I investigated pretreatment of rice straw at different concentrations of NaOH i.e 2%, 3% and 4% etc. The highest lignin removal was found with 2% NaOH. 4% NaOH gave less lignin removal than 2%. I think higher concentrations of alkali should remove more lignin than alkali with low concentration. Pretreament time and temperature was same for all concentrations.
An explanation might be the analytics of the lignin content after pretreatment.
Did you perform an acid hydrolysis and determine the lignin gravimetrically as solid residue (e.g. Klason-Lignin)? If yes there might be the following problem: If cellulose or hemicellulose components are altered during your pretreatment at high alkaline conditions, they might not go complete in solution during the lignin analysis via acid hydrolysis. This would result in an overestimation of lignin content at high NaOH loadings.
I have used Chesson's method for the determination of lignin, cellulose and hemicellulose.
Did you determined lignin content in the solid residue after the alkali pretreatment? If not, and you just determine the content of cellulose of the pretreated residue, it may be because of the reaction conditions (temperature, residence time, ...). If high concentrations are paired up with high temperature/reaction time it could work, not as a pretreatment, but as a alkaline hydrolysis. That would explain why the cellulose content is lower.
The below mentioned researcher has many articles on rice straw pulping, alkali treatment, and lignin removal. Happy new year.
https://www.researchgate.net/profile/Yehia_Fahmy
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