Fast activation of natural biomasses by microwave heating

Abstract Microwave was effectively applied as a fast heating tool for activating two natural biomasses, pine wood and Arabica coffee residues. The heated biomasses were tested for removing four food dyes: Allura Red E129, Sunset Yellow E110, Tartrazine E102, and Brilliant Black E151 from solution. The biomasses were heated by 2.45 GHz microwaves at different input powers 200–1000 W for 8 min. The results indicated that heating at high input powers is necessary to activate biomasses and to enhance dyes adsorption. Dielectrical properties (dielectric constant ɛ ′, dielectric loss ɛ ″, tangent loss tan  δ , and penetration depth dp ) indicated that Arabica coffee is a better microwave absorber leading to significant structural damage when heating at high input powers. Microwave-heated pine wood showed a better performance for dyes removal compare to coffee residue. Heating at 1000 W is not recommended for both biomasses where oxidation reactions destroyed the structure of the materials. Moreover, microwave heating for long times had a bad influence on dyes removal. %Removal–adsorption capacity plot indicated that 0.8 and 1.0 g/100 mL is the optimum adsorbent dosage for E110 and E129, respectively. Analysis by Langmuir equation revealed that saturation values were 9.3 and 7.2 mg/g for E110 and E129 at pH 2.0 and 25 °C, receptively. Dyes removal by microwave-heated wood was a fast process and more than 90% of adsorbent's capacity was utilised within 30 min.