Effects Of Calcium And Magnesium Ions On The Partition Of Tetracycline Between Water And Humic Acid

Tetracycline(TC)is a kind of widely used antibiotics.It coexists with dissolved organic matter(DOM)and metal ions in the environment and forms complex pollutants.It is of great significance to explore the superposition mechanism of the interaction among DOM,TC and metal ions.Based on the results of dialysis batch experiments and ~1H NMR analysis,the binding mechanism of components in the presence of humic acid(HA)-TC-metal ions in solution was studied.In order to explain the mechanism,the four TC rings were named A,B,C and D from the dimethylamine ring of TC.The main conclusions are as follows:(1)The oxygen-containing functional groups(carboxyl,hydroxyl)on the surface of HA and the protonated dimethylamine on the A ring of TC are the main groups that have specific effects.TC and the oxygen-containing functional groups of HA are bound by hydrogen bond and cation exchange.The change of the overall charge of HA and the change of TC form caused by the increase of pH value will affect the electrostatic interaction between them.And the change of HA surface functional group caused by the increase of ion strength will also affect its ability to adsorb TC.(2)The ~1H NMR results showed that the addition of Mg(Ⅱ)made the TC twisted conformation more stable,thus inhibiting the adsorption of TC on HA.When the complexation ratio of TC-Mg is 1:1,the inhibition of Mg(Ⅱ)on the adsorption capacity of TC on HA is weak.The reason is that the B and D rings of TC will preferentially complex with Mg(Ⅱ),and do not affect the binding of the protonated dimethylamine to HA.When the complexation ratio is 1:2,the second Mg(Ⅱ)will be complexed on the dimethylamine of A ring.The inhibition is enhanced by the deactivation of TC dimethylamine and the stabilization of TC twisted conformation,at this time,the main adsorption mechanism is electrostatic effect.(3)Under the condition of acid pH,because of the difficulty of TC-Mg complex formation,Mg(Ⅱ)ion has little effect on HA-TC,but the alkaline condition is the opposite.The binding sites of Ca(Ⅱ)on TC are on B,C,D ring,not on A ring containing dimethylamine,and TC exists in extended conformation after its addition.Meanwhile,Ca(Ⅱ)also plays the role of cation bridge,thus promoting the adsorption of TC on HA.(4)The influence of Mg(Ⅱ)on the absorption of TC by HA was predicted by using NICA-Donnan model.It was verified that the application scope of the model could be extended from the original humic substance-metal ions binary system to the humic substance-ionic organic pollutants-metal ions ternary system.The specific parameters obtained by fitting the experimental data show that HA and TC are mainly combined by specific actions.