Study On The Dissolution And Stability Of The Ba-Sr-Pb Containing Arsenate Solid Solutions In Aqueous Solution

Arsenic has been known from antiquity to be highly toxic for animals and the majority of plants.Chemical precipitation was often adopted to remove arsenic from wastewater which would emissions waste residue with arsenic in it.We must test its stability in environment.These arsenic waste residue often contained impurities,and existed in the form of solid solution.It was a meaningful work for environmental protection to study the solubility and stability of arsenic-containing solid solution in aqueous solution.In the present work,the dissolution and stability of three arsenic-containing solid solution series,i.e.,the BaHAsO4 H2O—SrHAsO4·H2O solid solution,the Morelandite—Alforsite[Ba5(AsO4)3Cl—Ba5(PO4)3Cl]solid solution and the Mimetite—Pyromorphite[Pb5(AsO4)3Cl—Pb5(PO4)3Cl]solid solution,have been studies through a series of batch experiments.Their solubility products(Ksp)and the corresponding free energies of formation(ΔGf0)were determined based on the experimental data.The Lippmann diagrams for the solid solutions were constructed to explain the aqueous evolution with time and to disscuss the reaction path during the solid solution-aqueous solution(SSAS)interactions.1.BaHAsO4 H2O-SrHAsO4 H20 solid solutionSix different members of the(BaxSr1-x)HAsO4·H2O solid solution were prepared and characterized by various techniques,and then dissolution of the synthetic solids was studied at 25℃ and initial pH 2,6 and 9 in a series of batch experiments for 4320h.With the increasing XBa,the morphology of the(BaxSr1-x)HAsO4·H2O solid solutions changed from short prismatic or granular crystals(XBa=0)to platy or blades crystals(XBa=1).The synthetic solids used in the experiments were found to have no obvious variation after dissolution.During the dissolution of the solid solution,the aqueous pH values and component concentrations increased rapidly at the beginning and then varied slowly with time,and finally exhibited stable after 2880h dissolution.With the increasing XBa,the aqueous pH value decreased and the aqueous Ba concentration increased.At XBa=0.21,the aqueous Sr and As concentrations had the highest values.Based on the data of the dissolution at 25℃ and initial pH=2,the solubility products(Ksp)for BaHAsO4·H2O and SrHAsO4·H2O were calculated to be 10-5.52 and 10-4.62,respectively.The corresponding free energies of formation(AGf0)were-1543.99 kJ/mol and-1537.94 kJ/mol.BaHAs04·H20 is less soluble than SrHAs04·H20.For the(BaxSr1-x)HAs04·H2O solid solution,the solubility decreased as XBa increased when 0.00<XBa<0.82 and increased as XBa increased when 0.82<XBa<1.00 The(BaxSr1-x)HAsO4·H2O solid solution had a minimum solubility product of 10-5.61 at XBa=0.82.The Guggenheim coefficients were determined to be a0=1.55 and a1=-4.35 for the(BaxSr1-x)HAs04 H20 non-ideal solid solution.The constructed Lippmann diagram was a typical Lippmann diagram for a non-ideal solid solution with a negative enthalpy of mixing,which produced a fall of the solutus curve with respect to the position of an equivalent ideal solutus,which indicated that the solubility of intermediate compositions is significantly smaller than that of an equivalent ideal solid solution.The system shows an "alyotropic" minimum at XBa2+,=0.87.At the end of the experiment,the dissolution followed the saturation curve for the pure end-member BaHAsO4·H2O and approached the intersection with the minimum stoichiometric saturation curve on the Lippmann diagram.2.Morelandite—Alforsite[Ba5(As04)3Cl—Ba5(P04)3Cl]solid solutionEleven different members of the Ba5(PxAs1-xO4)3Cl solid solution were prepared and characterized by various techniques,and then dissolution of the synthetic solids was studied at 25℃ and initial pH 2,6 and 9 in a series of batch experiments for 3600h.With the increasing XP,the morphology of the Ba5(PxAs1-xO4)3Cl solid solutions changed from acicular crystals(XP=0)to rodlike crystals(XP=1).The synthetic solids used in the experiments were found to have no obvious variation after dissolution.During the dissolution of the solid solution,the aqueous pH values and component concentrations increased rapidly at the beginning and then varied slowly with time,and finally exhibited stable after 2880h dissolution.With the increasing XP,the aqueous pH value decreased and the aqueous Ba and Cl concentration increased.At XP=0.00,the aqueous pH value and As concentrations(initial =2,9)had the highest values.At XP=0.21,the aqueous P and As concentrations(initial =6)had the highest values.Based on the data of the dissolution at 25℃ and initial pH=2,the solubility products(Ksp)for Ba5(AsO4)3Cl and Ba5(PO4)3Cl were calculated to be 10-33.77 and 10-43.33,respectively.The corresponding free energies of formation(ΔGf0)were-957.332 kJ/mol and-5996.42 kJ/mol.Ba5(PO4)3Cl is less soluble than Ba5(AsO4)3Cl.For the Ba5(PxAs1-xO4)3Cl solid solution,the solubility decreased as XP increased.The Guggenheim coefficients were determined to be a0=1.69 for the Ba5(PxAs1-xO4)3Cl non-ideal solid solution.At the end of the experiment,the dissolution was above the saturation curve for the pure end-member Ba5(AsO4)3Cl on the Lippmann diagram.3.Mimetite—Pyromorphite[Pb5(AsO4)3Cl—Pb5(PO4)3Cl]Eleven different members of the Pb5(PxAs1-xO4)3Cl solid solution were prepared and characterized by various techniques,and then dissolution of the synthetic solids was studied at 250C and initial pH 2 in a series of batch experiments for 3600h.The dissolution of the synthetic solids’ two end-members was also studied at 25℃ and initial pH 2,4,6 in a series of batch experiments for 3600h.With the increasing XP,the morphology of the Pb5(PxAs1-xO4)3Cl solid solutions changed from threadiness or acicular crystals(XBa=0)to grainy or massive crystals(XP=1).The synthetic solids used in the experiments were found to have no obvious variation after dissolution.During the dissolution of the solid solution,the component concentrations increased rapidly at the beginning and then varied slowly with time,and finally exhibited stable after 2160h dissolution.The aqueous pH values ranged 2.1 from 2.4.The aqueous P concentrations conformed to CPPH=2<CPPH=4<CPPH=6 when Pb5(P04)3Cl dissolved at different initial pH.The aqueous As concentration had the highest value 0.22 mmol/L when Pb5(As04)3Cl dissolved at initial pH 2 and the concentration could be ignored when Pb5(As04)3Cl dissolved at initial pH 4,6.Based on the data of the dissolution at 25℃ and initial pH=2,the solubility products(Ksp)for Pb5(As04)3Cl and Pb5(P04)3Cl were calculated to be 10-91.99 and 10-78.31,respectively.The corresponding free energies of formation(ΔGf0)were-2723.7 kJ/mol and-3776.94 kJ/mol.Pb5(P04)3Cl is more soluble than Pb5(AsO4)3Cl.For the Pb5(PxAs1-xO4)3Cl solid solution,the solubility increased as XBa increased.The Guggenheim coefficients were determined to be ao=-1.38 for the Pb5(PxAs1-xO4)3Cl non-ideal solid solution.At the end of the experiment,the dissolution was round the "Solidus(a0=0)" curve on the Lippmann diagram.