Adsorption Isotherm Studies of Ni (II), Cu (II) and Zn (II) Ions on Unmodified and Mercapto-Acetic Acid (MAA) Modified Sorghum Hulls

Aims: To compare the efficiency of Sorghum Hull of two different particle sizes, different concentrations modified with mercaptoacetic acid and the effect of each on the heavy metal removal to obtain optimum conditions for sorption process.

Study Design: The concentrations of the metal ions were monitored using Atomic Absorption Spectrophotometer (AAS).

Place and Duration of Study: This study was carried out at the Industrial Chemistry laboratory, Department of Pure and Industrial Chemistry, University of Port Harcourt between February and August 2013.

Methodology: The Sorghum Hulls (SH) (Sorghum bicolor) obtained from a Brewery, were washed and air dried, crushed to smaller particles and sieved to obtain particle sizes of 106 µm and 250 µm. They were activated by soaking in excess of 0.3M HNO3 solution for 24 h, filtered through a Whatman No.41 filter paper, rinsed with deionised water and air dried for 24 h. The air-dried activated sorghum hulls were divided into three parts, one part was left unmodified and the other two parts were modified by 0.5M and 1.0M mercaptoacetic acid (HSCH2COOH). Equilibrium sorption of Ni (II), Cu (II) and Zn (II) were carried out for each adsorbent (106 µm and 250 µm) at pH of 6.0 and temperature of 28°C to find the effects of initial concentration on the sorption process by preparing stock solutions of 1000 mg/L of Cu (II), Ni (II) and Zn (II) ions from CuSO4, Ni(C2H3O2)2, and ZnSO4 respectively. Working concentrations of 10, 20,30,40,50 mg/L were obtained by serial dilutions. The concentrations of these solutions were confirmed using Atomic Absorption Spectrophotometer (AAS). The difference in the metal ion concentration of the solutions before and after sorption gave the amount adsorbed by each adsorbent.

Results: Sorption efficiency followed the trend Cu2+>Ni2+>Zn2+ and Ni2+>Cu2+>Zn2+ for 106µm and 250µm, respectively. This was influenced by factors such as ionic radii, ionic charge, hydration energy, initial metal ion concentration, contact time, adsorbate pH and extent of acid modification. Sorption capacity followed the sequence: 1.0MSH>0.5MSH>USH. Equilibrium sorption of the three metals on the adsorbents using coefficient of determination (R2) showed that sorption of Zn2+ on 250μm size and sorption of Ni2+ on 106μm size followed Langmuir isotherm; sorption of Zn2+ on 106μm mesh and sorption of Cu2+ on 250μm size by Freundlich isotherm and sorption of Cu2+ on 106μm size and sorption of Ni2+ on 250μm mesh by Temkin isotherm. Sorption on 1.0MSH gave the highest value of constants: (qmax = 121.814 mg/l, KF = 1.5281 mg/l and bT = 0.245 kJ/mol) and (qmax = 148.932 mg/l, KF = 1.7246 mg/l and bT = 0.262 kJ/mol) for 106 µm and 250 µm size, respectively, for Langmuir, Freundlich and Temkin isotherms respectively.

Conclusion: Therefore, Sorghum Hulls were found to be good adsorbent for Ni, Cu and Zn ions with mercaptoacetic acid modification increasing the sorption capacity.