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Nanomaterials for Groundwater Remediation 323

typically have ionic strengths in excess of 10 4 M and frequently have
significant concentrations of calcium or magnesium, conditions should
tend to favor nanomaterial deposition. Even the electrosteric repulsions
provided by surface modifiers such as polymers or polyelectrolytes can
be overcome at high ionic strength and in the presence of divalent cations
2+
such as Ca 2+ and Mg . For example, the approximately 100 percent
transport of PMAA-PMMA-PSS modified RNIP through a 21-cm sand
column up to 10 mM of a monovalent cations (NaCl) (Figure 14a),
decreases at ionic strengths of greater than 100 mM. In the presence of
2+
divalent cations, such as Ca which are much more effective at screen-
ing the charge on particles, transport is reduced dramatically at con-
2+
centrations of Ca greater than 5 mM (Figure 8.14b). The electrosteric

Figure 8.14 (a) Effect of ionic strength (NaCl) and (b) cation
2
type (Ca ) on transport of polyelectrolyte-modified nano-
iron through a 21-cm water-saturated sand column.