To enhance the Cd2+ adsorption capacity on coconut shell-derived biochar by chitosan modifying: performance and mechanism

In this study, by using coconut shell and chitosan (CS) as raw materials, mesoporous chitosan@coconut shell-derived biochar composites (CS@BC) were prepared and were used for Cd2+ removal from aqueous solutions. The tests of SEM, BET, XRD, FTIR, and XPS were used to identify the adsorbent structure and adsorption mechanisms. Batch adsorption experiments shown that the Cd2+ adsorption by biochar was strongly dependent on the initial pH of the solution. The pseudo-second-order kinetic model and Langmuir isotherm model can describe the process of Cd2+ adsorption by biochars more accurately, which indicated that the adsorption process was a chemical reaction and single-layer adsorption. Under the conditions of initial pH 5.0, dosage 10 mg, contact time 360 min, and temperature 25 degrees C, the maximum Cd2+ adsorption capacity of BC and CS@BC was 48.84 mg/g and 63.88 mg/g, respectively. Notably, the Cd2+ removal efficiency of CS@BC in three different water bodies was almost 100%. In addition, after 5 times of adsorption-desorption experiments, the Cd2+ removal efficiency of CS@BC remained above 90%, which indicated that CS@BC had excellent regenerative properties. The adsorption mechanisms of Cd2+ by BC and CS@BC were mainly attributed to ion exchange (Q(ex)), co-precipitation (Q(pre)), complexation with O-containing functional groups (Q(co)), and Cd2+-pi interaction (Q(pi)). Hence, inexpensive, readily available and simply prepared CS@BC composites were highly efficient adsorbent materials that can be applied for the removal of Cd2+ from aqueous solutions.