Single and Multicomponent Heavy Metal Ion Adsorption from Aqueous System Using Biochar Derived from Date Seed Biomass

Abstract

Water contamination by heavy metal ions is a world-wide issue. Adsorption is an effective treatment method for heavy metal removal from contaminated aqueous solutions. Traditional adsorbents such as activated carbons are expensive and may prove un-economical. Therefore, it is important to develop effective low-cost adsorbents. The aims of this study are to prepare an effective low-cost adsorbent of biochar prepared from date seed biomass and to investigate the adsorption performance of the derived biochar for heavy metal removal from aqueous solutions. Single and multicomponent systems of lead, copper, and nickel were studied. The studies were conducted in two stages: the first was to investigate the preparation and operational conditions to identify the most promising biochar; the second was to investigate different methods to modify the biochar to enhance its adsorption characteristics. In the first stage, nine sets of biochar were prepared using slow pyrolysis at different temperatures (350–550 oC) and heating times (1–3 h). The influences of pyrolysis temperature and heating time on the biochar physiochemical properties were studied. One–way ANOVA and Tukey Post-hoc tests showed that the biochars prepared at different pyrolysis conditions were significantly different. Initial evaluation of the nine biochars showed that the adsorption uptake was positively correlated with the pyrolysis temperature and heating time. Biochar prepared at 550 °C for 3 h (referred to as DSB550-3) with particle size range of 0.6-1.4 mm was the best adsorbent for Pb2+, Cu2+, and Ni2+ removal. Thus, it was selected in further investigations. Batch experiments for single component systems indicated that the maximum adsorption capacities of DSB550-3 biochar were 0.718, 0.421 and 0.333 mmol g−1 for Pb2+, Cu2+, and Ni2+ respectively. These values were found to be higher than those of hickory wood biochar (0.016 for Pb2+, 0.041 for Cu2+, and 0.004 for Ni2+) mmol g-1 and Salisbury biochar (0.230 for Pb2+, 0.101 for Cu2+, 0.105 for Ni2+) mmol g-1. Fixed bed adsorption showed that DSB550-3 biochar effectively reduced the total amount of Pb2+, Cu2+ and Ni2+ by 97%, 70.3%, and 56%, respectively. The adsorption performance of DSB550-3 biochar for heavy metal removal was evaluated under a variety of conditions including solution pH, particle size, temperature, and presence of other cations. The removal efficiency increased with particle size until it reached a maximum in the particle size range (0.6-1.40 mm) after which removal efficiency declined. Solution pH strongly affects the adsorption uptake of biochar. The metal uptake increased as the pH increases and reached a plateau at around pH 6. The adsorption process was found to be thermodynamically favourable and spontaneous. Temperature also influenced the metal uptake and a change of 18-38% in uptake capacities within the temperature range of 25 and 45 oC on adsorption was observed. When compared to single component systems, the adsorption capacities of Pb2+, Cu2+, and Ni2+ in multicomponent systems were reduced by 48-75% in both batch and fixed bed experiments. In fixed-bed experiments, both Ni2+ and Cu2+ broke through earlier than Pb2+ ion, indicating that the functional groups on the biochar had a relatively stronger affinity for Pb2+ ion than Cu2+ and Ni2+ ions. The overall adsorption capacities of the fixed bed for all three ions were reduced by approximately 55% (on mill equivalents basis) when compared to Pb2+ adsorption capacity in single component system but only marginally when compared to Ni2+ and Cu2+. Post-adsorption characterization and chemical analysis indicated that the adsorption was mainly controlled by the mechanisms of ion exchange and metal complexation with carboxyl and hydroxyl groups of the biochar. The contribution of different adsorption mechanisms to the overall metal adsorption was different in the single and multicomponent systems. Ion exchange was the predominate mechanism for the adsorption of Pb2+, Cu2+, and Ni2+, which accounted for 57%, 69%, and 72%, respectively, of the total adsorption in single component systems and accounted for 37-40% of the total adsorption in multicomponent system. Equilibrium isotherms and adsorption kinetics were studied by using DSB550-3 biochar. The equilibrium isotherms of Pb2+, Cu2+, and Ni2+ adsorption in single component systems were well described by the Sips model. It was observed that for all three metals, equilibrium was reached within 3-6 h of contact. The kinetics for Pb2+ and Cu2+ and Ni2+ showed good agreement with the pseudo second order kinetic model suggesting that chemisorption might be one of the adsorption mechanisms. Quantitatively, the adsorption of Pb2+ and Cu2+ was faster than that of Ni2+ which might be either related to the binding strength of Pb2+ and Cu2+ with surface functional groups or due to the variance in ionic characteristics. Biochar re-usability for repeated applications was investigated in four consecutives regeneration cycles by using 0.5 M HCl as an elution agent. The adsorbent showed good re-use potential with only marginal reduction in its uptake capacity over the first 3 cycles. This has an important economic significance as the re-usability makes the material more comparable to the commercial adsorbents such as commercial activated carbon. The biomass and/or biochar were modified to enhance the adsorption properties for the biochar. The modification methods included pre-treatment of biomass prior to pyrolysis using an alkali (DSB-PB) or an acid (DSB-PA) as well alkali and acid post–treatment of biochar pyrolyzed at 550 oC for 3 h (DSB-BW and DSB-AW, respectively). The modified biochar was compared to unmodified one in terms of their metal uptake capacities, kinetic behaviour, and surface chemistry and morphology based on FTIR and SEM analyses. Of the four modified biochars investigated, DSB-PA biochar showed the highest adsorption capacities for Pb2+, Cu2+, and Ni2+. When compared to those of DSB550-3 biochar, the adsorption capacity of DSB-PA biochar for Pb2+, Cu2+, and Ni2+ increased by 27%, 66% and 98%, respectively. The adsorption rates of metal ions onto DSB-PA biochar were higher when compared to unmodified biochar, with almost 94% of the total amount of metals adsorbed within the first hour. Electro-assisted adsorption was also investigated for Pb2+, Cu2+, and Ni2+ by using biochar (DSB-Electro). Comparing to DSB550-3, the results showed that DSB-Electro effectively increased the adsorption capacity of the biochar for Pb2+, Cu2+, and Ni2+ by 21%, 36% and 94%, respectively. Metal ion adsorption onto electrode occurred rapidly, for example; with around 88% of Pb2+ and Ni2+ ions adsorbed within the first 3 h, while 96% of total adsorption of Cu2+ ion occurred at the first hour of contact. From the reversing of the electrical potential, it was observed that the adsorbed ions were not completely desorbed, and a large fraction of the ions were retained in the biochar electrode. This indicated that the only a small fraction of the ions were held by the electrostatic charge introduced by the current. It was likely that the enhanced charged facilitated other adsorption mechanisms by bringing the ions in contact with the biochar initially via electrostatic force. The results of this study indicated that date seed biochar is a suitable adsorbent which can effectively remove Pb2+, Cu2+, and Ni2+ from aqueous solutions. Findings of this study can provide a basis for preparing modified biochar in producing effective low-cost adsorbents.