Biochars were produced by pyrolyzing residue of a traditional Chinese medicine, Danshen, at different temperatures (250-.800 degrees C) and used as adsorbents for sulfamethoxazole (SMX). The biochar prepared at 250 degrees C (B250) exhibited 2-7 times higher sorption capacity to SMX than those prepared at higher temperatures. The high sorption capacity of 8250 was mainly accounted by the negative charge-assisted H-bond [(-)CAHB] between SMX and B250 due to its abundant O-containing groups and comparable pK(a) to that of SMX (pK(a,2) =5.7). The thermodynamic analysis suggested that SMX sorption on the biochars was a spontaneous and endothermic process. The higher increase of enthalpy and entropy for low-temperature chars were well correlated with the strong (-)CAHB between SMX and oxyl groups on the chars. This study indicates that (-)CAHB could be more easily formed on low-temperature chars which are rich in oxyl groups, and provides valuable information on design and synthesis of efficient biochars for various environmental applications. (c) 2014 Elsevier B.V. All rights reserved.