Enhanced adsorption of Cu(II) and Cd(II) by phosphoric acid-modified biochars

•H3PO4-modified biochars had more surface oxygen-containing functional groups and larger surface area.•Modified biochars sorbed Cu(II) and Cd(II) much more strongly than pristine biochars.•Surface area had significant effects on Cu(II) and Cd(II) adsorption by modified biochars.•Surface complexation was a major mechanism for metal sorption by modified biochars.

In this research, adsorption of Cu(II) and Cd(II) by biochars was investigated. To enhance the adsorption of these two metal ions, a simple modification of biochars by phosphoric acid (H3PO4) was carried out. The surface area was larger and the contents of oxygen-containing functional groups of modified biochars were more than pristine biochars. In comparison with pristine biochar, modified biochars sorbed Cu(II) and Cd(II) much more strongly. Surface area had significant effects on the sorption of Cu(II) and Cd(II) by modified biochars, it also resulted in the higher sorption for the pristine biochar at high pyrolysis temperature. X-ray photoelectron spectroscopy analyses indicated that the quantities of carboxyl (-COOH) and hydroxyl (-OH) functional groups of modified biochars were larger than those of pristine biochar at the same pyrolysis temperature. Compared with that of pristine biochars, the strong ability of -COOH and -OH of modified biochars to form complexes with Cu(II)/Cd(II) ions resulted in higher adsorption of these two metal ions. The phosphorus-containing groups of modified biochars, such as P=O and P=OOH from the result of Fourier transform infrared spectroscopy, interacted and also formed complexes with metal ions, possibly resulting in the enhanced adsorption of Cu(II) and Cd(II). Thus, sorption of metal ions by modified biochars was controlled by the mechanism of surface complexation between oxygen containing functional groups and metals. In general, the H3PO4 modification was an effective method to prepare biochars with a high affinity for the sorption of heavy metals.

Graphical abstractFor the H3PO4 modified biochars, the adsorption of Cu(II) and Cd(II) decreased with increasing pyrolysis temperatures at high concentration, and there was no difference at low concentration (Ce < 1 mg/L) (Fig. 1). The strong ability of modified biochars to form complexes with Cu(II)/Cd(II) resulted in higher adsorption due to the increased number of -OH and/or -COOH groups in modified biochars. The new formed phosphorus-containing groups, such as P=O and P=OOH, after H3PO4 modification also enhanced sorption of Cu(II)/Cd(II) because of more complexes being formed between the metals and the modified biochars. These results suggested that surface complexation between metals and oxygen-containing functional groups of H3PO4 modified biochars was a major adsorption mechanism in metal sorption process.Download high-res image (153KB)Download full-size image