A sustainable valorization of Kenaf stalk by maleic acid pretreatment towards lignin-based sunscreens preparation and papermaking

Lignin has a UV-blocking ability because it contains phenolic and ketone functional groups. However, the lignin produced in the paper industry undergoes extreme polycondensation, resulting in an extremely dark color, which impedes its value-added application in the field of UV-blocking cosmetics. In this study, maleic acid (MA) was used as a green and recyclable reagent to pretreat an agricultural waste lignocellulosic biomass, Kenaf stalk. Lignin underwent a carboxyl esterification reaction with MA to produce light-colored dissolved lignin (DL), which was further prepared into lignin nanospheres and used in lignin-based sunscreens. The results showed that when adding 5 wt% DL-lignin nanospheres (DL-LNPs) to a sunscreen with an actual measured sun protection factor (SPF) of 7.05, the SPF of the lignin-based sunscreen increased to 63.82 far exceeding the commercially available SPF 50 sunscreens. The prepared lignin-based sunscreen had good biocompatibility when applied to human skin and did not stain the skin. The remaining neutral solid after pretreatment was made into Kenaf fibers by PFI milling for papermaking. The ring crush strength index of the paper increased from 2.81 N m/g to 3.14 N m/g when 15 wt% Kenaf fibers was added to the softwood pulp. When the additions Kenaf fibers was 5 wt%, the tensile index and tearing index of the paper increased from 8.57 N m/g and 4.68 mN m 2 /g to 9.22 N m/g and 5.48 mN m 2 /g, respectively. Therefore, this study provides a new way for the value-added utilization of Kenaf stalk lignocellulosic biomass. Graphical abstract