Metal oxide-chitosan based nanocomposites for efficient degradation of carcinogenic PAHs

Abstract

Polycyclic-aromatic-hydrocarbons (PAHs) have been categorized as priority-pollutants due to their mutagenic, persistence and proven carcinogenicity. Owing to omnipresence and toxicity of PAHs and derivatives, their complete removal is highly-imperative. Therefore, iron-oxide based chitosan-nanocomposites mediated photo-oxidative degradation of toxic prototype PAHs such as anthracene (ANTH) and phenanthrene (PHEN) was explored. Such nanocomposites of distinct morphologies were synthesized via green route employing Azadirachta-indica as natural surfactant in aqueous medium. Synthesized materials showed excellent photocatalytic efficiency in exponential decay of ANTH and PHEN with time (12 h) under direct sunlight. At neutral pH, photodegradation of PAHs (2 mgL−1) by nanocomposites (20 mg) followed Ist order kinetics and Langmuir model (R2: 0.99; p value<0.05). Highest degradation observed for ZnFe2O4-CS (ANTH: 95 % ; PHE: 92 %) followed by CuO-Fe2O3-CS (93 %, 90 %), NiFe2O4-CS (90 %, 88 %), Co2O3-Fe3O4-CS (88 %, 85 %) and FeCr2O4-CS (83 %, 81 %), respectively was in accordance to zeta-potential charge and BET surface area values and involving vital role of “cation- π” interaction amongst nanocomposites and PAHs. Lower elimination of PHEN (92 %) than ANTH (95 %) might be due to its structural non-planarity and higher stability. Smaller and relatively safer by-products like (Z)-3-hydroxyacrylic acid, (Z)-4-oxobut-2-enoic acid and (1E,3E)-penta-1,3-dien-1-ol,prop-2-en-1-ol, propionic acid and acrylic acid were identified by GC–MS analysis. Minimum removal of ANTH (∼50 %) and PHEN (∼40 %) obtained in dark revealed the extent of adsorption of nanocomposites. Effect of various radical quencher’s revealed the role of OH, h+ and ˙O2- active species in the oxidation of selected PAHs. Overall, present study provides an alternative insight on preparing effective, green, reusable (n = 10), stable nanocomposites for the removal of carcinogenic PAHs.