Study on optimizing the preparation condition of CuAl-LDH functionalized maize stalk biochar and evaluating its performance in anionic dye adsorption

Abstract

The use of nanoparticle-modified biochar as adsorbent could eliminate the environmental and public health risks associated with water-soluble organic contaminants due to its superior textural attributes and high uptake capacity. This work optimized the preparation conditions of CuAl-layered double hydroxide functionalized maize stalk biochar (CuAl-LDH/MSB) for anionic dye (acid blue 25, AB25) adsorption using Taguchi design approach. CuAl-LDH/MSB composites were synthesized under 9 different conditions, according to Taguchi L9 (3⁴) orthogonal array design (molar ratio of Cu²⁺: Al³⁺: 2:1, 3:1 and 4:1, pyrolysis temperature: 500 °C, 600 °C and 700 °C, biomass loading: 2 g, 3 g and 4 g and pyrolysis duration: 1 h, 1.5 h and 2 h). The physicochemical and adsorption properties of the optimal CuAl-LDH/MSB sample were assessed using different analytical techniques and batch adsorption tests. Analysis of variance was performed to calculate the percentage of contribution of controllable parameters, which described their levels of influence on process response. Optimization results showed that the CuAl-LDH/MSB properties and adsorption performance were mostly influenced by Cu²⁺: Al³⁺ molar ratio (34.98%), followed by biomass loading (34.74%), pyrolysis time (29.03%) and pyrolysis temperature (1.25%). The optimal conditions for preparation of CuAl-LDH/MSB for adsorptive removal of AB25 were pyrolysis temperature of 600 °C, Cu²⁺: Al³⁺ molar ratio of 2:1, pyrolysis time of 1.5 h and biomass loading of 3 g, which resulted in 25.02 mg g⁻¹ AB25 dye uptake. The favourable adsorption of AB25 by the optimal adsorbent (CuAl-LDH/MSB_-opt) was achieved at pH of 6.0 and equilibrium time of 2 h. Further study revealed that Freundlich isotherm and pseudo-second-order kinetic model accurately anticipated the adsorption experimental data, confirming the multilayer coverage process of AB25 onto CuAl-LDH/MSB_-opt and chemisorption mechanism. The AB25 adsorption onto CuAl-LDH/MSB_-opt was feasible in nature, spontaneous and endothermic as indicated by thermodynamic parameters.