Humic acid gel drying with supercritical carbon dioxide

Humic acids (HAs) are brown, multifunctional biopolymers found in animals, plants, soils and sediments. HA solutions in aqueous NaOH deposit brown gels on addition of strong acids. We have dried aqueous HA gels isolated from two pears and two soils from different locations by vacuum oven, freeze drying and by supercritical fluid CO2 (SF) drying after replacing the gel water with acetonitrile. SF-dried HA aerogels from the pears, soils and the live alga Pilayella littoralis have much higher surface areas (36-188 m(2)/g) and lower bulk densities (72-160 kg/m(3)) than solids obtained from the same gel by vacuum oven and freeze drying. Gels redeposited from freeze-dried HA samples give products with similar surface areas as freeze-dried HAs after SF gel drying. Scanning electron micrographs of solid HAs from vacuum oven, freeze and SF gel drying show distinct morphology differences at the micron level. (C) 1998 Elsevier Science B.V. All rights reserved.