References
  -  T. Aman, A.A. Kazi, M.U. Sabri, Q. Bano, Potato peels as solid
    waste for the removal of heavy metal copper (II) from waste
    water/industrial effluent, Colloids Surfaces B Biointerf., 63
    (2008) 116–121. 
 
  -  X. Zhou, Y. Liu, J. Zhou, J. Guo, J. Ren, F. Zhou, Efficient
    removal of lead from aqueous solution by urea-functionalized
    magnetic biochar: Preparation, characterization and mechanism
    study, J. Taiwan Inst. Chem. Eng., 91 (2018) 457–467. 
 
  -  M. Bystrzejewski, K. Pyrzyńska, A. Huczko, H. Lange, Carbon-encapsulated magnetic nanoparticles as separable and
    mobile sorbents of heavy metal ions from aqueous solutions,
    Carbon, 47 (2009) 1201–1204. 
 
  -  Q. Qiu, X. Jiang, G. Lv, Z. Chen, S. Lu, M. Ni, J. Yan, X. Deng,
    Adsorption of heavy metal ions using zeolite materials of
    municipal solid waste incineration fly ash modified by microwave-assisted hydrothermal treatment, Powder Technol., 335
    (2018) 156–163. 
 
  -  Z. Li, L. Wang, J. Meng, X. Liu, J. Xu, F. Wang, P. Brookes, Zeolite-supported nanoscale zero-valent iron: New findings on
    simultaneous adsorption of Cd(II), Pb(II), and As(III) in aqueous
    solution and soil, J. Hazard. Mater., 344 (2017) 1–11. 
 
  -  X.M. Zheng, J.F. Dou, M. Xia, A.Z. Ding, Ammonium-pillared
	  montmorillonite-CoFe2O4 composite caged in calcium alginate
	  beads for the removal of Cs+ from wastewater, Carbohyd.
    Polym., 167 (2017) 306–316. 
 
  -  S. Babel, T.A. Kurniawan, Low-cost adsorbents for heavy
    metals uptake from contaminated water: a review, J. Hazard.
    Mater., 97 (2003) 219–243. 
 
  -  M. Wang, Y. Zhu, L. Cheng, B. Andserson, X. Zhao, D. Wang,
    A. Ding, Review on utilization of biochar for metal-contaminated
    soil and sediment remediation, J. Environ. Sci.-China, 63
    (2018) 156–173. 
 
  -  W. Zhang, S. Mao, H. Chen, L. Huang, R. Qiu, Pb(II) and Cr(VI)
    sorption by biochars pyrolyzed from the municipal wastewater
    sludge under different heating conditions, Bioresour. Technol.,
    147 (2013) 545–552. 
 
  -  E.B. Son, K.M. Poo, J.S. Chang, K.J. Chae, Heavy metal removal
    from aqueous solutions using engineered magnetic biochars
    derived from waste marine macro-algal biomass, Sci. Total
    Environ., 615 (2018) 161. 
 
  -  K.B. Cantrell, P.G. Hunt, M. Uchimiya, J.M. Novak, K.S. Ro,
    Impact of pyrolysis temperature and manure source on physicochemical
    characteristics of biochar, Bioresour. Technol., 107
    (2012) 419–428. 
 
  -  S. Wang, Y. Tang, C. Chen, J. Wu, Z. Huang, Y. Mo, K. Zhang,
    J. Chen, Regeneration of magnetic biochar derived from eucalyptus
    leaf residue for lead(II) removal, Bioresour. Technol.,
    186 (2015) 360–364 
 
  -  W. Yap, N.M. Mubarak, J.N. Sahu, E.C. Abdullah, Microwave
    induced synthesis of magnetic biochar from agricultural biomass
    for removal of lead and cadmium from wastewater, J.
    Ind. Eng. Chem., 45 (2017) 287–295. 
 
  -  M. Keiluweit, P.S. Nico, M.G. Johnson, M. Kleber, Dynamic
    molecular structure of plant biomass-derived black carbon
    (biochar), Environ. Sci. Technol., 44 (2010) 1247–1253. 
 
  -  J.H. Yuan, R.K. Xu, H. Zhang, The forms of alkalis in the biochar
    produced from crop residues at different temperatures,
    Bioresour. Technol., 102 (2011) 3488–3497. 
 
  -  J. Zheng, J. Dou, J. Yuan, W. Qin, X. Hong, A. Ding, Removal
	  of Cs+ from water and soil by ammonium-pillared montmorillonite/Fe3O4 composite, J. Environ. Sci.-China, 56 (2017) 12–24. 
 
  -  J. Lu, X. Jiao, D. Chen, W. Li, Solvo thermal synthesis and characterization
	  of Fe3O4 and γ-Fe2O3, Nanoplates, J. Phys. Chem.
    C., 113 (2009) 4012–4017. 
 
  -  I. Langmuir, The adsorption of gases on plane surfaces of
    glass, mica and platinum, J. Chem. Phys., 40 (2015) 1361–1403. 
 
  -  X. Zhou, Y. Liu, J. Zhou, J. Guo, J. Ren, F. Zhou, Efficient
    removal of lead from aqueous solution by urea-functionalized
    magnetic biochar: Preparation, characterization and mechanism
    study, J. Taiwan. Inst. Chem. E., 91 (2018) 457–467. 
 
  -  D. Karamanis, P.A. Assimakopoulos, Efficiency of aluminum-pillared montmorillonite on the removal of cesium and
    copper from aqueous solutions, Water Res., 41 (2007) 1897–1906. 
 
  -  S. Yang, C. Han, X. Wang, M. Nagatsu, Characteristics of
    cesium ion sorption from aqueous solution on bentonite- and
    carbon nanotube-based composites, J. Hazard. Mater., 274
    (2014) 46–52. 
 
  -  S. Yang, N. Okada, M. Nagatsu, The highly effective removal
    of Cs⁺ by low turbidity chitosan-grafted magnetic bentonite, J.
    Hazard. Mater., 301 (2016) 8–16. 
 
  -  A.G. Volkov, S. Paula, D.W. Deamer, Two mechanisms of permeation
    of small neutral molecules and hydrated ions across
    phospholipid bilayers, Bioelectrochem. Bioenergetics, 42 (1997)
    153–160. 
 
  -  E.R. Nightingale, Phenomenological theory of ion solvation
    effective radii of hydrated ions, J. Phys. Chem., 63 (1959) 1381–1387. 
 
  -  Z. Yin, Y. Liu, S. Liu, L. Jiang, X. Tan, G. Zeng, M. Li, S. Liu, S.
    Tian, Y. Fang, Activated magnetic biochar by one-step synthesis:
    Enhanced adsorption and coadsorption for 17β-estradiol
    and copper, Sci. Total Environ., 639 (2018) 1530–1542. 
 
  -  C. Souza, D. Majuste, M.S.S. Dantas, V.S.T. Ciminelli, Effect of
    zinc ion on copper speciation and adsorption on activated carbon,
    Hydrometallurgy, 176 (2018) 78–86. 
 
  -  K. Li, X. Wang, Adsorptive removal of Pb(II) by activated carbon
    prepared from Spartina alterniflora: Equilibrium, kinetics
    and thermodynamics, Bioresour. Technol., 100 (2009) 2810–2815. 
 
  -  C.K. Singh, J.N. Sahu, K.K. Mahalik, C.R. Mohanty, B.R.
    Mohan, B.C. Meikap, Studies on the removal of Pb(II) from
    wastewater by activated carbon developed from Tamarind
    wood activated with sulphuric acid, J. Hazard. Mater., 153
	  (2008) 221–228.