The sewage sludge was obtained from a municipal wastewater treatment plant in Beijing, China. The sewage sludge was collected after mechanical dewater treatment, stored in refrigerator at −18˚C before use. Main characteristics of sewage sludge are listed in Table 1.

Equilibrium sorption studies were performed to explore heavy metal adsorption capacity of the biosorbent. The adsorption isotherms for sewage sludge and alkali treated sewage sludge were shown in Figure 1 and Figure 2, respectively; q_e represented the amount of metal ion adsorbed per unit weight of biomass and C_e represented the metal ion concentration remaining in solution at equilibrium.

The initial pH value was 5.0 for all working solutions in order to prevent precipitation of heavy metals, the pH was not controlled during the equilibrium experiment. The final pH of heavy metal working solution after adsorption was in the range of 5.9 - 6.3 (results are not given). According to previous study [22], at the pH range of 6, Pb²⁺ accounts for about 98.6% of total lead, Cd²⁺ and Ni²⁺ accounts for 100% of total cadmium and total nickel, respectively. Thus, the precipitation of heavy metal ions can be neglected in the adsorption process.

For both alkali treated sewage sludge and sewage sludge, the Langmuir model yielded a little better fit than the Freundlich model (Table 2), and the good agreement with experimental data suggests that monolayer adsorption existed for the experiment, which is consistent with adsorption process between heavy metals and other biosorbent such as sugar beet pulp [23], dried activated sludge [24]. In addition, comparing with sewage sludge, the q_m value of alkali treated sewage sludge for Cd, Pb, Ni, increased by 0.51, 0.70 and 0.32 mmol/g, respectively. The higher q_m value of alkali treated sewage sludge indicates that the maximum heavy metal adsorption capacity of sewage sludge was significantly enhanced by alkali treatment.

For alkali treated sewage sludge, q_m and K_L values followed the order: Pb²⁺ > Cd²⁺ > Ni²⁺. This trend indicates the bonding affinity of alkali treated sewage sludge to heavy metals is in the order of Pb²⁺ > Cd²⁺ > Ni²⁺.

The alkali treatment can significantly enhance the heavy metal adsorption capacity of sewage sludge, and the

effect of NaOH concentration for alkali treatment on lead adsorption capacity was studied, as shown in Figure 3.

For all biosorbent prepared with different NaOH solution, the equilibrium data were well described with Langmuir model as shown in Table 3, which was consistent with previous study. The principal components in sewage sludge are polysaccharides and proteins. Alkali treatment (mainly NaOH) is an effective carbohydrate and protein extraction method. NaOH ionizes charged groups in proteins and polysaccharides [18]. The ionized functional groups of protein and polysaccharides, such as amino group, hydroxyl group, carboxyl group, complexes with heavy metals. Thus, comparing with sewage sludge before treatment, the q_m value of treated sewage sludge prepared by 0.125 mol/L NaOH solution increased by 0.7 mmol/g. However, when the NaOH concentration increased from 0.125 mol/L to 0.25 mol/L, the q_m of biosorbent decreased from 1.05 mmol/g to 0.84 mmol/g. This trend may be due to that the NaOH solution with higher concentration hydrolyzed and disintegrated part of protein and polysaccharides in treated sewage sludge [25]. Nevertheless, when the NaOH concentration continued to increase from 0.25 mol/L to 7.5 mol/L, the q_m values of treated sewage sludge showed increasing trend. This increasing trend indicates that the NaOH solution with higher concentration leads to the hydrolysis of bacteria cell of sewage sludge [26]. Then, more intracellular complexation sites expose to heavy metals.

The IR spectra obtained from sewage sludge prior to and after alkali treatment (Figure 4) were used to analysis the presence of main adsorption functional groups. For alkali treated sewage sludge prior to lead adsorption, the peak at 1657 cm⁻¹ was attributed to the stretching vibration of C=O and C-N groups. The peak at 1562 cm⁻¹ was due to the N-H bending vibration and C-N stretching vibration. These groups above were the characteristics spectra of protein. The peak at 1416 cm⁻¹ was assigned to the stretching vibration of C=O and deformation vibration of OH. The peak at 1014 cm⁻¹ was attributed to the stretching vibration of OH.

Comparing with the IR spectra of alkali treated sewage sludge prior to and after Pb²⁺ adsorption, the peak shapes were similar, and no new adsorption peak was observed. It indicated that the material structure did not change after Pb²⁺ adsorption. After Pb²⁺ adsorption, the peak at 1570 cm⁻¹ was moved for 20 cm⁻¹ toward infra-

red region, it was due to the complexation of Pb²⁺ and N-H group. In addition, the peak at 1408 cm⁻¹ was moved for 32 cm⁻¹ toward infrared region, it was due to Pb²⁺ binding with -COOH group. Thus, the main function groups works in the heavy metal adsorption process was N-H group and COOH group. These IR spectra data were similar to results of various activated samples obtained by Laurent et al. [12] [21].