EFFECT OF METALS ON THE DYNAMICS OF WATER AT THE BIOCHAR SOLID-LIQUID INTERFACE

Abstract

Previous studies revealed that water molecules are hooked to biochar surfaces through unconventional hydrogen bonds (De Pasquale et al., 2012; Conte et al., 2013). Next question to answer is how metals can affect water mobility as they are adsorbed on biochar surface. To this aim, an orchard pruning char obtained by pyrolysis at 500°C was treated with solutions of Cu(II), Cr(VI) and Cu(II)+Cr(VI). Two different types of water can be recognized in BC. Namely, a fast relaxing water (T1 = 99 ms) is differentiated by a slow relaxing one (T 1= 233 ms). The former is made by molecules interacting with biochar surface through formation of the unconventional Hbonds previously identified (De Pasquale et al., 2012; Conte et al., 2013). The latter is made by water molecules freely slipping on the immobilized water layer. After metal adsorption, no changes were observed in MTBC relaxogram shapes as compared to BC relaxogram (Fig. 1). Moreover, all the MTBC relaxograms were similar to each other, regardless of the metal adsorbed on the carbonaceous material. Nature of the metal appears not to affect water dynamics on the porous material. The shortening of the T 1values in MTBC as compared to BC sample (from 99 to 42 ms, and from 23 to 95 ms, respectively) has been interpreted by two different molecular models. In the first case, a water layer may be immobilized on the metals directly adsorbed on MTBC surface (Fig. 2A). Since metal-water interactionscan be stronger than those in the untreated BC, the result is a shortening of the T1values from 99 to 40 ms. As a consequence, also the H-bonds between the immobilized water and the freely moving one can be strengthened, thereby leading to shorter T 1 value. The second molecular model considers metal ions bridging two water layers (Fig. 2B). The first water layer is immobilized directly on the surface of the porous material and it is bridged to the second water layer through the metal cation. Further investigations are needed in order to differentiate among the different models accounting for water dynamics on the surface of biochar treated with metal cations.