The large water consumption (1.9-5.9 m(3) water per m(3) of biofuel) required by biomass processing plants has become an emerging concern, which is particularly critical in arid/semiarid regions. Seawater, as a widely available water source, could be an interesting option. This work was to study the technical feasibility of using seawater to replace freshwater in the pretreatment of date palm leaflets, a lignocellulosic biomass from arid regions, for bioethanol production. It was shown that leaflets pretreated with seawater exhibited lower cellulose crystallinity than those pretreated with freshwater. Pretreatment with seawater produced comparably digestible and fermentable solids to those obtained with freshwater. Moreover, no significant difference of inhibition to Saccharomyces cerevisiae was observed between liquids from pretreatment with seawater and freshwater. The results showed that seawater could be a promising alternative to freshwater for lignocellulose biorefineries in coastal and/or arid/semiarid areas.

Date palm residues are one of the most promising lignocellulosic biomass for bioethanol production in the Middle East. In this study, leaflets and rachis were subjected to hydrothermal pretreatment to overcome the recalcitrance of the biomass for enzymatic conversion. Evident morphological, structural, and chemical changes were observed by scanning electron microscopy, X-ray diffraction, and infrared spectroscopy after pretreatment. High glucan (>90% for both leaflets and rachis) and xylan (>75% for leaflets and >79% for rachis) recovery were achieved. Under the optimal condition of hydrothermal pretreatment (210°C/10 min) highly digestible (glucan convertibility, 100% to leaflets, 78% to rachis) and fermentable (ethanol yield, 96% to leaflets, 80% to rachis) solid fractions were obtained. Fermentability test of the liquid fractions proved that no considerable inhibitors to Saccharomyces cerevisiae were produced in hydrothermal pretreatment. Given the high sugar recovery, enzymatic digestibility, and ethanol yield, production of bioethanol by hydrothermal pretreatment could be a promising way of valorization of date palm residues in this region.

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Deep eutectic solvents (DESs), featured as promising green solvents, have increasingly drawn attention in the processing of a wide spectrum of lignocellulosic biomass (e.g., corn stover, wheat straw, and rice straw). However, the ineffectiveness of enhancing enzymatic digestibility of lignocellulosic date palm residues by pretreatment using DESs (choline chloride/glycerol) only was observed in this study. Inspired by the different observation with counterparts, it was hypothesized that the recalcitrance of date palm residues caused the ineffectiveness. A novel approach of reducing recalcitrance of biomass through hydrothermal pretreatment prior to DESs treatment was proposed to revive the effectiveness of DESs on enhancing cellulose digestibility. The results showed that hydrothermal treatment effectively triggered the functioning of choline chloride/glycerol to boost enzymatic digestibility (1.7 times increase) of date palm residues. In addition, this study indicated that the significant increases of both xylan (25%) and lignin (22%) removals rather than the modification of cellulose crystallinity by choline chloride/glycerol were responsible for the enhancement of enzymatic digestibility.

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