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This paper aimed at determining the quantity, nature and role of extracts during the sorption behavior of two Central African hardwood species: Nauclea diderrichii (Bilinga) and Erythrophleum suaveolens (Tali) at 30, 40, 50, 60 and 70°C using relative humidities (RH) between 5.3% and 97%. For this purpose, extract contents and their nature were determined by solvent extraction and mass spectrometric analysis (MALDI-TOF/MS) respectively. The ABC model whose parameters were used to determine the monolayer moisture capacity (EMCm) also described type II sigmoidal sorption isotherms. The results showed that acetone extracts were the highest in Tali (7.88%) and Bilinga (6.92%). MALDI-TOF results found 10 new chemical components out of 24 in Tali (saponins, flavonoids, glycosides, alkaloids and stilbenes) while 16 were identified as saponins, acids and flavonoids in Bilinga. Acetone-soluble extracts had the most significant effect on the hygroscopic nature of wood. Extracted wood recorded higher equilibrium moisture contents (EMC), monolayer moisture capacities and lower sorption differences. For native wood, water-repellent polyphenols like alkaloids and stilbenes and newly identified components there in, rendered Tali less hygroscopic than Bilinga. The nature of extractives would be an effective alternative to explore the chemical compounds, which can best improve the hygroscopic response of wood. [ABSTRACT FROM AUTHOR]

Volcanic soils cover 1% of the Earth's surface but support 10% of the world's population. They are among the most fertile soils in the world, due to their excellent physical properties and richness in available nutrients. The major limiting factor for plant growth in volcanic soils is phosphate fixation, which is mainly attributable to active species of aluminium and iron. The sorption and desorption of phosphate is studied on the surface horizons of two African agricultural soils, a silandic Andosol (Rwanda) and a vitric Andosol (São Tomé and Principe). Both soils are slightly acid. The silandic Andosol is rich in active aluminium forms, while the vitric Andosol has high amounts of crystalline iron and aluminium oxides. Sorption isotherms were determined by equilibrating at 293K soil samples with phosphate solutions of concentrations between 0 and 100 mg P L-1 in NaNO3; phosphate was determined by visible spectrophotometry in the equilibrium solution. To study desorption, the soil samples from the sorption experiment were equilibrated with 0.02 M NaNO3. The isotherms were adjusted to mathematical models. In almost all the concentration range, the adsorption of phosphate by the silandic Andosol was greater than 90% of the amount added, being lower in the vitric Andosol but always higher than 65%. The high sorption by the silandic Andosol is attributed to its richness in non-crystalline Fe and Al, while in the vitric Andosol crystalline iron species seem to play a relevant role in the adsorption. The sorption isotherms of both soils fitted to the Temkin model, the adjustment to the Langmuir or Freundlich models being unsatisfactory; throughout the range studied, the sorption increases with increasing phosphorus concentration, a maximum sorption is not predictable (as occurs when the sorption is adjusted to the Langmuir model). For an added P concentration of 100 mg L-1 (3.2 mmol L-1), the sorption is 47.7 µmol P g-1 in the silandic Andosol and 41.6 mmol P g-1 in the vitric Andosol. The desorption is low and the comparison of the sorption and desorption isotherms reveals a pronounced hysteresis, that is, the irreversibility of the sorption. The high phosphate sorption and its irreversibility are comparable to those published for other volcanic soils with high contents of allophane, active aluminium and free iron. The strong phosphate adsorption is a serious limiting factor for plant growth, which requires a careful management of phosphorus fertilization. [ABSTRACT FROM AUTHOR]