The effect of pH and ligands on the sorption of heavy metal ions by cellulose

The adsorption of copper, lead, cadmium and zinc ions by cellulose suspensions has been examined at pH values ranging from 3.5 to 11. The chemical form of the metal ions has been changed by adding ligands such as oxalic acid, citric acid, tartaric acid, ethylenediaminetetraacetic acid, ethylene- diamine, glycine, cysteine and α,α'-bipyridyl. Stable anionic complexes were not sorbed by the cellulose, and at a given pH cationic complexes were sorbed less than hydrated ions. The presence of excess ligand generally served to mask the precipitation of the metal- hydroxy species which can occur in the pH 6-7 region. ��� The amount of metal cation sorbed increased with pH. This has been attributed to increased ionization of acid functional groups having a pKa value of around 4.4. ��� The implication of the results, in respect to aqueous metal ion levels in natural systems, has been considered.

The sorption of lead and cadmium species by clay minerals

The effect of pH changes and the presence of ligands on the uptake of lead and cadmium ions by three types of clay mineral (kaolinite, illite and montmorillonite) has been investigated. ��� In the absence of ligands, metal ion adsorption increases gradually with increasing pH until a threshold pH value is exceeded, whereupon total precipitation/sorption (attributed to formation of hydroxy species) occurs. With ligands present, the threshold point tends to shift to higher pH values, the magnitude of the effect depending on the stability of the metal complex formed. Adsorption of cationic metal complexes is subject to competition from charged protonated ligand species; anionic complexes are not sorbed.

The sorption of copper species by clays. I. Kaolinite

The effect of changes in pH and the presence of ligands on the uptake of copper ions by kaolin has been investigated. In alkaline media the clay suspension acts as a nucleation centre for hydroxy-bridged copper species, and the major role of many ligands is to 'mask' this precipitation reaction. Uncharged and negatively charged complex species are not sorbed to any measurable extent. In acidic media, the adsorption capacity of the clay for cationic species increases with pH. Probable mechanisms are discussed and the feasibility of predicting the effects of ligands on sorption considered.

The sorption of copper species by clays. II. Illite and montmorillonite

The effect of changes in pH and the presence of ligands on the uptake of copper ions by an illite and a montmorillonite type clay has been investigated. The observed adsorption behaviour on montmorillonite can be interpreted in terms of an ion exchange model, i.e. all positively charged solution species compete for available sites. For illite (and kaolinite) the controlling process appears to be the formation of polymeric hydroxy species attached to particular sites on the clay. The solution pH controls the development of hydroxy bridges, and ligands influence uptake through a masking action, i.e. prevention of hydroxy species formation. Stable anionic complex ions are not adsorbed by either clay.

The sorption of zinc species by clay minerals

The effect of changes in pH and the presence of ligands on the uptake of zinc ions by three types of clay mineral (kaolinite, illite and montmorillonite) has been investigated. In alkaline media the clay suspension acts as a nucleation centre for polymeric hydroxy species, and the major role of many ligands is to mask the precipitation process. Uncharged and negatively charged species are not sorbed to any measurable extent. In acidic media the adsorption capacity of the clays for zinc increases with pH and possible mechanisms are considered. For kaolinite and illite the controlling process appears to be the attachment of hydroxy species to particular sites on the particle edges; with montmorillonite ion exchange at negative lattice sites appears predominant. Of particular interest is the apparent affinity between montmorillonite and species containing nitrogen functional groups.