Effect Of Heavy Metal Elements On Formation And Crystal Form Conversion Of Aluminum Scale

With the development of society,the problem of drinking water safety has become increasingly prominent.As a common pollutant in water environment,heavy metals have become the focus of attention in the field of drinking water purification due to their various types,complex forms and toxicity.In addition,due to the late development of water purification technology and the long service life of water supply network in China,most water supply networks have serious deposition phenomenon.Among all kinds of pipe deposits?pipe scales?,the existence of aluminium-containing pipe scale is relatively common,which is an important matrix for the accumulation of various heavy metals,and has an important impact on the transfer and transformation of heavy metals in the pipe network.Recent studies by our group have shown that besides the influence of aluminium scale on the accumulation and release of heavy metals,heavy metals also have some influence on the formation types of aluminium scale.In order to investigate the morphological changes of aluminium-containing pipe scale under different environmental conditions systematically when heavy metals coexist,this experiment simulated the pipe network system with amorphous Al?OH?₃ scale and aluminosilicate compound scale respectively.And the morphological characteristics and changing law of aluminium scale under different heavy metal ion content,water environment pH value and aging time were observed when Cr?VI?,Cd?II?and Cu?II?coexisted.The research shows that in the amorphous Al?OH?₃ scale system without adding heavy metal ions,the aluminium-containing scale is mainly Al₂?OH?₆·H₂O?am?in the acidic environment of pH=6.5.When the environmental pH of the pipe network becomes8.5,the Tohdite/5Al₂O₃·H₂O component will appear in the pipe scale.For the aluminium-containing scale system without adding heavy metal ions,the scale is mainly amorphous Al?OH?₃ and aluminosilicate in acidic environment with pH=6.5.When the pH is 8.5,a small amount of bayerite will appear in the composite pipe scale.At pH=8.5,when the molar ratio of Cr?VI?,Cd?II?and Cu?II?to Al in amorphous Al?OH?₃ scale system is 1:10,the scale components become stable on the 100th,60th and 5th day,respectively.And in the aluminosilicate composite pipe scale system,scale components in systems with three heavy metals become stable at the 60th day of operation.And further research found that in the amorphous Al?OH?₃ scale system,when the molar ratio of Cr/Al is 1:100¹:10,Cd/Al is 1:10³:1 or the molar ratio of Cu/Al is 1:10¹:1,all three heavy metal elements can promote the formation of bayerite crystal under pH=8.5.And The order of promoting effect from strong to weak is Cd?II?,Cu?II?and Cr?VI?.And for aluminosilicate scale system,when the molar ratio of Cr/Al is 1:100¹:10,Cd/Al is 1:100³:1,or the molar ratio of Cu/Al is 1:100¹:10,the formation of bayerite can also be promoted by the three heavy metals.And then the order of promoting effect from strong to weak is Cr?VI??Cd?II?and Cu?II?.And When the molar ratio of Cd/Al is1:10³:1,Cd can promote the formation of silica-aluminate?Na₂Al₂SiO₆?.On the premise that the molar ratio of heavy metals to Al is 1:10,the pipe network system with coexistence of Cr?VI?and Cd?II?is within the range of pH=6.5-7.5,and the components of pipe scale are not affected by pH.The pipe network system with Cu?II?coexists in the range of pH=7.5⁸.0,and the components of pipe scale are not affected by pH.By comparing the effects of three kinds of heavy metals on the formation and transformation of two types of typical pipe scale under different pipe network conditions,this study finds that When aluminosilicate component exists in the aluminium scale of pipe network,it can increase the formation amount and crystallization rate of bayerite when Cr?VI?coexists.When Cd?II?coexists in the pipeline network,aluminosilicate component can promote the formation of trace bayerite at pH<8.0.When the molar ratio of Cu?II?to aluminium is 1:10 in the official network,for amorphous Al?OH?₃ scale system and aluminosilicate compound scale system?amorphous Al?OH?₃+aluminosilicate?,the pH value of water environment have little effect on on each component of aluminium-containing pipe scale.The research results fill in the blank of domestic research on the transformation law of aluminium scale morphology in pipeline network,and provide important theoretical basis for the operation and management of urban water transmission and distribution system.