Carbon-coated sepiolite clay fibers with acid pre-treatment as low-cost organic adsorbents

Abstract

Carbon-nanoparticle-coated sepiolite hydrous-magnesium-silicate clay fibers are prepared by 220°C-autoclave hydrothermal carbonization of cellulose using 1.2-mol/L-hydrochloric-acid-pretreated sepiolite fibers. The deposited amorphous carbon nanoparticles (30–150 nm) nonuniformly and partially covering the sepiolite surface have surface functional groups (CH2, CH3, CC and CO), which enhance the organophilicity. Compared to a mixture of pristine sepiolite and hydrothermal carbon prepared without sepiolite, the carbon-coated sepiolite exhibits lower specific surface area but greater organic adsorption, due to the smaller pore size and particle size of the hydrothermal carbon coating compared to the hydrothermal carbon prepared without sepiolite. The acid pre-treatment decreases the sepiolite crystallinity and increases the deposited carbon content from 24.67 to 31.63 vol%, while increasing the specific surface area from 26.53 to 33.98 m2/g, increasing the fraction of phenol removed from 47.1% to 57.1%, and increasing the monolayer phenol adsorption capacity from 4.1 to 5.26 mg/g. Compared to the acid treated but uncoated sepiolite, the carbon coating decreases the density from 2.227 to 1.696 g/cm3 and decreases the specific surface area from 118 to 34 m2/g, but increases the fraction of phenol removed from 6.1% to 57.1%. The acid pre-treatment gives greater adsorption than the addition of acid to the hydrothermal process liquid.