Solubility at 5 - 75 C and thermodynamic parameters of halogenated mimetites Pb5(AsO4)3X
Authors: Puzio Bartosz , Sordyl Julia , Manecki Maciej 
Published: March 2020

&lt;p&gt;Solubility at 5 - 75 &amp;#176;C and thermodynamic parameters of halogenated mimetites Pb&lt;sub&gt;5&lt;/sub&gt;(AsO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;X&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;B. Puzio&lt;sup&gt;*&lt;/sup&gt;, J. Sordyl and M. Manecki&lt;/p&gt;&lt;p&gt;AGH University of Science and Technology, Department of Mineralogy, Petrography and Geochemistry, Krak&amp;#243;w, Poland (*correspondence: bpuzio@agh.edu.pl)&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Mimetite Pb&lt;sub&gt;5&lt;/sub&gt;(AsO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;Cl, apatite supergroup member, is a mineral of very low solubility. A&amp;#160;very flexible structure of apatite allows for substitution of Cl by F, OH, Br, or even I. Due to lack of solubility constants K&lt;sub&gt;sp&lt;/sub&gt;, and other thermodynamic parameters (enthalpy of&amp;#160;formation &amp;#916;H&lt;sup&gt;o&lt;/sup&gt;&lt;sub&gt;f&lt;/sub&gt;, specific heat capacity C&lt;sup&gt;o&lt;/sup&gt;&lt;sub&gt;p&lt;/sub&gt; entropy of formation S&lt;sup&gt;o&lt;/sup&gt;&lt;sub&gt;f&lt;/sub&gt;, Gibbs free energy of&amp;#160;formation &amp;#916;G&lt;sup&gt;o&lt;/sup&gt;&lt;sub&gt;f&lt;/sub&gt;), it is unclear which of the investigated phases is the most soluble or most stable. Answers to these questions have multiple environmental and&amp;#160;technological consequences.&lt;/p&gt;&lt;p&gt;The objective of this study was to run dissolution experiments of synthetic halogenated analogs of mimetite: Pb&lt;sub&gt;5&lt;/sub&gt;(AsO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;F, Pb&lt;sub&gt;5&lt;/sub&gt;(AsO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;OH, Pb&lt;sub&gt;5&lt;/sub&gt;(AsO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;Cl, Pb&lt;sub&gt;5&lt;/sub&gt;(AsO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;Br, and&amp;#160;Pb&lt;sub&gt;5&lt;/sub&gt;(AsO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;I, and determine their solubility at 5&amp;#160; - 75 &amp;#176;C which allows to calculate thermodynamic functions of state.&lt;/p&gt;&lt;p&gt;Pure phases have been successfully synthesized by precipitation from aqueous solutions. Batch dissolution and dissolution&amp;#8211;recrystallization experiments were conducted for up to 9 months in triplicates at 5, 15, 25, 35, 45, 55, 65 and 75 &amp;#176;C, at pH = 3.5 (to avoid crystallization of secondary phases during dissolution), in a 0.05 M&amp;#160;NH&lt;sub&gt;4&lt;/sub&gt;NO&lt;sub&gt;3&lt;/sub&gt; background electrolyte. A plateau in the [Pb] evolution patterns was used to determine equilibrium. The&amp;#160;ion activity products (IAP) of the mimetites were calculated based on the dissolution reaction:&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Pb&lt;sub&gt;5&lt;/sub&gt;(AsO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;X&amp;#160; &lt; = &gt; 5{Pb&lt;sup&gt;2+&lt;/sup&gt;}&amp;#160; +&amp;#160; 3{AsO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;3-&lt;/sup&gt;}&amp;#160; +&amp;#160; {X&lt;sup&gt;-&lt;/sup&gt;}&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;where the brackets denote activity and X means F&lt;sup&gt;-&lt;/sup&gt;, OH&lt;sup&gt;-&lt;/sup&gt;, Cl&lt;sup&gt;-&lt;/sup&gt;, Br&lt;sup&gt;&amp;#226;&amp;#128;&amp;#145;&lt;/sup&gt; or I&lt;sup&gt;-&lt;/sup&gt;. The new, experimentally determined values of logK&lt;sub&gt;sp&lt;/sub&gt; at 25 &amp;#176;C for mimetites are: -76.45&amp;#177;0.72; -77.71&amp;#177;0.38; -76.82&amp;#177;0.55; -76.13&amp;#177;0.54 and -72.48&amp;#177;0.45 respectively. The logK&lt;sub&gt;sp&lt;/sub&gt; of&amp;#160;Pb&lt;sub&gt;5&lt;/sub&gt;(AsO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;Cl determined here is in very good agreement with the logK&lt;sub&gt;sp&lt;/sub&gt; determined by&amp;#160;Bajda, 2010 (the discrepancy equals to 0.62%). The nonlinear regression of logK&lt;sub&gt;sp&lt;/sub&gt; versus temperature allowed for calculation of &amp;#916;H&lt;sup&gt;o&lt;/sup&gt;&lt;sub&gt;f&lt;/sub&gt;, C&lt;sup&gt;o&lt;/sup&gt;&lt;sub&gt;p&lt;/sub&gt;, S&lt;sup&gt;o&lt;/sup&gt;&lt;sub&gt;f&lt;/sub&gt; and &amp;#916;G&lt;sup&gt;o&lt;/sup&gt;&lt;sub&gt;f&lt;/sub&gt;. The calculated &amp;#916;G&lt;sup&gt;o&lt;/sup&gt;&lt;sub&gt;f&lt;/sub&gt; for mimetites increases linearly with the increase of ionic radius of&amp;#160;X&lt;sup&gt;-&lt;/sup&gt;. Thus, the most stable phase is F-mimetite while the least stable, in terms of Gibbs free energy of formation, is I-mimetite. The thermodynamic data reported in this study supplement existing databases used in geochemical modeling.&lt;/p&gt;&lt;p&gt;Financial support for the research was provided to B.P. by the Polish National Science Centre (NCN) grant No. 2017/27/N/ST10/00776.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Bajda T. (2010) Solubility of mimetite Pb&lt;sub&gt;5&lt;/sub&gt;(AsO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;Cl at 5&amp;#8211;55 &amp;#176;C. Environ. Chem. 7, 268&amp;#8211;278.&lt;/p&gt;