Deciphering and optimizing the process of removal and recovery of Pb(II) and Cd(II) from wastewater by biosorbent derived from fruit waste

Abstract

Lead (Pb) and cadmium (Cd) are the two most hazardous heavy metals in consideration of their prevalence in industries and high toxicity to living organisms. Biosorption by using abundant fruit waste is an environmentally benign and cost-effective approach for heavy metal removal from wastewater. In this study, the rind of Citrullus lanatus (watermelon rind, shorten as WR) and the seed of Mangifera indica (mango seed, shorten as MS) were employed for Pb(II) and Cd(II) biosorption in aqueous solutions. For comprehensive mechanistic elucidation, which is crucial for process control and commercial (bio)sorbent development, a hybrid methodology of surface and structure analysis, spectroscopic techniques, chemical methods and quantum chemistry simulation (QCS) was established in combination with macroscopic biosorption studies. In batch studies, the maximum biosorption capacities of 231.57 ± 1.09 mg-Pb/g-WR at pH 5.0 and 98.51 ± 1.15 mg-Cd/g-WR at pH 7.0 were achieved by WR. The Redlich-Peterson (R-P) and Dubinin-Radushkevich (D-R) isotherms provided the best description to the equilibrium data of Pb(II) and Cd(II), respectively, suggesting the heterogeneous surface of WR. Besides, Pb(II) and Cd(II) were rapidly sequestered by WR, with the kinetic processes well described by the pseudo-first-order and pseudo-second-order equations, respectively. For MS, the maximum biosorption capacities were determined to be 263.63 ± 0.06 mg-Pb/g-MS at pH 5.0 and 93.53 ± 1.43 mg-Cd/g-MS at pH 7.5. The R-P isotherm provided the best description to Pb(II) and Cd(II) biosorption by MS, implying multilayer loadings of Pb(II) and Cd(II) on the heterogeneous surface of MS. Rapid biosorption of Pb(II) and Cd(II) was observed in the kinetic studies with the pseudo-first-order and pseudo-second-order equations as the best fitting models, respectively. The sequestered Pb(II) and Cd(II) were easily eluted from MS by dilute acids including HNO3 and citric acid with a good biosorption capacity remained in the regenerated MS. Unlike WR, the biosorption capacity of MS dramatically decreased as its particle size increased. In the continuous mode, the WR-packed bed column reactor maintained superior performance of Pb(II) biosorption over 10 cycles with a breakthrough time ranging from 8.3 h to 13.0 h, and 95% of the sequestered Pb(II) was desorbed in 1.3-2.3 h by 0.05 M HCl. To the best of my knowledge, advanced solid-state nuclear magnetic resonance (NMR) spectroscopy was employed for the first time to investigate the interactions between heavy metals and biosorbents in this study. Coupling with conventional spectroscopic techniques such as Fourier-transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS), carboxyl, hydroxyl, amine and ether groups from cellulose, pectin and hemicellulose in WR, and cellulose, hemicellulose and tannins in MS, were distinguished as the binding sites for Pb(II) and Cd(II) sequestration. Together with analytical methods such as zeta potential measurement, scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM-EDX), powder X-ray diffraction (PXRD) and ion exchange experiments, the underlying mechanisms were identified as complexation, electrostatic attraction, ion exchange (with Ca²⁺ and Mg²⁺) and microprecipitation. The QCS at a molecular level verified the feasibility of heavy metal complexation with the identified binding sites and indicated the preference of heavy metal binding to carboxyl groups. The mechanistic studies reveal that pectin, hemicellulose and amorphous cellulose are promising matrices for developing commercial (bio)sorbents for heavy metal sequestration. Compared with other fruit-waste-derived biosorbents in the literature, WR and MS in this study exhibited much higher Pb(II) biosorption capacities and comparable Cd(II) biosorption capacities. In particular, the high selectivity of WR towards Pb(II), the stability of the biosorption process, and the excellent column performance reveal the great practicability of WR in scale-up treatment of Pb(II)-bearing wastewater. Moreover, the hybrid mechanistic methodology established in this study can comprehensively decipher biosorption processes at both macroscopic and microscopic scales. It also provides important insights into other environmental studies, as most environmental processes are related to interactions between adsorbates and biological surfaces with varying degrees.