Tailoring of spherical nanocellulose via esterification with methionine followed by protonation to generate two different adsorbents for mercuric ions and Congo red.

Herein, we report two different adsorbents from spherical nanocellulose (SNC) in successive steps, for the adsorption of Hg ²⁺ ions and Congo red (CR). Cellulose extracted from pine needles was subsequently converted to SNC through mixed acidic hydrolysis. As-obtained SNC was esterified with methionine at C6 of the anhydroglucose unit to produce SNC-methionine ester (SNC-ME). The amino group of methionine residue in SNC-ME was protonated to SNC-PME with positive surface charge. The SNC-ME and SNC-PME were evaluated as Hg ²⁺ ions and CR adsorbents, respectively. The SNC, SNC-ME, SNC-PME, Hg ²⁺ -loaded SNC-ME, and CR-loaded SNC-PME were characterized by FTIR, XRD, XPS, Zeta potential, BET, FESEM, EDS, and surface charge analysis. SNC-ME showed Hg ²⁺ ions removal efficiency of 94.8 ± 1.9 % in 40 min, while SNC-PME showed CR removal efficiency of 96.1 ± 3.8 % in 90 min. The adsorption data of both the adsorbents fitted best into pseudo-second order kinetic and Langmuir isotherm. The maximum adsorption capacity of SNC-ME for Hg ²⁺ ions was 211.5 ± 3.1 mg/g and that of SNC-PME for CR was 281.1 ± 7.1 mg/g. The astounding recyclability of the adsorbents for ten repeat cycles with significant cumulative adsorption capacity of 760.9 ± 12.8 mg/g for Hg ²⁺ ions and 758.8 ± 12.7 mg/g for CR endorses their spectacular potentiality for wastewater treatment.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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