Environmental Behavior Of Copper Oxide Nanoparticles Under The Influence Of Dissolved Humic Acid

Compared with traditional bulk metals,Copper oxide nanoparticles?CuO NPs?have better physicochemical properties that are widely used in catalysts,semiconductor materials and photovoltaic materials.The extensive use of CuO NPs makes it released into the environment inevitably in the process of production,application or processing,and thus poses potential threats to the environment,biology and human health.Dissolved Humic acid is one of the most common dissolved organic matter in the environment.It contains a large number of functional groups such as phenolic hydroxyl groups and carboxyl groups,which can complex with metal ions that can reduce the concentration of free metal ions in the liquid medium.It is the focus of this study whether the presence of DHA will make the solubility equilibrium of CuO NPs move towards the direction of copper ion generation.The Selective adsorption of DHA will change the surface area exposed by nanoparticles in the suspension and affect the solubility equilibrium of CuO NPs.Therefore,the following two hypotheses are raised:As the amount of adsorption continues to increase before equilibrium,the migration of metal ions from the surface to the bulk solution would become more difficult which eventually leads to inhibited ion release.On the other hand,DHA will also reduce the hydrodynamic particle size of metal-based NPs suspended in the solution.As the dispersion of CuO NPs is increased by DHA,their specific surface area may also be increased.This would promote the ion release.The apparent CuO NPs dissolution thus results from the balancing between the exposed?increased dissolution?and the covered?decreased dissolution?surface area when the particles are dispersed by DHA?hypothesis 1?.However,the effective surface area of metal-based NPs exposed in a suspension containing DHA cannot be quantified directly by existing experimental means.To check the correlation between dispersion and adsorption in the presence of DHA and their contributions to the dissolution of CuO NPs,hypothesis 2 was raised.Hypothesis 2 assumes that the amount of dissolved CuO NPs would decrease with the increase of adsorption.Based on the above Hypothesis,DHA prepared from the sediment of Dianchi Lake was selected as the dissolved organic matter required for the experiment.Under the condition of pH=7,influences of Dissolved humic acid?DHA?on the dissolution,speciation,and stability of CuO NPs was investigated.The results found that:?1?The amount of released Cu from CuO NPs was found to increase over time with increasing concentrations of DHA,96%of which was present as organic complexes after 72 h,indicating that the release of copper ions from CuO NPs in the experiment is mainly in the form of organic complex;?2?DHA can disperse CuO NPs or reduces the size of its aggregates.With the increasing of DHA concentration,the hydrodynamic diameter and the?-potential of CuO NPs decreased.The adsorption of DHA on CuO NPs increased with the increasing of DHA concentrations,and the adsorption of DHA on CuO NPs surface inhibite the aggregation of CuO NPs and increase its dispersion;?3?When DHA concentration is higher than 16.09 mg C/L,DHA has little effects on dispersing CuO NPs and increasing exposure area of CuO NPs.The adsorption amount of DHA on CuO NPs is still increasing,and the ratio of total copper concentration to CuO NPs still increased linearly.Hypothesis 1 is rejected which cannot identify the CuO NPs dissolution driving force by the CuO NPs exposed surface area.Hypothesis 2 is also rejected that the dissolution of CuO NPs does not decrease with the increasing amount of DHA adsorption on CuO NPs,and the surface of CuO NPs is not Completely occupied by DHA.As the result,DHA promotes the dissolution of CuO NPs due to the complexation of copper,rather than enhancing the dispersion of particles by the adsorption of DHA on the surface of CuO NPs;?4?Under the high concentrations of DHA,homo-conjugation of DHA promoted by the negative charge-assisted H-bond would competes with copper ions for DHA-Cu²⁺binding with increasing DOM concentration,which the carbon standardization DHA and copper ion complexing ability?log K?is reduced.Similarly,the reduction of active sites from DHA surface functional groups may also lead to weaker ligand exchange capacity between DHA and CuO NPs,so the adsorption capacity of DHA on CuO NPs and the dispersion of CuO NPs in solution are both reduced.In this experiment,it was found that the multiple increase of DHA concentration did not increase the dissolution,adsorption and other environmental behavior processes of CuO NPs,and the intermolecular force of DHA may be the reason for the flattening of the dissolution and adsorption trend at high concentrations.The conclusion of this experiment is helpful to better understand the environmental behavior of CuO NPs under the action of DOM and the potential mechanism of their kinetics.It also provides a theoretical basis for the morphological transformation and toxicity of CuO NPs in the environment.