inner-sphere complexes through phosphoric acid group with goethite surface,accompanied with intramolecular hydrogen bond to neutralize the positively charged amino groups on the other end of GLU,thus enhancing the stability of the complexes.The possible conformation of GLU on goethite,as well as with the presence of Cu2+are shown in the figures below.Conformation of Figure b is mainly in acidic condition;conformation in Figure c mainly exist in the neutral to alkaline conditions.3.The study of glufosinate-ammonium/glyphosate impact on toxicity of heavy metal(1)Under the appropriate pH condition of testing organism(pH6.5 for luminescent bacteria,pH8.0 for daphnia magna),GLU and GLY showed no toxic effect on luminescent bacteria and daphnia magna,but under acidic conditions,GLY had certain toxic effect on luminescent bacteria,which might be associated with the morphological changes of GLY;(2)The GLU almost had no effect on the toxicity of Cd2+ and Zn2+ to luminescent bacteria and daphnia magna,but GLU could significantly reduce the toxic effect of Cu2+ and with the increase of GLU concentration,the inhibition effect on metals toxicity became more obvious.GLY had different degree of relief on the toxicity of Cu2+,Cd2+ and Zn2+,and the detoxification of GLY on Cu2+ and Zn2+ was found to be better than that of 2+.In general,GLU and GLY have antagonism at different levels to the toxicity of different heavy metals.(3)Through the calculation of free metal ions in different concentration of GLU/GLY,we found that as complexing ability between GLU/GLY and metal increased,the concentration of the free metal ions became lower,resulting more obvious inhibition effect on metal toxicity.This might be caused by the complexation of GLU/GLY with metals which changed the distribution of heavy metals formation,thus affecting the metal biological effectiveness.The inhibiting ability of GLU on the toxicity of heavy metals was generally lower than that of GLY,probably due to the low complexing ability of GLU with heavy metal than GLY.(4)When GLU or GLY coexisted with Cue+,the toxicity towards the two testing organisms could be predicted by the availability of free state Cu2+ in the system.In conclusion,due to the complexation of functional groups on GLU,its environmental behavior could be affected by the coexisting metal ions,such as increasing adsorption on the surface of the iron oxide through bridging effect with metals,and relieving the toxic effects of metal ions.The strength of the GLU-metal ions interaction is related to the complexation ability of the metal ions,in general Cu2+>Zn2+?>Cd2+.Therefore,when evaluating environmental behaviors and risks of GLU,factors such as pH and the chemical condition of water environment should be considered.