Locally Sourced Charcoal Removes Excess Fe from Seawater - Effect of Sorbent Size on the Adsorption Properties

The status quo of the iron (Fe) concentration in the seawater (0.55 mg/L) of the Araromi coastal area in Nigeria exceeds the permissible limit set by the World Health Organization (WHO; 0.3 mg/L) for drinking water. To lower the Fe content, the adsorption potential of locally sourced mangrove wood-based charcoal (WC) with three particle sizes (0.38dp, 1.18dp, and 2.00dp) was investigated, where dp means particle diameter in mm, under varied parameters (dosages, contact time, and temperature). The adsorption efficiency (ζ) improved with an increase in each parameter. The WC with a particle size of 0.38dp evinced the highest ζ (91.0%, 88.8%, and 81.1%) under the parameters of 60 g/L dosage, 50 min of contact time, and 60 °C of temperature, respectively. Generally, the ζ followed the order of 0.38dp > 1.18dp > 2.00dp. Through modeling, 0.38dp WC would achieve an ζ of 100% if the dosage, contact time, and temperature were 4.04 g/L, 56 min, and 81.2 °C, respectively. Furthermore, the adsorption favored the Freundlich model (R2 = 0.893 – 0.999) over the Langmuir model (R2 = 0.558 – 0.994), fitting pseudo-second-order (R2 = 0.974 – 0.992) rather than pseudo-first-order (R2 = 0.804 – 0.989) kinetics. Also, the predominance of chemisorption over physisorption was established by the enthalpy change value (ΔH = 19.05 – 40.47 kJ/mol). The Fe adsorption was thermodynamically feasible (ΔG = -10.3 – -7.3 kJ/mol) and endothermic, confirming the positive linearity between ζ and temperature. Consequently, low-cost, readily available WC was effective in lowering the Fe levels in seawater to a potable level.