Citric acid modified wood membranes for efficient adsorption of tetracycline: Effect of alkali pretreatment concentration and adsorption mechanism

Abstract

Abstract Innocuous citric acid (CA) modified wood membranes (WMs) with carboxylic groups (–COOHs) hold promise for application in water purification. In spite of this, it remains challenging to make –COOHs located deep inside wood cell walls due to the recalcitrant structure of the lignocellulosic materials. Herein, the WMs were modified via the pretreatment with various NaOH concentrations followed by esterification with CA (x-CAW) for tetracycline (TC) typical of antibiotics with amide carbonyl group (HN–C = O). As NaOH concentration increased from 2% to 6%, wood lignin decomposed and finally cellulosic structure began to dissolve. As a result, the H bonds of hydroxyl on C3 and C6 changing to C2 and C6 made cellulose I transform to cellulose II. It facilitates the esterification between cellulose and CA to enhance the –COOH content. The TC adsorption breakthrough curves showed that the x-CAW had an 8–12-fold enhancement of the effective treatable volume with high adsorption capacity compared with raw wood. The largest amount of TC was adsorbed at pH 5 through the formation of H–bonding between the Ocarboxyl–H group of x-CAW and the O = Camide group of zwitterionic TC. The study provides effective strategies to overcome critical limitations related with the application of WM and design a competitive membrane material for environmental remediation.