Photochemical Transformation Study Of Lead-based Pigments In Water

The widespread use of lead increases the risk of lead exposure.Lead-based pigments remain one of the major sources of lead exposure for children worldwide.Mainly through interior decoration,children's toys and other contact.International concern about the use of pigments containing lead began in 2009.Some countries have enacted laws to prohibit the use of lead additives in pigments.By September 30,2019,38%of all countries had legally binding controls,85 countries do not have legally binding control measures in place and the remaining 35 do not have relevant information.The minimum total lead concentration limit for pigments containing lead in every countries is 90 ppm.This provision may underestimate the environmental risks by failing to take into account the process of conversion of pigments containing lead into the environment.This research mainly starts from the environmental transformation of minium pigments and white lead pigments,and considers the effects of fulvic acid,common inorganic salts,pH and light on pigments in water.Lead ion dissolution of lead pigment was studied by atomic absorption spectroscopy(AAS).The morphology transformation of lead was studied by means of X-ray photoelectron spectroscopy(XRD),X-ray photoelectron spectroscopy(XPS)and transmission electron microscopy(TEM).The main contents are:(1)The effects of fulvic acid(FA)and pH on lead ions releaseThe semiconductor minium is photoactive under ultraviolet light.We controlled the concentration of fulvic acid in the reaction solution of the sample,adjusted the pH of the reaction solution,irradiated by ultraviolet light,and studied the regularity of lead ion released.It can be found that lower pH value(pH=4.67)and higher FA concentration(20 mg/L)are beneficial to lead ion release.When significant amounts of FA exist in the water,they will complex with the released lead ions preferentially.Then,lead ions react with CO2 and proton resulting in the formation of insoluble Pb3(CO3)2(OH)2 after FA consumed.Correspondingly,the concentration of dissolved lead ion in water increased firstly and then decreased.These results further prove that FA plays an important role in the photolysis of minium.Based upon characterization and experimental results,lead ions release and lead species transformation mechanisms of minium were eventually speculated.(2)Effect of inorganic anions on minium pigment in waterThe minium suspension was reacted in different inorganic anions(SO42-,Cl-,HCO3-,NO3-,PO43-),and controled the concentration of anions.From the AAS we konw,in the presence of different inorganic anions,lead ion release is different,which is mainly attributed to the solubility of lead complexes formed by lead and different inorganic salts and the pH of salt solution environment.In general,the higher the salt concentration,the more soluble lead ions are released.XRD results show that PbCO3 exists in all the photoreaction products of minium in different salt solutions(except in chloride ion and sulfate ion salt solutions,where corresponding lead salts are generated).Light is the main factor affecting lead ion release and lead species transformation.Further study on the photolysis of minium in natural water under natural sunlight detected the release of lead ions,and obtained a similar rule to that under laboratory conditions,which verified our hypothesis.(3)Effect of FA and inorganic anions on white lead pigment in water under ultraviolet light White lead pigment was selected as the research object for further investigation.The presence of FA or inorganic anions(SO42-,Cl-,HCO3-,NO3-,PO43-)in the reaction solution was controlled for light/dark experiments.The conclusion was drawn through the characterization test.We can know that white lead is responsive to most ultraviolet light regions.AAS shows that FA has a significant effect on lead ion release of lead white.In the presence of inorganic anions and FA in the solution under ultraviolet light,lead ions release obviously.Lead ion release is different in different inorganic salt environments.By XRD detection of reaction products,the photoreaction products in chloride,phosphate,nitrate and bicarbonate were all PbCO3,while the photoreaction products in sulfate were lead sulfate hydrate.In the presence of different inorganic anions,the dark reaction products are still Pb3(CO3)2(OH)2.This is show that light accelerated the reaction of the white lead with the salt solution.The influence of inorganic anions and FA factors on the release and morphologic transformation of lead ions from lead pigments were studied by controlling variables.It is helpful to understand the environmental behavior and risk of lead pigments in natural water and provide a reference for photochemical risk assessment.