Adsorption is one of the best methods for wastewater purification. The fact that water quality is continuously decreasing requires the development of novel, effective and cost available adsorbents. Herein, a simple procedure for the preparation of a magnetic adsorbent from agricultural waste biomass and ferrofluid has been introduced. Specifically, ferrofluid mixed with wheat straw was directly pyrolyzed either by microwave irradiation (900 W, 30 min) or by conventional heating (550°C, 90 min). Magnetic biochars were characterized by X-ray powder diffraction, Mössbauer spectroscopy, textural analysis and tested as adsorbents of As(V) oxyanion and cationic methylene blue, respectively. Results showed that microwave pyrolysis produced char with high adsorption capacity of As(V) (Qm= 25.6 mg g–1 at pH 4), whereas conventional pyrolysis was not so effective. In comparison to conventional pyrolysis, one-step microwave pyrolysis produced a material with expressive microporosity, having a nine times higher value of specific surface area as well as total pore volume. We assumed that sorption properties are also caused by several iron-bearing composites identified by Mössbauer spectroscopy ([super] paramagnetic Fe2O3, α-Fe, non-stoichiometric Fe3C, γ-Fe2O3, γ-Fe) transformed from nano-maghemite presented in the ferrofluid. Methylene blue was also more easily removed by magnetic biochar prepared by microwaves (Qm=144.9 mg g–1 at pH 10.9) compared to using conventional techniques.

Abstract
 1. Introduction
 2. Materials and Method
  2.1. Preparation of magnetic carbon biochar
  2.2. Product analysis
  2.3. Zeta potential
  2.4. Sorption experiments
 3. Results
  3.1. Elemental analysis, X-ray powder diffraction and Mössbauer spectroscopy
  3.2. Textural properties and particle surface
  3.3. Sorption properties versus pH and adsorption isotherms
 4. Discussions
  4.1. Comments to the microwave pyrolysis
  4.2. Discussion on phase analysis and physicochemical properties
  4.3. Adsorption of arsenic (V) and MB with magnetic biochar
 5. Conclusions
 References

• Anton Zubrik(Institute of Geotechnics, Slovak Academy of Sciences) Corresponding Author
• Marek Matik(Institute of Geotechnics, Slovak Academy of Sciences)
• Michal Lovás(Institute of Geotechnics, Slovak Academy of Sciences)
• Katarína Štefušová(Institute of Geotechnics, Slovak Academy of Sciences)
• Zuzana Danková(Institute of Geotechnics, Slovak Academy of Sciences)
• Slavomír Hredzák(Institute of Geotechnics, Slovak Academy of Sciences)
• Miroslava Václavíková(Institute of Geotechnics, Slovak Academy of Sciences)
• František Bendek(Institute of Geotechnics, Slovak Academy of Sciences)
• Jaroslav Briančin(Institute of Geotechnics, Slovak Academy of Sciences)
• Libor Machala(Department of Experimental Physics, Faculty of Science, Palacký University)
• Jiři Pechoušek(Department of Experimental Physics, Faculty of Science, Palacký University)