Specific chemical interactions between metal ions and biological solids exemplified by sludge particulates

• pH plays an universal role on metal uptake by sludge particulates.
• Dissolved organic matter can compete with sludge particulates for metal ions at specific pH value.
• The surface acidity of sludge particulates is defined in acidity strength and acidity capacity.
• Surface complex formation can describe metal ions and sludge particulates well.

The adsorption of metals onto biological surfaces was studied exemplified by municipal sludge particulates of the primary, the secondary, and the tertiary sludge types from four regional wastewater treatment plants. Major factors affecting the extent of metal adsorption including pH, DOM, total biomass, and total metal loading were studied. The acidity–basicity characteristics of the DOM, the metal ions (Lewis acids), and the surface of the sludge particulates make pH the most important parameter in metal adsorption. Change in pH can modify the speciation of the metal ions, the DOM, and the surface acidity of the sludge particulates and subsequently determines the degree of metal distribution between the aqueous phase and the sludge solids. Information on the acidity–basicity characteristics of the DOM and the sludge particulates are used to calculate the stability constant of metal ion–sludge complexes.