Effect Of Speciation Transformation Of Manganese On Aggregation And Deposition Of Graphene Oxide

Due to the extensive existence of manganese（Mn）in natural aquatic environments and the increasing applications of graphene oxide（GO）products,the co-existence of Mn and GO in natural waters would be inevitable.However,the effect of their interactions on the transportation and fate of GO is not clear.In this work,the impact of different species of manganese on the aggregation of GO was investigated systematically through batch experiments under different chemical conditions（e.g.,Mn concentration and p H）.In addition,the dissolved oxygen（DO）in the shallow water and deep-water areas changed relatively large.We studied the effects of normal DO concentration and anoxic conditions on the oxidation rate of Mn（II）.Finally,we verified that whether in-situ formation of manganese oxide（MnO₂）is better than ex-situ formation of MnO₂.In view of the above discussion,the main contents and results of this paper are as follows:（1）When p H of solution at neutral condition,the experiments about the effect of different concentrations of GO（5 to 100 mg/L）and different Mn concentrations（0.01 to 6 m M）were conducted.The results showed that the critical coagulation concentration（CCC）of Mn（II）to GO was 3 m M in the GO concentration range of 5 to 100 mg/L.With the increase of Mn（II）concentration in the solution,the zeta potential of GO gradually decreased.When the CCC of Mn（II）approached 3 m M,the surface charge of GO tended to zero and GO formed aggregation and deposition because of the combined of van der Waals attractive force and gravity.（2）When the solution including the normal level of DO,the experiment of the effect of Mn（II）at different solution p H values on GO aggregation and deposition was performed.The experimental results showed that when the solution p H exceeded 8.0,the residual concentrations of GO solution decreased apparently.At the same time,Mn（II）concentration in the supernatant of the reaction system was gradually decreased as the function of p H increased.This indicated that GO aggregation was inseparable with manganese oxidation.（3）The effects of Mn（II）concentration on the aggregation and deposition of GO under different p H conditions under anoxic conditions（DO ≈ 0.12 mg/L）was introduced.The experimental results showed that GO aggregated when p H of the solution exceeded 9.6.At the same time,Mn（II）concentration in the supernatant after the reaction gradually decreased with the increase p H.Compared with aerobic conditions,Mn（II）oxidation in waters under anoxic conditions required higher p H values.This showed that at the same p H,the oxidation rate of manganese was limited by the content of DO;at the same level of DO concentration,the higher the p H,the faster the manganese oxidation rate.（4）Comparing the effect of pre-formed MnO₂ and in-situ MnO₂ on GO aggregation under different p H conditions,the experimental results showed that only in-situ fresh formed manganese dioxide could promote the aggregation and deposition of GO.This showed that when Mn and GO coexist in natural water body,only the in-situ Mn oxidation in the water can promote GO aggregation,while the MnO₂ already existed in natural waters had little effect on GO aggregation.