Fast activation of natural biomasses by microwave heating

• Microwave is effectively employed for activating pine wood and Arabica coffee residues.
• Microwave-heated pine wood has a better adsorption of food dyes.
• ɛ′, ɛ″ and tangent loss (tan δ) showed that Arabica coffee is a better microwave absorber.
• Microwave heating produced active surfaces in a short time.

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.

Effect of microwave input power (200–1000 W) on dyes adsorption by pine wood (heating time 8.0 min).Figure optionsDownload as PowerPoint slide