Objectives : The physicochemical characteristics of Mg-biochar composites derived from kelp and pine after pretreatment with MgCl2 were analyzed, and their adsorption capacities for an anionic dye, Congo red (CR), were evaluated.Methods : After pretreating 60 g of kelp and pine sawdust in 1 L of 0.1 M MgCl2･6H2O, the raw materials were pyrolyzed at 500℃ to produce Mg-biochar composites (kelp based KB-Mg and pine based PB-Mg). The fundamental physicochemical characteristics of the Mg-biochar composites were examined, and their adsorption capacities for CR were investigated using different initial pH values, adsorption kinetic models, and adsorption isotherm models.Results and discussion : The Mg-biochar composites showed the development of uniform deposits of Mg minerals primarily as MgO crystal on the surface by the surface modification with MgCl2. When the pristine biochars were surface-modified with MgCl2, their adsorption capacities for CR were significantly increased over the entire pH range tested. The CR adsorption process by all biochars was best described with the pseudo-first order kinetics model, and the adsorption isotherm characteristics were better described with the Langmuir isotherm model for all biochars. The Langmuir maximum adsorption capacities for KB-Mg and PB-Mg were 423.0 mg/g and 394.7 mg/g, respectively. It is suggested that the main mechanism for CR adsorption on the Mg-biochars is electrostatic attraction between CR and the biochars.Conclusions : The results showed that surface modification with MgCl2 could greatly enhance the CR adsorption capacity of biochars, and the results demonstrated the great potential of KB-Mg and PB-Mg for CR removal.

Objectives:This study investigates the adsorption capacity of magnetic biochar (KB-Fe) derived from kelp for two dyes, Congo red (CR) and crystal violet (CV), which have different charge characteristics. The adsorption capacities were compared to magnetic biochar (PB-Fe) derived from pine.Methods:Pristine biochars (KB, PB) were produced through the pyrolysis of kelp and pine sawdust at 500℃. Magnetic biochars (KB-Fe, PB-Fe) were produced by treatment of 60 g of biomass in 1 L of 0.1 M FeCl₃･6H₂O and pyrolysis of the raw materials at 500℃. The fundamental characteristics of the biochars were analyzed, and the adsorption capacities of the biochars for the CR and CV dyes were evaluated based on two adsorption isotherm models, two adsorption kinetics models, and adsorption characteristics based on changes to the pH between 5 and 11.Results and discussion:CR adsorption by all biochars decreased with an increase in pH. When KB and PB were surface-modified with magnetic iron oxides, their adsorption capacity for CR increased. CV adsorption by all biochars increased with pH. When KB and PB were surface-modified with magnetic iron oxides, their adsorption capacity for CV considerably decreased. According to the Langmuir isotherm model, the adsorption capacity for CR was in the order of KB-Fe, KB, PB-Fe, and PB, and the adsorption capacity for CV was in the order of KB, PB, PB-Fe, and KB-Fe. The CR adsorption was better described by the pseudo-second order kinetics model for all biochars, with the exception of KB-Fe. CV adsorption was also better described by the pseudo-second order kinetics model for all biochars, with the exception of KB.Conclusions:The results show that kelp biochar has great potential for the adsorptive removal of CR and CV compared to pine-based biochar. In addition, it was demonstrated that surface modification of kelp biochar with magnetic iron oxides can significantly improve its adsorptive capacity for anionic dyes; however, its adsorptive capacity for cationic dyes decreases.