Factorial experimental design for optimizating the removal conditions of lead ions from aqueous solutions by three wastes of the olive-oil production

Olive stone (OS), two-phase olive mill solid (OMS) and also olive tree pruning waste (OTP), three wastes of the olive-oil production, were tested as low-cost adsorbents for Pb (II) removal from aqueous solutions. In order to reduce the total number of experiments to achieve the best conditions of the batch biosorption procedure, two 32 factorial designs of experiments were carried out for each biosorbent. The factors considered were biosorbent dosage, pH, initial lead concentration and temperature. Three levels for each factor were used; biosorbent dosage (2 g/L; 10 g/L and 22 g/L), pH (4; 5 and 6), initial lead concentration (10 mg/L; 40 mg/L and 70 mg/L) and temperature (25 °C; 40 °C and 60 °C). Two responses, the lead removal percentage and biosorption capacity after 120 min of contact time, were then analyzed statistically using the Statgraphics Plus 5.1 software. A second-order quadratic model suggested the optimum conditions. Three dimensional plots demonstrated relationships between the lead ion uptake with the paired factors, describing the behaviour of biosorption system in a batch process.

Research highlights
► Three solid wastes of the olive-oil production were tested as low-cost adsorbents for Pb(II) removal from aqueous solutions.
► The effect of biosorbent dosage, pH, initial lead concentration and temperature were tested.
► Two responses, the lead removal percentage and biosorption capacity were analyzed statistically.
► A second-order quadratic model described the behaviour of biosorption system and suggested the optimum conditions.
► Three dimensional plots demonstrated relationships between the lead ion uptake with the paired factors.