The solubility of metal arsenates
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THE residues from nonferrous mining and processing operations often contain appreciable quantities of arsenic compounds which are in many cases considered to be stable. Arsenic is often "stabilized" as ferric arsenate, calcium arsenate, magnesium arsenate and other metal arsenic (v) compounds and disposed of in slag, tailings and residues dumps. The solubility of the metal arsenic (v) compounds is generally considered to be extremely low. Seidell t gives solubility data for a number of metal arsenates but the original source of the data is often very old and, for reasons given later, probably most unreliable. In 1956 Chukhlansev 2,3 published work on the solubility of a number of metal arsenates, the results of which have been used in several compilations of thermodynamic data both in the form of solubility products 4 and free energies of formations: Chukhlansev's experiments were performed in aqueous solutions using NaOH, H2SO4, H N O 3 or HC1 to adust pH. The solubilities show a general linear relationship between log solubility and pH and this has been used by other workers for extrapolating to conditions of higher pH and as the basis of the opinion that metal arsenates under specific conditions of pH are extremely insoluble. Direct extrapolation of solubility products without considering the particular equilbria involved can also lead to large errors when assessing solubilities at various pH values. Recently, Tozawa Umetsu and Nishimura 6 and Nishimura and Tozawa 7 reported a variation to Chukhlansev's linear relationship between log solubility and pH for ferric, calcium and magnesium arsenates which indicated that extrapolation of Chukhlansev's results to higher pH could lead to solubilities which are two low by up to two orders of magnitude. Nishimura and Tozawa explained their results as being due to the decomposition of the particular arsenate to the hydroxide at higher pH, liberating the appropriate arsenic (v) oxyanion. The solubilities of ferric, calcium and magnesium arsenates at various pH values can be fairly accurately predicted using available free energy data for the various substances in each system.
T H E R M O D Y N A M I C STABILITY D I A G R A M S A convenient approach to this problem of metal arsenate solubility is to consider all of the known equilibria in the particular system and to construct the appropriate stability diagram. A general equation for all hydrolitic equilibria can be written: aA
+
xH + =
bB
+
cH20
and the standard reaction isotherm AG ~ =
K
-- R TIn
expanded by taking T = 298.15 K, pH = - log all§ and all20 = 1, to obtain the equation
xpH = - AG ~9s15K(kJ) abB 5.708 = -- log - This equation applied to each of the known or deduced equilibria in a particular system results in a straight line on a plot of log activity of the dissolved ion vs pH (approximately log solubility v s pH). A number of metal arsenate-water systems are considered here in this way and appropriate "solubilit y " / p H diagrams drawn.
T H E IRON (111) A R S E N A T E - W A T E R SYSTEM Free ene
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