Biosorption of strontium from aqueous solutions onto spent coffee grounds

A set of experiments was carried out to evaluate the strontium uptake potential of spent coffee grounds (SCG) by batch tests in aqueous medium. Adsorption of Sr2+ as a function of contact time and adsorbent dose, pH, particles size, agitation speed, temperature and co-ions presence was investigated. Obtained results revealed that the maximum adsorption took place at pH range of 5-8 and temperature values between 283 and 333 K. Particles size effect was not very significant and agitation speed influenced on the equilibrium time. Competitive adsorption experiments allowed us to classify the negative effect on the Sr2+ uptake according to this order Al3+ ≫ Co2+ > Mg2+ > Ca2+ ≫ Na+ > K+ > Cs+. Kinetic study indicated that the Sr2+ uptake was fast and it was well fitted by the pseudo second order reaction model. Adsorption isotherm was well interpreted by Langmuir model. The maximum adsorption capacity was found to be 69.01 mg g−1 at pH 7, 293 K, particles sizes = 200-400 μm and agitation speed 250 rpm. The thermodynamic study revealed that the process was spontaneous (ΔG0 < 0), exothermic (ΔH0 < 0) with a raised affinity for Sr2+ (ΔG0 < 0, ΔS0 > 0) and occurred by physical adsorption (Ea = 8.37 kJ mol-1). FTIR analysis showed carboxylic acid and amino group presence on SCG surface playing a vital role in Sr2+ biosorption. (author)