posted on 2024-03-07, 04:04authored byLing Zhang, Min Xu, Lingzhen Li
The development of low-cost, efficient,
and environmentally
friendly
adsorbents is the key to highly toxic hexavalent chromium [Cr(VI)]
removal by adsorption. In this paper, amino-functionalized lotus stem
hydrochar (ALSHC) was prepared from an agricultural waste lotus stem
(LS) for the adsorption removal of Cr(VI) from water. The effects
of the initial Cr(VI) concentration, contact time, temperature, coexisting
anions, and reusability of ALSHC on Cr(VI) removal were examined in
detail. The adsorption mechanism was further discussed by investigating
the impact of the solution’s initial pH, the relation between
the pH change in solution and Cr(VI) removal during the process, the
changes of chromium (Cr) species in solution and on ALSHC during adsorption,
and the XPS characterization. The results demonstrated that ALSHC
effectively removed Cr(VI) from water with rapid adsorption (the removal
rate reached 80.90% in only 10 min) and in situ detoxification. Most
importantly, ALSHC still had better adsorption performance (adsorption
capacity of 30.95 mg g–1) than commercially activated
carbon, even at pH = 9.00. The adsorption of Cr(VI) by ALSHC accorded
with the pseudo-second-order kinetic model and Langmuir isotherm model,
indicating a monolayer chemisorption process. The adsorption process
was shown to be spontaneous and endothermic based on the thermodynamic
characteristics (ΔG0 < 0, ΔH0 > 0, and ΔS0 > 0). The mechanism of Cr(VI) removal was mainly composed
of three
parts in sequence: Firstly, Cr(VI) in solution was quickly adsorbed
onto ALSHC with protonated –NH2 through electrostatic
attraction; subsequently, the adsorbed Cr(VI) on ALSHC was mostly
detoxicated by in situ reduction; and finally, the reduced Cr(III)
and the remaining Cr(VI) were fixed on the ALSHC surface by complexation.
The prepared ALSHC displayed a certain superiority in Cr(VI) adsorption
and had the prospect of further development.