Quantitative relationships between the adsorptivity of carbonaceous materials in soil for Pb(II) and soil organic matter content

Strong adsorptivity of manufactured carbonaceous materials (MCMs) mediates the behavior of heavy metals in soil. Laboratory-reported adsorptivity of MCMs often deviates from their actual abilities in soil, because soil organic matter (SOM) can change the adsorptive abilities of MCMs by coating dissolved organic matter (DOM) on the surface of the MCMs. It was considered that the influence of SOM on the adsorptivity of MCMs in soil follows a sequential pathway of SOM releasing DOM in soil solution and subsequent DOM binding onto MCMs, thereby altering MCM surface acidity and hence changing MCM adsorptivity for heavy metals. In this study, we first extracted DOM from ten topsoils collected over a broad region of China with a wide range of SOM. The DOM solutions were then used to load DOM onto four MCMs including activated carbon (AC), multiwalled carbon nanotube (MWCNT), and two biochars (BC400 and BC700), respectively, obtaining a total of 44 MCM-DOM complex samples with known amounts of bound DOM. These MCM-DOM complex samples were then determined for their surface acidities and adsorptive abilities for Pb(II). We found that there were significant correlations between DOM concentration and SOM content, between DOM binding onto MCMs and DOM concentration, between surface acidity of MCM-DOM complexes and DOM binding onto MCMs, as well as between Pb(II) adsorption on MCM-DOM complexes and surface acidity of MCM-DOM complexes. With understanding of these individual linear correlations, linear relationships between the Pb(II) adsorption and SOM content were established by combining individual correlations and by directly plotting the former against the latter. These relationships may be used to accurately predict the adsorptive abilities of MCMs for heavy metals in soils via simply determining SOM.