Geopolymers have potential to be used as adsorbents for water treatment applications similar to zeolites. They can remove heavy metals and other ions from water through cation exchange and adsorption onto their surface. The document discusses how geopolymers can be synthesized from various source materials like fly ash or metakaolin, and outlines methods used to prepare geopolymer adsorbents. Preliminary results show that geopolymers have higher adsorption capacities and surface areas than the raw source materials. Further research is needed to optimize synthesis conditions and evaluate performance for different water matrices.

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Geopolymers & water treatment
Tero Luukkonen, 9.7.2014, Geopolymer camp, St. Quentin

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How can geopolymers be utilized in water treatment (as adsorbents)?
•Adsorbent = material used to attach molecules (in this case) from water to its surface.
•Geopolymers are known to be amorphous analogues of zeolites.
•Zeolites can be used as ion exchangers in various water treatment applications: soluble heavy metal removal, water softening (Ca2+, Mg2+), ammonium removal etc. depending on the pore size distribution.
•Therefore: it could be assumed that geopolymers could have similar properties.
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Number of publications
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Database: Scopus
Keyword: geopolymer AND *sorbent
Relatively new approach?

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Mechanisms how geopolymers work as sorbents
•Geopolymers work similarly to zeolites:
–Cation exchanger (releasing Na+ or K+ if prepared by geopolymerzation in alkaline medium)  only cations can be removed from water!
–Adsorbent (chemisorption or physisorption).
–Increase of pH due to residues of alkalis used in synthesis  precipitation of metal hydroxides. This effect decreases as geopolymer is being washed.
•Smaller particle size / more porous structure  more surface area  more active sorbent.
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Geopolymers in water treatment: some process options
1. As an active filter medium
•Geopolymer can be re-used (regenerated)
2. As a slurry made from powder
•To be dosed to sedimentation process.
•Geopolymer not re-usable.
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Water to be treated
Filter containing geopolymer adsorbents, diameter ≈ 2 – 3 mm
Treated water
Water to be treated
Treated water
Sludge
Geopolymer adsorbents, diameter = 10 - 100 μm
Water
Geopolymer slurry

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How to prepare geopolymer adsorbents (some examples)?
•Dissolution of rice husk ash and NaOH pellets (97% purity) in distilled water. Metakaolin was added after the dissolution of silica. (Chem. Lett. 2014, 43, 128–130)
•The zeolitic tuff and kaolinitic soil were mixed in different ratios, and then the sodium hydroxide solution was added. After molding, compacting, curing at 80ºC, the samples were ground and sieved into aggregate size between 250-500 μm. Then the product was washed with excess amount of distilled water (to remove unreacted alkali), dried at 100ºC and kept in a desiccator. (Advances in Materials Physics and Chemistry, 2012, 2, 119-125)
•Fly ash was mixed with 14M NaOH solution using a mass ratio of 1.25. Geopolymer paste then started to form, which was mixed for 5–10 min to give complete homogenization. Vibration with ultrasonification to de-foam and enhance the dissolution of Al–Si material (fly ash) in the alkaline solution. The mixtures were then procured for 24 h at room temperature. The paste was then poured in a cylindrical container which was closed for curing at a temperature of 105 ◦C for 24 h in an oven. (Journal of Hazardous Materials 188 (2011) 414–421)
•How we did it: mixing of metakaolin with alkaline solution (10 M NaOH + Na-silicate). Curing at room temperature for 48 h. Crushing to required particle size. Washing with distilled water. Drying at 105 C.
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d > 1 mm
50 μm < d < 1 mm
d < 50 μm

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What geopolymer adsorbents have been used to remove (examples)
•Cs + (Chem. Lett. 2014, 43, 128–130)
•Cu2+, Ni2+, Zn2+, Cd2+ and Pb2+ (Advances in Materials Physics and Chemistry, 2012, 2, 119-125)
•Pb2+ (Journal of Hazardous Materials 188 (2011) 414–421)
•Pb2+, Cu2+, Cr3+, and Cd2+ (Applied Clay Science 56 (2012) 90–96)
•Cs+ (Applied Clay Science 87 (2014) 205–211)
•Cu2+, Ni2+ and Pb2+ (Chemical Papers 67 (5) 497–508 (2013))
•Cu2+ (Journal of Hazardous Materials B139 (2007) 254–259)
•Our research: Ni, As and Sb in real mine wastewater matrix.
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Adsorption studies @ Kajaani University of Applied Sciences
•Preparation of fly ash, metakaolin and blast furnace slag based geopolymer adsorbents.
•Batch and continuous sorption tests with (real) mine wastewaters:
–Adsorption capasities, q [mg/g]
–Adsorption isotherms (Langmuir, Freundlich, etc.)
–Kinetics (reaction kinetics equations)
–Regeneration
•Characterization of geopolymers:
–XRD (identification of crystalline phases)
–XRF (elemental composition)
–SEM-EDS (surface morphology, elemental composition)
–BET (surface area, pore volumes)
–IR (identification of surface chemical groups)
–Total surface charge
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XRD: metakaolin geopolymer
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•Amorphous halo at 20 to 40 °2Theta

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BET: surface area
•Specific surface area:
–Metakaolin 11,50 m2/g
–Metakaolin GP 22,42 m2/g.
–(For comparison: activated carbon typically 1000 m2/g).
•Average pore width
–Metakaolin 18,16 nm
–Metakaolin GP 30,97 nm
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Some preliminary results: removal of Ni
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Removal of Ni with BFS geopolymer: kinetics
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Removal of Ni with BFS and MK geopolymers: effect of pH and comparison with starting material
Adsorbent dosing: 5 g/L
Real mine wastewater
pH = 8
Adsorbent dosing: 5 g/L Real mine wastewater Shaking 24 h

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Conclusions and remarks at this point
•Geopolymerization as a process to produce efficient adsorbents (cation exchangers) seems promising.
•Possible to reuse via regeneration (for example concentrated NaCl solution).
•Geopolymerization increases adsorption capacity, specific surface area and average pore width.
•Some further research questions:
–Na vs K hydroxide / silicate in alkaline geopolymerization?
–Applicability of acidic medium geopolymers?
–Optimization of geopolymer synthesis in terms of specific surface area?
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Kaolinite
Metakaolin
Metakaolin geopolymer
SEM-EDS, x700, red=Al, green=Si, blue=O
THANKS FOR YOUR ATTENTION!

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