The hydroxyethyl cellulose derived from biomass is made by preparing a biomass by extracting resins and waxes, acid leaching, and alkali treatment to obtain a cellulose-rich residue, and then bleaching the cellulose-rich residue, followed by hydroxyethylation of the cellulose to obtain hydroxyethyl cellulose. The use of hydroxyethyl cellulose in brine treatment include reacting concentrated brine and hydroxyethyl cellulose with an effluent gas stream to capture CO2 and reduce salinity of the concentrated brine. The reduced salinity brine may then be useful for enhanced oil recovery methods.