| Activated sludge, composed of microbial groups, larger flocs and inorganic matter, has been proved to be an economically effective sorbent due to its low cost and high availability. In the present paper, Pb(II) was the heavy metal selected in this study to investigate the sorption behavior of metals onto activated sludge and distribution of metal in the chemical fractions. The overall objective of this study were:(1) to investigate the Pb(II) sorption characteristics of activated sludge taking into account kinetic and equilibrium;(2) to investigate the adsorption mechanism of Pb(II) on activated sludge by XPS, SRF, SEM and FT-IR.(3)to investigate the effect of initial pH, initial Cu(II) ion concentration, temperature and contact time on the distribution of Pb(II) in the chemical fractions of activated sludge after sorption equilibrium in accordance with the BCR sequential extraction procedure;(4) to study the removal of Pb(II) in activated sludge using ultrasound-assisted nitric acid.According to the experimental results, the main conclusion as follows:(1) Sorption characteristics of Pb(II) on activated sludgeThe removal of Pb(II) by the sorbent increased quickly at the initial stage of sorption, but later the sorption rate slowed.96h was selected as optimum shaking time for further experiments. The initial concentration of Pb(II) don’t product visible effect on sorption equilibrium time.With the initial concentration of Pb(II) increased, the adsorption capacity of Pb(II) increased while the adsorption rate decreased.There is an agreement between experimental and calculated qe values for the pseudo-first-order model and pseudo second-order mode. The correlation coefficient (R2) between experimental data and the two models values predicted by the two models are both above0.99. Therefore, both the pseudo-first-order and pseudo-second-order models are able to model the sorption process of Pb(II) onto activated sludge.The equilibrium sorption data was well modeled by both the Langmuir and Freundlich isotherms at10℃,20℃,30℃. The maximum adsorption capacity of activated sludge is28.74,31.45and33.41mg/g at10℃,20℃,30℃. The adsorption capacity of activated sludge increased as the temperature rose, indicating that sorption process is endothermic. The sorption intensity (RL) values and magnitude of1/n quantifies the favorability of the sorption and the degree of heterogeneity of the activated sludge surface.pH is one of the major parameters affecting the sorption process of metal ions. In the low pH range, the low removal efficiency of Pb(II) can be attributed to high concentration of H+that competes with Pb2+for active sites. As pH increases, the overall surface charge of the activated sludge becomes more negative and sorption capacity increases because of a higher electrostatic attraction between the positively charge metal ion and negative surface charge of activated sludge.Interference of Cu(II) may have a detrimental effect on the sorption of Pb(II) by activated sludge. With the initial concentration of Cu(II) increasing, Cu(II) has a more negative effect on sorption of Pb(II).(2) Adsorption mechanism of Pb(II) on activated sludgeThe thermodynamics study stated that the spontaneous nature of sorption of Pb(II) on the activated sludge was inversely proportional to the temperature, exothermic and degrees of freedom increased. The results also showed the chemisorption and physisorption coexist in the adsorption of Pb(II) onto activated sludge.The analysis of the FT-IR spectra showed the presence of functional groups able to interact with Pb(II). It should be noted that the results obtained at this stage were sufficient to hypothesise what chemical bonding reactions occurred between the functional groups and Pb(II).The measured Zeta potential values and XRF indicate the presence of electrostatic interactions, hydrophobic interactions and ion exchange. SEM measurements showed the activated sludge was heterogeneity and crystals were observed on the nickel(II) loaded sludge.(3) Chmical fraction of Pb(II) transportation in activated sludgeThe content of Pb(II) in the F1, F2and F4fraction increased rapidly during the first0.1h. Further increase was slow after this initial period. Maximum content of Pb(II) in F3fraction was reached within0.1h and the value decreased with the increasing contact time.The temperature had a significant influence on the distribution of Pb(II) in the chemical fractions of activated sludge during sorption. The content of F1, F2, F3and F4fraction in activated sludge increased as the temperature increased. When the initial concentration of Pb(II) was20mg/L, about58%of total Pb(II) was extracted in the reducible fraction (F2).The pH had a significant influence on the distribution of Pb(II) in the Chemical fractions of activated sludge during sorption. The content of F1increased from0.62mg/g to2.91mg/g as the pH rose from2to9. The maxmum content of F2, F3and F4is4.75,3.45and2.81mg/g at neutral condition.Cu(II) had severe influence on the transformation of Pb(II) fraction in the sludge. In the single-ion situation, the distribution order of Pb in different fractions was F2(4.75mg/g)>F3(3.45mg/g)>F4(2.81mg/g)>F1(1.07mg/g). Cu(II) facilitates Pb(II) transformation to F2fraction while at the same time inhibitingPb (II) transformation in the F3and F4fractions. However, the F1fraction is polytropic.X-ray photoelectron spectroscopic (XPS) measurements confirmed the existence of acid soluble/exchangeable and oxidizable fractions of Pb(II) in the activated sludge at the molecular level.(4) Removal of Pb(II) in activated sludge using ultrasound-assisted nitric acidDissolved quantity of Pb(II) in activated sludge increased with increasing sonication time. When sonication time was5,10,20and30min, removal amount of Pb(II) was7.55,8.17,9.08and9.33mg.Ultrasound alone was not effective enough to extract Pb(II) from the sludge. The extraction efficiency was promoted when ultrasound was used to assist nitric acid in extracting Pb(II) from activated sludge.Leaching rate of Pb(II) in activated sludge would increase with the acoustic energy increasing. However, the intension of acoustic energy don’t promote extraction efficiency of Pb(II) from the activated sludge.The different chemical fraction of Pb(II) in activated sludge showed different removal efficiencies using ultrasound-assisted nitric acid. The order of chemical fraction of Pb(II) removal efficiencies was residual fraction (F4)<oxidizable fraction(F3)<reducible fraction(F2)<acid soluble/exchangeable fraction(F1). |
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