Sludge of wastewater treatment plants as Co2+ ions sorbent

Sludges produced in huge amounts by wastewater treatment plants (WWTP) display high fertility properties; however, the presence of heavy metals restricts their use for agricultural purposes. Sorption capacity of sludge is generally much higher and it can also be considered as a cheap sorbent of heavy metals. The paper describes cobalt sorption by dried activated sludge (DAS) obtained from the aerobic phase of a WWTP. DAS was characterized by FT-IR spectroscopy, cation exchange capacity (CEC), and atomic absorption spectrometry (AAS) analysis. Sorption capacity of DAS (Q _eq) increased with the initial concentration (C ₀) of Co²⁺ (CoCl₂) within the range from 100 μmol g^?1 to 4000 μmol g^?1, reaching 15 μmol g^?1 and 200 μmol g^?1, respectively. The maximum uptake capacity (Q _max) at pH 6.0 calculated from the Langmuir isotherm model was (256 ± 9) μmol g^?1 for Co²⁺ ions. Obtained Q values were dependent on pH within the range from 3.0 to 7.0. Competitive effect of other bivalent cations such as Ni²⁺ in Co²⁺ sorption equilibrium was confirmed; which is in agreement with the hypothesis of the decisive role of ion-exchange mechanism in metal sorption. The obtained data are discussed from the point of view of potential utilization of sludges as sorbents, i.e. in non-agricultural application.