The Energetics of Ion Sorption onto Biochars for Soil Remediation.

Rice husk biochar and compost mixture produced in Ghana have been reported to retard the mobility of toxic elements and are thus desirable amendments for the remediation of contaminated soils. It is therefore necessary to characterize their physicochemical properties and elucidate the impact of mixing compost with rice husk biochar on the overall adsorption performance. To that end, we used operando in situ flow microcalorimetry (FMC) to quantify the energetics of ion sorption unto rice husk biochar (BC) and biochar compost (BCO) and Attenuated Total Reflectance Fourier Transform Spectroscopy (ATR-FTIR) to characterize their surface functional groups. The heat of exchange (Qexch) of anion exchange (AE) and cation exchange (CE) were used to probe the surface positive and negative charges respectively. Qexch of AE were measured using 100mM of NaCl and NaNO3: Qexch of CE were measured using 100mM of NaNO3 and KNO3. All solutions were prepared at a pH value of 5.8. Preliminary results showed that both Qexch AE and CE were smaller for BC than those for BCO: and that Qexch for AE are smaller than Qexch for CE on both materials. ATR-FTIR showed higher absorbance of hydrogen-bonded (Oh), carboxylate anions (COO-) and aliphatic group on BCO than BC. Two preliminary conclusions can be derived. First, the pH at which the point of zero charge occurs (pHZPC) for both materials is below 5.8 as Qexch CE is greater than Qexch of AE, and second mixing compost with biochar significantly changes the surface charge properties of the materials. Further studies will ascertain the contribution of functional groups to the exchange energetics differences by probing the surface chemistry of BCO/BC at a range of pH values.