Time constraints on experimental studies of lead apatites

Abstract

As so ci ate Ed i tor: Jacek Szczepañski The pa per pres ents the re sults of ex per i men tal stud ies on the syn the sis and ther mo dy namic sta bil ity of se lected Pb-apatites in terms of cri te ria de ter min ing ter mi na tion of the ex per i ments. Based on the case study, we in di cate dif fi cul ties in ana lys ing the ob tained ex per i men tal data. Time-re solved sam pling of pre cip i tate formed dur ing a dropwise syn the sis of pyromorphite was per formed and the re sults were com pared to the lit er a ture data. It has been con cluded that the Ostwald rip en ing time for syn the sized sol ids de pends pri mar ily on the chem i cal com po si tion of the in tended Pb-ap a tite phase. We pre sented that heter o ge ne ity of pre cip i tate af fects its dis so lu tion in terms of re peat abil ity of the re sults and equilibrating time. A unique 9-year-long ex per i ment on vanadinite sta bil ity at a pH range from 2.0-6.0 re vealed that among all tested dis so lu tion con ditions only the re ac tions at the pH = 3.5 can per form as the ba sis for some ther mo dy namic cal cu la tions. It has been con cluded that the rate of phase tran si tions in the Pb-apatites group can be mis lead ing in terms of de ter min ing the equi lib rium of the sys tem, and the ex per i men tal setup de signed par tic u larly to pro vide re li able con trols in this as pect should be in volved. Means in this re spect have been pro posed. Key words: pyromorphite, vanadinite, ex per i men tal min er al ogy, dis so lu tion, sol u bil ity syn the sis.

Solubility Product of Vanadinite Pb5(VO4)3Cl at 25 °C—A Comprehensive Approach to Incongruent Dissolution Modeling

“…The experiments were performed in triplicate at pH = 3.5, adjusted using 0.1 M HNO 3 (MERCK, supra pure). This pH value was indicated in our previous studies as adequate for the potential thermodynamic modeling of vanadinite stability [55]. The dissolution was carried out for 12 years.…”

“…For the wet chemical analysis, -small portions of synthetic vanadinite (~50 mg) were digested in 0.02 M EDTA (2,2 ,2 ,2 -(ethane−1,2diyldinitrilo)tetraacetic acid) (POCH.SA, pure for analysis) and analyzed for Pb, V and Cl, as described in the next section. Compared to other reagents e.g., HNO 3 , the EDTA is an effective solvent for mineralization of Pb-apatites yielding relatively quick and complete dissolution of the solid [14,36,37,39,55].…”

“…Standard empirical thermodynamic studies of apatites last several weeks, as this has been widely accepted to be sufficiently long for the solids to equilibrate with the solutions [36,37,39,68]. However, vanadates tend to polymerize, inducing re-precipitation of vanadinite Pb 5 (VO 4 ) 3 Cl into chervetite Pb 2 V 2 O 7 ; the system is highly pH and temperature dependent and the kinetics of these reactions are poorly understood [55]. This caused difficulties in the interpretation of dissolution data and in unambiguous determination of the equilibrium of the system with the use of the dissolution-precipitation method (e.g., [37]).…”

“…During the incongruent dissolution of vanadinite-as the system approaches thermodynamic equilibrium, a key factor in determining the properties of the minerals-chervetite is formed. Recrystallization is relatively slow and occurs at different rates, which highly depend on the pH and temperature; however, this process is still poorly understood in the context of vanadinite solubility [55]. In the carbonaceous environments, the amount of adsorbed (or possibly precipitated as chervetite) vanadium is greatly affected by the dissolved organic compounds (humic substances, organic acids) because of reduction of vanadate V(V) to vanadyl (V(IV)) VO 2+ ions [56][57][58][59].…”