BACKGROUND: There is growing interest in employing sludge-based carbons (SC) from waste treatment as adsorbents. However, the application of SC is limited by its poor porosity caused by large amounts of minerals blocking the pores. It is thus, highly desirable to develop a post-washing process to remove the minerals in SC. RESULTS: In this paper, SC was washed with deionized water (DW), hydrochloric acid (HCl), citric acid (CA) and a mixed solution of hydrochloric acid and citric acid (HCl-CA), respectively. Results showed that HCl-CA washing had the highest demineralization efficiency (63%), so that HCl-CA washed SC had the lowest ash content (32%) and largest total pore volume (0.455 m(3) g(-1)), and presented the highest adsorption capacities of 319 mg g(-1) for acid orange II (AOII) and 250 mg g(-1) for methylene blue (MB). CONCLUSION: HCl-CA washing has the highest demineralization efficiency. The high efficiency is attributed to the synergetic effects of both protonation and chelation. Demineralization has a strong influence on the development of pore structures, which further influences adsorption capacity. (C) 2012 Society of Chemical Industry