Surface Physical-Chemical Characteristics of Sludge Particulates

The surface physical-chemical characteristics, specifically surface acidity and surface electrical properties of sludge particulates, govern the fate and transport of heavy metals in wastewater treatment systems. Surface acidity (i.e., site density and acidity strength) and surface electrical properties such as pH at the zero point of charge (${\rm pH}_{{\rm zpc}}$) and surface charge of primary, secondary, and tertiary sludge particulates were investigated. Alkalimetric titration was used to determine the surface site density and acidity constant. Electrophoresis was used to determine the surface electrical characteristics. Results show that the sludge surface site can be treated as a weak monoprotic acid under neutral and low pH conditions. The site density ($\Gamma _{m}$) was determined to be$2.4\times 10^{-3}$mol/g suspended solids (SS) for primary sludge and$1.7\times 10^{-3}$mol/g SS for secondary and tertiary 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 particulates have strong affinity toward metal ions, including potassium, sodium, and lithium ions. At the field ionic strength, the${\rm pH}_{{\rm zpc}}$value was approximately 2.9 ± 0.3 for all sludge particulates investigated. It was also revealed that the carboxyl moiety is the major surface functional group contributing the surface characteristics. The significance of surface physical-chemical characteristics of sludge particulates is exemplified by the metals uptake as effected by pH.