Lamellar inorganic solids and biochar nanocomposites for sorptive removal of metal ions and nitrophenol

Abstract

In this research, the adsorbents of ternary-component composites were developed to remove various types of toxic chemicals. The selected three components are clay, MgAl-layered double hydroxide (MgAl-LDH) and biochar because they are nontoxic and low cost and also possess high adsorption capacity and high surface area. The composites were synthesized by the combination of post-pyrolysis and co-precipitation methods, and characterized by XRD and SEM-EDS. The result shows that clay and LDH particles distributed and deposited on the biochar matrix, confirming the coexistence of three phases in micrometer and nanometer scales.

Ni2+, CrO42- and 4-Nitrophenol were chosen as the representatives of chemical contaminant species of cation, anion, and organic compounds, respectively, for the adsorption study. The kinetic adsorption models and the isotherm adsorption models were used to explain the adsorption capacity and behavior of clay/MgAl-LDH/bagasse biochar composite (CLB) and clay/MgAl-LDH/rice straw biochar composite (CLR). The result of Ni2+ adsorption agreed with the pseudo-second-order kinetic model and the Langmuir isotherm model. The adsorption mechanism was proposed to be a cation exchange between Na+ and Ca2+ in the interlayer of clay and the adsorbate Ni2+. Meanwhile, the adsorption behavior of an anionic adsorbate CrO42- was also well fitted with the pseudo-second-order kinetic model and the Langmuir isotherm model. The proposed adsorption mechanism was the exchange of CrO42- with NO3- and CO32- in the interlayer of MgAl-LDH. The maximum adsorption capacities (Qmax) of CLB were 13.2 mg g-1 and 7.5 mg g-1 towards Ni2+ and CrO42-, respectively. Those of CLR were 12.7 mg g-1 and 6.1 mg g-1, respectively. The adsorption of 4-Nitrophenol could be explained by the pseudo-second-order kinetic model and the Freundlich isotherm model. The adsorption intensity (n) of CLB and CLR were 1.7 and 1.8, respectively, indicating favorable adsorption.

In addition, this study explored the fixed bed adsorption technique by using the CLB and CLR for 4-nitrophenol removal. The adsorption performance was presented by the breakthrough curve corresponding to the ratio of the initial concentration, the residue concentration and the effluent volume of 4-nitrophenol. The cumulative adsorbed amounts of CLB and CLR were 0.88 mg g-1 and 0.37 mg g-1, respectively, obtained using 2 cm bed thickness, 0.3 mL min-1 and the initial concentration of 10 mg L-1. These ternary composite adsorbents were demonstrated as promising sorbents for the removal of various contaminant species.