Combination of alkalinity and porosity enhances formaldehyde adsorption on pig manure -derived composite adsorbents

Carbonaceous porous adsorbents of a high content of an inorganic phase (40–80%)were prepared by the zinc chloride activation of waste bio-char from the liquefication of pig manure at 900 °C. To increase both the content of the carbon phase and conductivity 10% nanographite was added to bio-char before the carbonization/activation process. Even though no strong effect of a char/activation agent ratio on the materials' porosity was found, they slightly differ in the surface areas and in the contents of the inorganic phase, which was found to be rich in calcium and magnesium. The synthesized materials were used as formaldehyde adsorbents from its low concentration of ∼2 ppm. The measured breakthrough capacity reached 0.78 mg/g and was higher than those measured on commercial carbons of high porosities. Since upon exposure to formaldehyde a marked surface acidification was found, the good performance of the composite adsorbents was linked not only to the porosity of the carbon phase but also to the effect of an alkaline environment in mesopores and to that of transition metals oxides on formaldehyde oxidation to formic acid. That acid likely formed salts with alkaline and alkaline earth metals present in our adsorbents. There was an indication that nanographite increased the extent of the formaldehyde oxidation reaction, which was demonstrated in a marked decrease in the surface pH of the spent adsorbent with the moderate amount of formaldehyde adsorbed.