Discard the dross and select the essence: Purified orange peel with coupled alkali-acid hydrolysis as potential Cd(II)-capturing adsorbent

Abstract

Turning agricultural wastes into green adsorbents is a compatible and cost-effective strategy for solid waste disposal and environmental pollution control. Herein, the discarded orange peel (OP) sample was purified via coupled alkali-acid hydrolysis and the obtained adsorbents were applied for eliminating toxic cadmium (Cd(II)) from the liquid phase. Based on the single factor experiments, the optimal Cd(II)-capturing adsorbent (i.e., OP-OH-H-25) was generated under the purification condition of soaking raw OP in 2.0 M NaOH solution at 25 °C followed by an acid hydrolysis treatment with 2.5 M HCl solution at 25 °C. Spectral analyses reflected the efficient elution of hemicellulose/pectin and lignin impurities, the transformation of cellulose type I to cellulose type II, and the exposure of sulfhydryl functional groups. The kinetics removal process of Cd(II) by OP-OH-H-25 reached equilibrium after 0.5 h with a removal efficiency of ∼88%. The maximum Cd(II) sorption capacity of OP-OH-H-25 (i.e., 124.90 mg/g) was higher than a series of previously reported adsorbents. The sorption behavior of Cd(II) onto the prepared OP-OH-H-25 material was mostly interfered with by the coexisting Mg2+, Ca2+, and heavy metals. The desorption data and spectroscopic analysis jointly pointed to the inner-sphere binding of Cd(II) with OP-OH-H-25 functional groups. Overall, the extracted OP-OH-H-25 sample could act as a promising decontaminant for the remediation of Cd(II)-polluted water environment.