Release of dissolved organic carbon from biochar and formation of humic-like component during photoreaction: Effects of Ca2+ and pH.

The photochemical reactivity of dissolved organic carbon from biochar (DBC) was higher than dissolved organic matter (DOM), but the photo-transformation of DBC in the presence of DOM under various conditions are poorly understood. Here, we studied the effects of Ca2+ and pH on the photo-induced changes in the optical and structural properties of DOM in source water with biochar. During DBC photobleaching, the DBC released from bulk DBC while the humic-like component formed. The release of DBC and the formation of humic-like component were inhibited by the presence of Ca2+ attributed to the inhibition of triplet excited state of DOM (3DOM*) and singlet oxygen (1O2) generation. Moreover, the 3DOM* yield increased while the 1O2 generation decreased as pH decreased from neutral, resulting in the increased formation of humic-like component and decreased release of DBC. The characterizations of ultrafiltration-isolated colloidal DOM after irradiation showed that hydrophilicity and the colloidal size of released DBC decreased in the presence of Ca2+. Additionally, the colloidal size of released DBC decreased while the hydrophilicity of DBC enhanced with increasing pH from neutral. This study not only gives insight into the DBC photo-transformation in the presence of DOM under various conditions but also reveals the influence of DBC on the variation of DOM properties during irradiation.