The contamination of water by toxic heavy metals including lead, cadmium, copper and zinc is a global problem. The release of these metals into the environment has become a serious health problem due to its toxicity. Progressively stricter discharge regulations on heavy metals have accelerated the search for highly efficient but economically feasible or alternative treatment methods for its removal. The use of low-cost and bio-waste or agro-waste as biosorbents for dissolved metal ions removals has shown potential to provide economic solutions to this environmental setback.
Garden grass (GG), cabbage waste (CW), banana peels (BP), maple leaves (ML) and palm oil fruit shells (POFS) have been identified as potentially low cost and efficient biosorbent for the removal of toxic heavy metals from aqueous solution. Very simple methods were used to prepare these biosorbents. The collected GG, CW, BP, ML and POFS were washed, cut into pieces, dried in oven at 105°C, grounded into powder and used for experiments.
The biosorbents were characterized by SEM, XRD, FTIR and BET tests. Surprisingly, all biosorbents showed that the surfaces of biosorbents’ particles are porous, heterogeneous structures, with uneven, asymmetric steps and pores which contained high internal spaces and posed higher specific surface area. The BET surface areas are 21.28, 1.027, 22.59, 10.94 and 39.76 m²/g for GG, CW, BP, ML and POFS, respectively. These biosorbents possesses many hydroxyl, carbonyl and phenyl functional groups (by FTIR test) and therefore, all these biosorbents are good contenders for water treatment and purification utilizations.
Biosorption of all four metals [Pb(II), Cd(II), Cu(II) and Zn(II)] by GG, CW, BP, ML and POFS was found to be dependent on experimental conditions. The optimum biosorption were noted at pH 6-6.5, shaking speed of 120 rpm, initial concentration of 10 mg/1, dose of 5g/1, contact time of 2 h and particle sizes < 75μm. The increase of temperature negatively affected the metals biosorption and at room temperature the metals biosorption process is spontaneous and exothermic in nature. The acid medium (0.1N H₂SO₄) was found be a better eluent for regeneration of exhausted biosorbents and it could be reused 5-7 times with minor deviation of efficiency except CW.
The efficient metal removing ability of biosorbents in both batch experiments and continuous flow fixed bed column bioreactors used to produce a biosorbent based metals removal system. It suggests that these novel biosorbents could lead to the development of a viable and cost-effective technology for metals removal from water and wastewaters. The prepared biosorbents were evaluated for the adsorption of Pb(II), Cd(II), Cu(II) and Zn(II) ions from single and multimetals aqueous solution by the batch method. The biosorption data were evaluated by equilibrium isotherms models. GG isotherm data posed better fitness with Langmuir, Freundlich and SIPS models as the R² lies between 0.991 and 0.999. Three-parameter models (Redlich-Peterson, Koble-Corrigan and SIPS) and two-parameter models (Langmuir and Freundlich) showed good fitness (R²: 0.991-1.0) with equilibrium adsorption data from CW. The biosorption data from BP are evaluated by Langmuir, SIPS, Redlich-Peterson, Radke-Prausnitz, and Brouers-Sotolongo and results showed a good fitness as the R² were between 0.998 to 1. Among the models three-parameter models such as Sips, Redlich-Peterson and Unilan showed good fitness with isotherm data from ML as the R² are 0.988-1.00. Likewise the equilibrium data from POFS posed proper agreement with three parameter models for biosorption of Cu(II). Significantly low RMSE and χ² values are found from all the used models which also signify the models fitness.
The maximum Pb(II) adsorption capacities (qm) are 54.205, 61.267, 120.096 and 50.267 mg/g for GG, CW, BP and ML respectively; whereas it were 41.66, 22.123, 50.459 and 39.599 mg/g for Cd(II) biosorption. Among the biosorbents GG, ML and POFS showed good biosorption capacities for Cu(II) ion and the values are 58.34, 34.534 and 59.502 mg/g. The Zn(II) adsorption capacities are moderate and the magnitude of capacities are 57.53, 12.236, 51.896 and 29.94 mg/g for GG, CW, BP and ML respectively. A strong antagonisms were found among the metals ions [Pb(II), Cd(II), Cu(II) and Zn(II) ions] in the multimetals adsorption systems though Pb(II) and Cd(II) ions dominated. Surprisingly, the maximum reduction in capacities were also found for Pb(II) and Cd(II). The equilibrium and kinetics data for desorption were also evaluated the by isotherm models and kinetics models. Some data from GG and ML showed good agreement with models but most of the data did not pose appropriate fitness.
The kinetics of metal removal by all biosorbents was extremely fast, reaching equilibrium in about 15-60 minutes which is showed the practical potentiality. The both pseudo-first-order and pseudo-second-order models was found to be the best fit (R²: 0.991-1.00) to describe the biosorption mechanism of Pb(II), Cd(II), Cu(II) and Zn(II) ions onto the biosorbents. This is implies that the adsorption mechanisms are both physisorption and chemisorption. As reaction constant, k₂ and nAV (from order and Avrami equation) values were greater than 1 suggested that biosorption reaction is more than one order. Intraparticle diffusion also involved for biosorption process. Along with this, Elovich and Fraction power models also posed good fitness with kinetics data.
The metal removing capacity of CW was also tested in continuous flow fixed-bed column bioreactors for artificial wastewater. The removal capacities were 15.72, 62.23, 68.23 and 70.71 times higher than that obtained in a batch system for Pb(II), Cd(II), Cu(II) and Zn(II) ions. The appropriate service times to breakthrough and metals ions concentration were 5-10 h and 10 mg/1, respectively. The design of a continuous fixed bed column treatment system with CW biosorbent for Pb(II), Cd(II), Cu(II) and Zn(II) laden wastewater can be reached using the BDST, Yoon-Nelson and Clark breakthrough models.
Elucidate the biosorption mechanisms is one of the aims for biosorption of metals. Fitness of pseudo-first-order and pseudo-second-order models suggested the biosorption are both physisorption and chemisorption. FTIR tests showed the functional groups which responsible ions exchange. Isotherm data fitted more with three-parameter models which signifies the adsorption onto heterogeneous surface. It is also found from SEM and XRD data. Thus, it could not be ascertained from the results which single mechanism involved for metals biosorption. However, it could presume that combination of all mechanisms with complexation are responsible for Pb(II), Cd(II), Cu(II) and Zn(II) ions and therefore, high biosorption capacities were found.