« It « : of zero charge (PZC). Tadros and Lyklema [3) and of the aof silica below pH 3; also the electrokinetic of silica at very low pH values is positive [5). Recently Koopal [6] has of silica can be only negative and at

doi:10.1201/9781482269703-27

ABSTRACT

Apparently, silica should be more efficient as adsorbent of cations than metal oxides, e.g., hematite and alumina whose surfaces are positively charged in the neutral pH range, but actually silica is a weak adsorbent of heavy metal cations compared with metal oxides, and significant adsorption on silica is observed at pH values only slightly lower than the pH of precipitation of the corresponding metal hydroxide in absence of silica. In natural systems, uptake of heavy metal cations by iron or manganese oxides is often more significant than adsorption on silica even when the latter is much more abundant. The fact that metal cations prefer to adsorb on positively charged surfaces of alumina or hematite rather than on a negatively charged surface of silica is intriguing and not fully understood.