Valorization of mango peel waste into phosphoric acid treated activated carbon for efficient removal of methyl orange from aqueous solution

Industrial wastewater contaminated with dyes pose a serious problem to the environment. So, it should be treated for dye removal before its release into the environment. Adsorption technique is a preferable method because of its efficiency and low-cost. The method becomes more attractive when various low-cost materials are used as adsorbents. Due to presence of various negatively charged functional groups on their surfaces, these materials are more specific towards cationic/positively charged pollutants. After suitable modification of the material, negatively charged pollutants can also be removed. Methyl orange (MO) is an anionic/negatively charged dye commonly found in most of the industrial wastewater. Mango peel (MP) is a significant waste produced from food processing industries. So the main objective of the present research is to remove MO from aqueous solution using MP as the adsorbent through adsorption technique. However, MO uptake on unmodified MP was only 22% and MP modified with phosphoric acid followed by carbonization (PMP-AC) showed 97.21% removal from a solution containing 50 mg/L MO. The MO adsorption on PMP-AC is optimized (contact time = 60 min and pH = 2.0). Both Langmuir and Freundlich isotherms are obeyed well with Langmuir monolayer uptake capacity of 23.2 mg/g. The modified material PMP-AC is properly characterized. P-O/P-O-C stretching from phosphate group bonded to C surface, aromatic C = C stretching, and aliphatic C-H bond are the functional groups present on the material surface. The interaction between PMP-AC and MO take place mainly through pore filling, electrostatic attraction, π-π interaction and hydrogen bonding. MO could be desorbed from the material using a 0.1 N NaOH solution, allowing reuse for up to two additional cycles. As per the observed outcomes, use of PMP-AC for MO removal is a successful, low-cost and eco-friendly process.