Surface physical-chemical characteristics of sludge particulates

The surface physical-chemical. characteristics, specifically surface acidit y and surface electrical properties of sludge particulates, govern the fate and transport of heavy metals in wastewater treatment systems. Surface aci dity (i.e., site density and acidity strength) and surface electrical prope rties such as pH at the zero point of charge (pH(zpc)) and surface charge o f primary, secondary, and tertiary sludge particulates were investigated. A lkalimetric titration was used to determine the surface site density and ac idity constant. Electrophoresis was used to determine the surface electrica l characteristics. Results show that the sludge surface site can be treated as a weak monoprotic acid under neutral and low pH conditions. The site de nsity (Gamma (m)) was determined to be 2.4 X 10(-3) mol/g suspended solids (SS) for primary sludge and 1.7 X 10(-3) mol/g SS for secondary and tertiar y sludges. The acidity constant (pK(H)) was estimated to be 6.1 +/- 0.5 for all sludge particulates studied. Results also showed that all sludge parti culates have strong affinity toward metal ions, including potassium, sodium , and lithium ions. At the field ionic strength, the pH(zpc) value was appr oximately 2.9 +/- 0.3 for all sludge particulates investigated. It was also revealed that the carboxyl moiety is the major surface functional group co ntributing the surface characteristics. The significance of surface physica l-chemical characteristics of sludge particulates is exemplified by the met als uptake as effected by pH.