Optimization of the castor fruit biomass chemical delignification by implementing the response surface methodology

Biomass delignification is a critical preliminary process to remove lignin from lignocellulosic residues for a further valorization of lignin and/or usage of cellulose. In this context, among the vast existing sources of lignocellulosic materials, castor plants and their fruits have been scarcely studied. Hence, this work is devoted to identifying by XRD the parts of castor fruit, i.e., endosperm, seed coat, pericarp and pericarp's needles, with the lowest number/amount of (in)organic crystalline phases, and then to carrying out a chemical delignification process. The XRD results indicate that the pericarp displayed only the distinctive reflections of native cellulose, while the other fruit's sections contain a mixture of amorphous xylan, calcite, dolomite and/or kaolinite. The pericarp was subjected to an alkaline chemical delignification assessing the concomitant influence of three key reaction variables (temperature, alkali concentration and time) on the delignification extent by applying sequentially a 2^k full factorial and a faced-centered central composite design. The design of experiments results showed that only four sources of variation are statistically significant, namely, the linear effect of temperature, alkali concentration and time, and the quadratic effect of alkali concentration with the binary interactions being non-significant. The response surface plots display a rising ridge-type topology implying that the maximum pericarp delignification requires operating at the reaction largest severity; in fact, numerically this value amounts to ca. 64 % at 70 °C, 3.7 wt/vol. of NaOH in the alkali solution, and 10 min.