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Air Quality Criteria for Lead Volume II of II - (NEPIS)(EPA) - US ...

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Acidity (pH)<br />

Freshwater<br />

Most <strong>of</strong> the Pb in aquatic environments is in the inorganic <strong>for</strong>m (Sadiq, 1992). The<br />

speciation <strong>of</strong> inorganic Pb in freshwater aquatic ecosystems is dependent upon pH and the<br />

available complexing ligands. Solubility varies according to pH, temperature, and water<br />

hardness (Weber, 1993). <strong>Lead</strong> rapidly loses solubility above pH 6.5 (Rickard and Nriagu, 1978)<br />

and as water hardness increases. In freshwaters, Pb typically <strong>for</strong>ms strong complexes with<br />

inorganic OH ! and CO3 2! and weak complexes with Cl ! (Long and Angino, 1977; Bodek et al.,<br />

1988). The primary <strong>for</strong>m <strong>of</strong> Pb at low pH (#6.5) is predominantly Pb 2+ and less abundant<br />

inorganic <strong>for</strong>ms include Pb(HCO)3, Pb(SO4)2 2! , PbCl, PbCO3, and Pb2(OH)2CO3<br />

(Figure AX7-2.2.1). At higher pH (∃7.5), Pb <strong>for</strong>ms hydroxide complexes (PbOH + , Pb(OH)2,<br />

Pb(OH)3 ! , Pb(OH)4 2! ).<br />

Figure AX7-2.2.1. Distribution <strong>of</strong> aqueous lead species as a function <strong>of</strong> pH based on<br />

a concentration <strong>of</strong> 1 µg Pb/L (U.S. Environmental Protection<br />

Agency, 1999).<br />

AX7-119

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