Still smaller and more highly charged ions react with water to produce relatively
large and stable anions (so called oxyanions), such as sulphate (SO 42 - ), by abstract-
ing oxygen ions from water and again liberating hydrogen ions (Fig. 5.1c), for
example:
eqn. 5.2
The net effect is to produce large anions, which dissolve readily since the charge
is spread over a large ionic perimeter. Other important oxyanions are nitrate
(NO 3 - ) and carbonate (CO 32 - ).
The general pattern of element solubility can be rationalized in terms of
charge and ionic radius (z/r) (Fig. 5.2). Ions with low z/rvalues are highly soluble,
form simple ions in solution and are enriched in the dissolved phase of river-
water compared with the particulate phase, for example Na+. Ions with interme-
diate z/rvalues are relatively insoluble and have a relatively high particulate :
dissolved ratio in riverwater, for example Fe^3 +, Al^3 +. Ions with large z/rvalues
form complex oxyanions and again are soluble, for example SO 42 - , NO 3 -.
Some oxyanions exist in solution as weak acids and will ionize or dissociate
(see Box 4.5) depending on the pH, as shown here for phosphorus.SVI()()s+Æ+ 42 HO 21 () SO^24 - ()aq H+()aq144 Chapter Five
S6+(SO2– 4 )C4+(CO2– 3 )P5+(PO3– 4 )V5+(VO 4 3–)Si4+(H 4 SiO 4 )B3+(BO3– 3 )N5+(NO– 3 )0–0.2 +0.2 +0.4 +0.6 +0.8 +1.0 +1.2 +1.4 +1.6 +1.8
Log |z| /r01234567Logriver particulate concentrationriver dissolved concentrationmg g–1
¥^10–3mg I–1ÊÁˈ ̃ ̄Ti4+Mn4+Al3+
Fe3+Sc3+Co2+Ni2+
Cd2+
Zn2+
Mg2+
Ca2+K+Li+
Na+F–Br–Cl–Cu2+Fig. 5.2Ratio of average elemental riverine particulate to dissolved concentrations plotted against the ratio of
charge to ionic radius for the most abundant ions of those elements. In the case of dissolved oxyanions, the
relevant dissolved species are shown in brackets. Concentration data from Martin and Whitfield (1983), other
data from Krauskopf and Bird (1995).