Removal of natural organic matter from potential drinking water sources by combined coagulation and adsorption using carbon nanomaterials

Abstract

In recent years, carbon nanomaterials have become promising adsorbents for water treatment. In this study, combined coagulation–adsorption treatment using single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) was employed for the removal of natural organic matter (NOM) from various water sources; powdered activated carbon (PAC) was used for comparison. Synthetic seawater and brackish water were produced, with humic acid as the source of NOM. Natural surface water was obtained from Broad River in Columbia, South Carolina. Synthetic landfill leachates were created to reflect the typical characteristics of leachate produced in young and old landfills. Glucose and humic acid were used as the NOM sources for the young and old leachates, respectively. Adsorption isotherm experiments were conducted to determine the adsorption capacity of each carbon nanomaterial, and jar testing was conducted to simulate the combined coagulation–adsorption treatment processes. The adsorption capacity of SWCNTs was greater than that of MWCNTs for the source waters. The adsorption capacity of PAC (log Kf=2.28–2.82) was generally higher than that of SWCNTs (log Kf=1.98–2.27) and MWCNTs (log Kf=1.08–1.16) in the case of source waters with low NOM content (i.e., seawater, brackish water), but was lower than those of SWCNTs and MWCNTs in the case of natural water and synthetic landfill leachates. NOM removal using the combined coagulation–adsorption process was more effective than the sole use of the adsorbent, regardless of the type of adsorbent. However, combined coagulation–adsorption treatment using SWCNTs, except in the case of the young leachate, resulted in the highest NOM removal (>90%) among the various combinations. PAC often outperformed MWCNTs in these combined treatment processes, except in the case of the young leachate. While MWCNTs have often been viewed as less-effective adsorbents, they were the most effective nanomaterial for removing NOM from the young leachate in the combined process (35.1% with aluminum sulfate; 17.4% with ferric chloride). When employing the combined coagulation–adsorption process, increases in adsorbent doses had little effect on the overall NOM adsorption. The results of this study show that combined coagulation–adsorption treatment using carbon nanomaterials can be effective in removing NOM from various water sources.