Table 1.

Target and Competitor Oxo-anion pKa’s, Geometry, and Common Occurrence Range

Oxo-anion	pKa’s	Protonated Structure at pH 6	Oxo-anion Geometry	Common Occurrence Concentration Range in Groundwater	Refs.
Target Oxo-anions
Arsenate As(V) H2AsO4−	pKa1 = 2.2 pKa2 = 7.0 pKa3 = 11.6		Tetrahedral	Background: < 0.01 mg/L Affected waters: 0.08 to 5 mg/ L	29–31
Arsenite As(III) H3AsO3	pKa1 = 9.2		Trigonal pyramid	Background: < 0.01 mg/L Affected waters: 0.01 to 5 mg/ L	30,32
Selenate Se(VI) SeO42-	pKa1 = −3.0 pKa2 = 1.7		Tetrahedral	Background: < 2×10−5 mg/L Affected waters: 0.14 to 1.4 mg/L	33–35
Selenite Se(VI) HSeO3−	pKa1 = 2.6 pKa2 = 8.2		Trigonal pyramid	Background: < 2×10−5 mg/L Affected waters: 0.14 to 1.4 mg/L	33–35
Chromate Cr(VI) HCrO4−	pKa1 = −0.98 pKa2 = 6.5		Tetrahedral	Background: 0.002 to 0.014 mg/L Affected waters: 0.05 to 0.43 mg/L	36,37
Competitor Oxo-anions
Phosphate H2PO4−	pKa1 = 2.1 pKa2 = 7.2 pKa3 = 12.7		Tetrahedral	Background (total phosphorus): 0.05 to 2 mg/L	29,38–40
Sulfate SO42-	pKa1 = −3.0 pKa2 = 2.0		Tetrahedral	Background: 1 to 250 mg/L	41–43
Silicate H4SiO4	pKa1 = 9.8		Tetrahedral	Background: 1 to 30 mg/L	42 44
Bicarbonate HCO3−	pKa1 = 6 pKa2 = 10.3		Trigonal planar	Background: 150 to 450 mg/L	42,45
Nitrate NO3−	pKa1 = −1.3		Trigonal planar	Background: 0.2 to 10 mg/L	39,41,46