Harnessing solar light to photodegrade azo dyes from wastewater by metal free biocompatible amino acid functionalized graphitic carbon nitride and statistical modelling

Abstract

Azo dyes are notable for their carcinogenic nature and are highly resistant to self degradation. Consequently, to address dye wastewater issues, a novel solar light responsive metal free amino acid functionalized graphitic carbon nitride photocatalyst (CNM) has been synthesized hydrothermally. Graphitic carbon nitride mostly doped with metal and metal oxides to enhance its photocatalytic activity, which may introduce toxicity, or these materials may degrade into harmful by-products. On the contrary, amino acid functionalization maintains a green and safe profile. Amino acids are biodegradable and non-toxic, making this method ideal for photocatalytic application. After functionalization due to extended conjugation the band gap reduces from 2.7 eV in CN to 2.62 eV in CNM which in turn effectively inhibits charge carrier recombination. Experimental batch studies demonstrated that CNM effectively removes 20 mg/L (96.6 %) and 15 mg/L (93.8 %) of Congo Red (CR) and Allura Red (AR) dyes from an aqueous solution within 25 and 35 min respectively, converting them into harmless products giving TOC (Total organic carbon) of 65.6 % and 58.5 %. Data obtained from kinetic studies fits well in pseudo-first-order rate equation. Bulk CN showed only 48 % and 43 % removal efficiency for CR and AR dyes under identical optimal conditions. Statistical analysis of dye degradation, using ANOVA with the Box-Behnken design model, revealed parameters that closely match those observed under the batch experimental studies. Studies involving radical scavenging have shown that holes (h⁺) and hydroxyl radicals (OH^•) are the major species responsible for degradation.