An adsorption study of nickel and cobalt on saprist Newfoundland peat

Air dried saprist Newfoundland peat, harvested from a private peat bog owned by Traverse Nursery in Torbay, St. John's, was employed as the sole adsorbent in laboratory batch and column experiments for the removal of Nickel and Cobalt cations from stock solutions. The saprist peat type has not been widely investigated as an adsorbent, unlike the fibrist peat. Characterization using non-destructive methods showed the peat as being acidic, consisting of small, overlapped and collapsed cellular masses. The cation exchange capacity was 70 meq/100g, with a wet bulk density of 0.65 g/cm³ and ~90 % organic content. The peat matrix was dominated by oxygenated functional groups and to a lesser extent amine/amide groups which are all capable of complexing / co-ordination with Ni²⁺ and Co²⁺. -- The response surface method of the Box-Behnken design showed that peat dose, concentration, solution pH and contact time were the significant factors that influenced the peat Ni²⁺ and Co²⁺ adsorption capacity with the interaction effect of the factors being metal specific. The batch test investigation showed that the metal uptake reaction was a pseudo-second order type and attainment of equilibrium was metal concentration and pH dependent. Kinetic equilibrium adsorption over 12 h gave a maximum adsorption capacity of 385 mg/g for Ni²⁺ at a peat dose of 21 g/L, pH of 5.5 and Ni²⁺ concentration of 125 mg/L while for Co²⁺ it was 33.44 mg/g at a peat dose of 2g/L, pH of 8 and Co²⁺ concentration of 200 mg/L with regression coefficients near unity. -- Equilibrium adsorption data gave good fits with both the Langmuir and Freundlich isotherms from which corresponding adsorption parameters were determined. The percent metal removed was nearly 100% for the two metals at solution pH 3, especially at metal concentrations below 50 mg/L while, between 35 % and 75% removal were obtained at concentrations between 125 and 200 mg/L. Competitive sorption tests showed that at higher concentrations and peat doses, more Ni²⁺ and Co²⁺ were removed in the presence of Cd²⁺, Pb²⁺, and Zn²⁺ with the order of removal being Pb²⁺ > Ni²⁺ > Cd²⁺ > Zn²⁺ > Co²⁺. Breakthrough was metal and flow rate dependent. The maximum adsorption capacity of the 12.5 cm long peat bed was 72 g/L and 24.7 g/L for Ni²⁺, at a flow rate of 1.0 Uh and 2.0 Uh respectively. For Co²⁺, 17 g/L and 6.7 g/L was the computed maximum adsorption capacity at a flow rate of 1.0 L/h and 2.0 Uh. The adsorbent exhaustion rate was metal and flow rate dependent with Ni²⁺, at a flow rate of 1.0 L/h, 0.69 g/L was computed and for Co²⁺, at a flow rate of 1.0 L/h, 2.16 g/L was computed. -- The metal uptake mechanism on the saprist Newfoundland peat was strongly pH dependent and based on the experimental data; complexation was the dominant reaction at acidic conditions especially at pH ~3.0, while ion exchange was the main reaction at basic conditions. Kinetics and equilibrium sorption data especially desorption with 0.1 M to 2 M HC1, showed that the two reactions occurred simultaneously at the pH of 5.5 and 8 for Ni uptake while, Co uptake occurred predominantly by ion exchange at pH 10 and by complexation at pH of 5.5 and 8