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exchangeable base cations, and decomposition of aluminosilicate minerals
(McBride 1994). For the Peerless iron, the pH change can be attributed primarily
to corrosion of the iron, which is explained by the following reaction:
Fe° + 2H20-> Fe 2+ +H2(g)+ 20H-           (7.1)
where Fe° is zero-valent iron, Fe2+ is ferrous iron, H2 (g) is hydrogen gas, and
OH- is a hydroxide ion.
7.1.3 Zinc Mass Removal by Sorption and Precipitation
Serial control tests were conducted at several different initial pHs to
separate the contributions of sorption and precipitation to removal of Zn. No
solids (i.e., foundry sand or Peerless iron) were used in the control tests so that
zinc would be removed only by precipitation. A wide range of pH (3.0 through
10.0) was considered. The initial pH was adjusted with 1.0 M of HCI or 1.0 M of
NaOH.
Zinc removed via precipitation in the control tests is shown in Fig. 7.4 as a
function of the initial solution pH. Precipitation of zinc begins between pH 5.0 and
6.0. Also shown in Fig. 7.4 is the zinc removal from the batch kinetic tests, which
was computed from the last concentration that was measured. The removal data
are also summarized in Table 7.2. Comparison of the zinc removal in the control
tests and the batch kinetic tests indicates zinc removal by foundry sands is
primarily by sorption for pH less than 5, and by a combination of sorption and
precipitation for pH greater than 6.0.