Adsorption of lead ions by green waste compost and its mechanism

Purpose: In this study, green waste compost (GWC) and peat were selected as adsorbents for the adsorption of lead ions from aqueous solution, and the governing mechanisms were elucidated. Materials and methods: The effect of the experimental factors on adsorption performance, including pH value, dosage, and initial metal concentration, was determined by means of batch experiments. The GWC and peat were extensively characterized using SEM images, estimating Brunauer–Emmet–Teller (BET) surface areas and Barrett–Joyner–Halenda (BJH) pores size distribution, and performing elemental analysis by using Fourier transform infrared spectroscopy (FTIR) and X-ray diffractometer (XRD). The adsorption isotherm models and kinetics equation have also been applied to fit the data results. Results and discussion: SEM showed uneven surface and distributed pores, while the BET results also presented the mesoporous structure with larger pores diameter on both adsorbents. FTIR and XRD analysis showed that carboxyl, hydroxyl, and acylamino groups are the main functional groups involved in lead ions binding, and minerals of PbAsO were detected. When the initial pH value exceeded 3.0, the removal rate remained above 90%, while the removal rate increase with high pH became insignificant for both adsorbents. The maximum lead ions removal rate was found at 4 g/L dosage, corresponding to 96.34 and 98.21%, respectively. The kinetics adsorption was conformed to the pseudo-second-order equation, showing that the process is controlled by chemical adsorption for two adsorbents. The adsorption of lead ions showed good degree of fitting with Langmuir and Freundlich adsorption isotherm equations, even though the fitting of GWC adsorption data with Langmuir is overall better, suggesting the presence of both heterogeneous and monolayer adsorption of molecules. Conclusion: In conclusion, research findings concluded that the GWC is a promising biosorbent for removing lead ions from the aqueous solution. Graphical abstract: [Figure not available: see fulltext.].