Highlighting the solubility in high pH solutions of the EPA 13 metals of concern (and the RCRA 8) and a selection of ions.

1. The metals I selected are from the RCRA 8 list and the 13 “metallic pollutants of concern” as well
as gold. Can’t evaluate metals recovery without thinking about gold!
Source: 1996 Handbook of Chemistry and Physics Student Edition or listed websites when no
information was in the Handbook.
Pgs 8-60 to 8-62 and 5-4 to 5-23.
A little background, the numbers I am giving are “Gibbs Free Energy” values at 25 C, 1 atm
pressure which is considered “standard conditions”. A larger negative value suggests that that
specific compound will be more common under standard conditions (because more energy is
released during formation versus being absorbed).
The list of soluble metal salts doesn’t have many Gibbs Free Energy values because I suspect
that either they are 1. too insignificant or 2. the values for GFE are only given for naturally
occurring metal salts.
All metals are provided with a list of the anions they combine with to form salts and the
corresponding GFE values and notes of importance.
Robert, the CC’d email addresses are for Karen Frye, my CFO and Dr.Randall Tagg, my Science
and Technology Consultant who is also the Chair of the CU Denver Physics Department.
The list:
Arsenic Considered a heavy metal and fairly insoluble, but becomes much less toxic in waters
with pH >8
http://www.webelements.com/arsenic/compounds.html
Antimony Insoluble in waters above pH 7 when combined with the following anions: Arsenate
AsO4, Arsenite AsO3, Oxide O3, Sulfate SO4, Sulfide S3.
Antimony Sulfide is the most common form per
environmentalchemistry.com/yogi/periodic/Sb.html
Barium Insoluble in waters above pH 7 when combined with the following anions: Arsenate,
Carbonate CO3, Chromate CrO4, Citrate (C6H5O7)2, Ferric (Fe(CN)6)2, Flouride F2 (-1156.8 –
second most common form), Oxalate C2O4, Phosphate (PO4)2, Sulfate SO4 (-1362.2 – most
common form), Tartrate C4H4O6.
Cadmium Insoluble in waters above pH 7 when combined with the following anions: Arsenate,
Carbonate, Chromate, Citrate, Ferric, Ferrous Fe(CN)6, Hydroxide (OH)2 (-897.5 – 4th most
common), Oxalate C2O4, Oxide O (-603.3), Phosphate (-3884.7, most common by a value of
200%), Sulfide (-477.4), Tartrate.
Beryllium not evaluated for solubility, however magnesium is in the same group and should have
similar properties
Magnesium Insoluble in waters above pH 7 when combined with the following anions:
Arsenate, Carbonate, Flouride (BeF2, -979.4 second most common by 100 kJ/mol), Hydroxide,
Oxalate , Oxide (-580.1, Phosphate, Silicate SO4 (-2055.1, lowest value by 100% meaning by far
the most common), Tartrate
Chromium Insoluble in waters above pH 7 when combined with the following anions: Bromide
Br3, Chloride Cl3, Flouride (-1088.0), Oxide (Cr2O3) (-1058.1), Phosphate, Sulfate SO4, Ferrous
Chromate FeCr2O4 (-1343.8). These values are the three lowest and therefore the most
common.
Copper Insoluble in waters above pH 7 when combined with the following anions: Arsenate,
Arsenite, Benzoate (C7H5O6), Cyanide, Ferric, Ferrous, Flouride F2, Hydroxide, Iodide (-69.5),

2. Oxalate, Oxide (-129.7 as CuO, third most common, -146.0 as Cu2O, second most common),
Phosphate, Silicate, Sulfate, Sulfide (-53.6 as CuS, -86.2 as Cu2S), Tartrate
Lead Insoluble in waters above pH 7 when combined with the following anions: Arsenate,
Benzoate, Carbonate, Chromate, Cyanide, Ferric, Ferrous, Flouride, Hydroxide, Oxalate, Oxide
(-129.7 as CuO, third most common, -146.0, second most common), Phosphate, Silicate, Sulfate,
Sulfide (-98.7, second most common), Tartrate, Thiocyanate
Mercury I Insoluble in waters above pH 7 when combined with the following anions: Acetate
(C2H3)2), Arsenate, Benzoate, Bromide, Chloride, Carbonate, Chromate, Citrate, Cyanide,
Formate, Iodide (-101.7), Oxalate, Oxide (-58.5), Phosphate, Sulfate, Sulfide (-50.6), Tartrate,
Mercury II Insoluble in waters above pH 7 when combined with the following anions: Acetate,
Arsenate, Benzoate, Chromate, Cyanide, Ferric, Ferrous, Flouride, Hydroxide, Iodide (-111.0),
Oxalate, Oxide, Phosphate, Silicate, Sulfate (-625.8, lowest value by %600, by far the most
common), Sulfide, Tartrate,
Nickel Insoluble in waters above pH 7 when combined with the following anions: Arsenate,
Arsenite, Benzoate, Carbonate, Chromate, Cyanide, Ferric, Ferrous, Flouride, Hydroxide,
Oxalate, Oxide, Iodide, Oxalate, Oxide, Phosphate, Sulfide (-79.5, second most common),
Tartrate
Selenium
The various forms of selenium behave differently in the environment. Heavy metal
selenides, selenium sulphides, selenites, and selenates all vary in solubility (that is, their
ability to dissolve in water). Selenates (salts containing one Se atom and four atoms of
oxygen) are among the most mobile Se compounds due to their high solubility and
inability to cling onto soil particles. Thus, they are readily taken up by microorganisms in
the soil or leached through the soil. Selenites (salts containing one Se atom and three
atoms of oxygen) are less soluble than selenates. Elemental Se is common in some soils,
but does not dissolve much in water and therefore is not likely to be taken up by living
organisms.
http://www.ec.gc.ca/ceqg-rcqe/English/Html/GAAG_Selenium.cfm
Zinc Selenide ( -163.0), Thallium (-59.0) only two compounds provided.
Silver Insoluble in waters above pH 7 when combined with the following anions: Acetate,
Arsenate, Arsenite, Benzoate, Bromide (-96.9), Carbonate, Chloride (-109.8), Chromate (-641.8
lowest value, most common), Citrate, Cyanide, Ferric, Ferrous, Iodide (-66.2), Oxalate, Oxide
(-11.2), Oxalate, Oxide, Phosphate, Sulfate (-618.4 second most common), Sulfide (-40.7),
Tartrate, Thiocyanate
Thallium little solubility information Selenium Thallide (-59.0)
Gold I Insoluble in waters above pH 7 when combined with the following anions: Bromide
(-53.3), Chloride (-117.6), Cyanide, Iodide, Oxide, Sulfide
Gold II Insoluble in waters above pH 7 when combined with the following anions: Bromide,
Chloride, Hydroxide, Iodide, Oxide, Sulfide