Amination of biochar surface from watermelon peel for toxic chromium removal enhancement

Abstract

Watermelon peel residues were used to produce a new biochar by dehydration method. The new biochar has undergone two methods of chemical modification and the effect of this chemical modification on its ability to adsorb Cr(VI) ions from aqueous solution has been investigated. Three biochars, Melon-B, Melon-BO-NH2 and Melon-BO-TETA, were made from watermelon peel via dehydration with 50% sulfuric acid to give Melon-B followed by oxidation with ozone and amination using ammonium hydroxide to give Melon-BO-NH2 or Triethylenetetramine (TETA) to give Melon-BO-TETA. The prepared biochars were characterized by BET, BJH, SEM, FT-IR, TGA, DSC and EDAX analyses. The highest removal percentage of Cr(VI) ions was 69% for Melon-B, 98% for Melon-BO-NH2 and 99% for Melon-BO-TETA biochars of 100 mg·L−1 Cr(VI) ions initial concentration and 1.0 g·L−1 adsorbents dose. The unmodified biochar (Melon-B) and modified biochars (Melon-BO-NH2 and Melon-BO-TETA) had maximum adsorption capacities (Qm) of 72.46, 123.46, and 333.33 mg·g−1, respectively. The amination of biochar reduced the pore size of modified biochar, whereas the surface area was enhanced. The obtained data of isotherm models were tested using different error function equations. The Freundlich, Tempkin and Langmuir isotherm models were best fitted to the experimental data of Melon-B, Melon-BO-NH2 and Melon-BO-TETA, respectively. The adsorption rate was primarily controlled by pseudo-second–order rate model. Conclusively, the functional groups interactions are important for adsorption mechanisms and expected to control the adsorption process. The adsorption for the Melon-B, Melon-BO-NH2 and Melon-BO-TETA could be explained for acid–base interaction and hydrogen bonding interaction.