Adsorption of Lead, Manganese, and Copper onto biochar in landfill leachate: implication of non-linear regression analysis

16 The feasibility of using wood-derived biochar (BC) to remove Pb, Mn, and Cu from landfill 17 leachate was investigated and modeled in this study. BC was produced under the pyrolytic 18 temperature of 740 °C. The effect of contact time, BC dosage and particle size on adsorption of 19 the heavy metals onto BC was examined. BC was used in two forms i.e., pulverized (PWB) and 20 crushed (CWB) to evaluate the effect of BC particle size on adsorption characteristics. The 21 kinetics of Pb, Mn, and Cu adsorption onto PWB and CWB were assessed using the pseudo 22 second-order and Elovich models, where both applied models could well describe the adsorption 23 kinetics. Removal efficiencies of the heavy metals were increases by 1.2, 1.4, and 1.6 times, 24 respectively, for Pb, Mn, and Cu, when PWB content of the leachate increased from 0.5 to 5 g L - 25 1 . Equilibrium adsorption capacity of the heavy metals onto BC in leachate system was evaluated 26 using the Langmuir, non-linearized Freundlich, linearized Freundlich, and Temkin isotherms and 27 found to have the following order for PWB: Non-linearized Freundlich > Temkin > Langmuir > 28 Linearized Freundlich. The Langmuir and linearized Freundlich models could not adequately 29 represent adsorption of the heavy metals onto BC, especially for CWB. The highest removal of 30 88% was obtained for Pb, while the greatest adsorption intensity was found to be 1.58 mg g -1 for 31 Mn. Using the non-linearized Freundlich isotherm significantly reduced adsorption prediction 32 error. The adsorption affinity of PWB for Pb, Mn, and Cu was greater than that of CWB in all 33 treatments. Wood-derived BC is suggested to be used for the removal of heavy metals from 34 landfill leachate as an economical adsorbent.


39
Landfill leachate may contain a wide range of contaminants at levels enough to raise serious 40 environmental and human health concerns. The majority of published research has focused on 41 removal of ammonia and organic fraction of landfill leachates, such as using biological reactors Kahrizak landfill and used for the adsorption experiments. Overflow of the fresh leachate from 108 newly-filled trenches was directly collected in four 10-L plastic containers. Collected leachate 109 can be classified as relatively fresh leachate based on the low pH values (5.11). Leachate 110 samples were immediately transported to the laboratory. Samples were kept refrigerated at 4 °C 111 without exposure to the ambient air for not more than three days before conducting relevant 112 analysis to prevent potential chemical and biological changes. (0.5%), S (0.1%) and ash (3.4%). The produced BC had particle density of 1.5 g cm -3 . PWB and 149 CWB had bulk densities of 0.93 and 0.69 g cm -3 , respectively. The pH of the BC (9.1) was 150 determined following the method of Singh et al. [31]. BET surface area was measured using a 151 Brunauer-Emmett-Teller Surface Area & Porosity Analyzer (NOVA 4200e) by nitrogen gas 152 sorption analysis at 77 K. Samples were vacuum degassed prior to analysis, at 300 °C for 5 to 15 h, based on the required time to reach a stable surface area measurement [32]. BET surface area 154 of the PWB and CWB were determined to be 335 and 281 m 2 g -1 , respectively. Where, C0 and Ce are, respectively, the initial and final concentrations of Pb, Mn and Cu in 169 leachate samples (mg L -1 ). Kinetic solutions were stirred on a shaker at constant rate of 120 rpm 170 at room temperature of 24 ± 2°C to provide effective interaction of sorbate with sorbent material.

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At the end of the specified agitation period, obtained mixtures were centrifuged for 15 min at 172 6000 rpm to separate liquid and solid phases, filtered by Whatman Paper Filter No. 1 (11 μm 173 pore size) and the filtrates were then analyzed for the heavy metal concentrations. The adsorption 174 isotherms were studied in actual leachate system for Pb, Mn and Cu. Certain quantities of PWB 175 and CWB (0.05 to 5 g) were separately weighted and added to a 100 mL fresh landfill leachate at initial pH of 5. in Fig. 1a and 1b. The adsorbent dosage was fixed at 1 g 100 mL -1 (10 g L -1 ) and the pH value of state to be established when BC with smaller particle size, i.e., PWB was used, implying slower 215 occupation of adsorption sites on the surface of PWB due to the greater specific surface provided 216 by PWB relative to CWB. The highest removal efficiency of 88% by PWB was obtained for Pb.

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As reaction time prolonged, repulsive forces between the metal ions adsorbed to BC and 218 those in the aqueous phase might be increased. In addition, unoccupied adsorption sites and 219 therefore adsorption efficiency will be quickly declined until the establishment of dynamic 220 balance in the system. The same observation was found for Ni uptake from aqueous solution by 221 AC derived from sugar bagasse [37]. From the adsorption diffusion viewpoint, two distinct 222 adsorption stages could be distinguished for the uptake of Pb, Mn, and Cu onto BC in landfill 223 leachate; surface diffusion during which the mass transfer is rapid and physical processes control 224 the adsorption, followed by intra-particle diffusion that is characterized by slow adsorption.

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Greater adsorption efficiency for heavy metals was observed for all the applied dosages of BC at 226 initial stages of the experiment, that may be attributed to the higher availability of adsorption amount. The highest removal efficiency was obtained for Pb followed by Mn and Cu due to 242 addition of PWB (Fig. 1c) and CWB (Fig. 1d).
Removal efficiency of Mn and Cu was comparable, with slightly higher elimination for Mn.

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Amount of Pb, Mn, and Cu adsorbed to each gram of BC reduced with rising adsorbent dosage, 245 likely due to the availability of more adsorption sites on the surface of both PWB and CWB.

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Optimum AC dosage of 7 g 100 mL -1 was found to effectively adsorb COD and NH3 from  The non-linear form of pseudo second-order model is represented as follow: Where k2p is the second-order adsorption constant (g mg -1 min -1 ), qe is the amount of heavy If the boundary conditions of qt = 0 to qt = qt and t = 0 to t = t is applied, the model can be using the pseudo second-order expression are given in Table 1 Where α is the initial adsorption rate (mg g -1 min -1 ) and β is defined as desorption constant    describe the equilibrium data perfectly in most cases.

Linearized and non-linearized Freundlich isotherms 379
The Freundlich isotherm has been widely applied to characterize the adsorption of organic 380 and inorganic pollutants using various adsorbents [44]. Freundlich isotherm constants found 381 through plotting ln qe vs ln Ce are given in Table 4. The ratio of the amount of adsorbate     was clearly observed; first the migration of metals from the leachate system to the external 500 surface of BC during which the mass transfer is very rapid and physical processes control the 501 adsorption, followed by the prolonged intra-particle diffusion characterized by slow adsorption.

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The non-linear Freundlich isotherm best describes the equilibrium adsorption data, followed by 503 the Temkin isotherm. Linearized Freundlich model could only moderately describe adsorption of 504 the heavy metals onto PWB, while it was not able to represent the adsorption by CWB.

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Linearization method for the Langmuir isotherm also affects the error structure suggesting that 506 the linearization of non-linear isotherm models may violate the theory behind an isotherm and 507 alter error distribution. It is recommended to use wood-derived BC as an effective adsorbent to 508 remove heavy metals from landfill leachate.