GP/AAMs compositions for water and wastewater treatment reported in literature.
The EU has the ambitious goal to transition from linear to circular economy. In circular economy, the old saying of “one’s waste is the other’s treasure” is being implemented. In this chapter, valorisation of industrial side streams, traditionally branded as waste, is discussed with respect to their applications as raw materials for new adsorptive products – geopolymers (GP) and alkali-activated materials (AAM) – as adsorbents in wastewater treatment. The chemical nature and structure of materials generally have great influence on GP/AAM adsorption capability. The approaches used for the raw materials preparation (chemical or physical) prior geopolymerization to increase the adsorption capacity of the final products will be discussed. Adsorption properties and performance of GPs/AAMs towards various contaminants are described, and the latest research on testing those materials as water remediation are reviewed. Special attention is paid to regeneration of exhausted materials and available resource recovery options that the regeneration approach opens. New forms of geopolymer adsorbent such as foams or core-shell structures are described and in the last part of the chapter, a short economic evaluation of resource recovery models is provided.
- metal removal
- nutrient recovery
- geopolymer composite
- wastewater treatment
The United Nations have ratified 17 goals of sustainable development, of which responsible consumption and production is directly, while economy, innovative industry, infrastructure, and climate action are indirectly related to circular economy and the need of sustainable production . Sustainability in the processing industries can be applied along the main value chain, e.g. from metal extraction to metal recycling, but can also be applied to the associated waste materials. Copper and iron mining alone are estimated to generate yearly about 5 bn tons of tailings , i.e. the fraction of the processed ore, after extraction of the valuable minerals. Finding a way to successfully reuse vast amount of this material and other waste sources is a great step towards circular economy.
While recycling an initial waste or side-stream, the material can be upcycled, meaning the newly derived product is of higher intrinsic value and properties or downcycled, where the new material has lower value. A prominent example for downcycling is the reuse of plastic bottles as fleece and carpet material. The material has less intrinsic value, because the carbon chains of the plastic polymer are shortened. Geopolymerization of industrial side streams is an upcycling process, as the geopolymer (GP), utilized as concrete and binder, has a higher value than the initial industrial by-products. To obtain upcycling, energy is put into the system, however, since tailings have usually been milled, the material has already undergone energy intensive steps and can therefore readily be used as starting blocks for geopolymerization. Utilizing tailings for upcycling into GPs, is therefore beneficially in terms of waste management, process energy, and emission of greenhouse gases, as the energy used in the beneficiation process is passed onwards into the geopolymerization process. Upcycling often requires further energy sources to achieve higher valuable material. In geopolymeration or alkaline activation this means the addition of chemicals, and in some occasions, such as analcime tailings, the addition of thermal energy .
In this chapter the valorization of high volume, inorganic side streams from mining, chemical industries, steel processing, and waste incineration into new adsorbents useable for water treatment is discussed. The purpose is to show how the material undergoes value change from side stream to potentially highly functional material.
As every tailing and every ash has a different chemical and mineral composition, tailoring of the properties of resulting adsorbents is possible by careful choosing of precursor materials. Aluminosilicates form the backbone of the geopolymer structure, but ion exchange, channel size, and physical properties are affected by the minerals used for geopolymerization [4, 5, 6]. Lastly, by controlling of the geopolymerization conditions, also the macroscopic structures can be developed by using various manufacturing methods from foaming to granulation.
The ultimate goal of using GPs/AAMs in water purification is to be able to recover valuable materials such as nutrients or battery chemical metals from contaminant-rich wastewater streams. In other words, the target is to use one industrial side stream to recovery of valuable material from another side stream or waste water in order to multiply circular economy potential.
2. Raw materials and preparation of GP/AAM materials for water treatment applications
This section summarizes different types of aluminosilicate precursors, occuring naturally or derived from industrial processes. Materials, which are currently abundant and/or urgent to dispose of, fall within the ambit of the section, but cover only water treatment applications not geopolymer production for construction industry, e.g. substitutes for Portland cement or as tailings’ covering.
2.1 Conventional and new sources of aluminosilicate precursors for GP/AAM preparation
The composition of FA varies widely as it is derived initially from various primary sources: municipal waste/sludge co-incineration, different coal types, or subspecialized byproducts from industrial treatment plant (paper, forestry industry or agriculture). The combustion and cooling processes have profound impact on the characteristics of FA (particles size, shape, surface area, uniformity, etc.) as well as its composition and impurities’ inclusion.
Mainly, ASTM C 618 specification is applied to indicate the class of FA used for geopolymer preparation; however, a local/field or an unspecified labelling is also common. Coal FA (class F [8, 9] and C ) has been extensively considered as an aluminosilicate source for GP production, while the exploitation of biomass and co-incinerated FAs is less common [11, 12]. On the other hand, the utilization of these FAs particularly in the GP production for water treatment sector might be also beneficial. It would reduce the FA accumulation in landfills, and improve adsorbents’ LCA in comparison with metakaolin-based GPs.
Although FAs were studied as adsorptive materials previously [7, 13, 14], concerns on potential toxicity of impurities and convenience of use have encouraged to seek more suitable forms of FA-based materials for water treatment sector.
Pre-treatment of FAs and IBAs with various chemicals were suggested in order to reduce their toxicity and to meet the environment requirements of pristine materials or/and GPs/AAMs based on them [14, 25, 26, 27].
The accumulation of BOFS has become a significant issue due to its generation in large quantity, high disposal costs, and unsuitableness in cement industry due to high iron oxide content. Sarkar et al. adopted BOFS as a raw material for obtaining of GPs and investigated Ni2+ , Zn2+ , and F−  removal. BOFS was used by Sithole et al. as a precursor for AAM preparation [42, 43]. In order to achieve highly porous structures for percolation column tests, a foaming agent (hydrogen peroxide) was added.
Recently, much attention has also been paid on how zeolite could be synthesized from low-cost materials . GP-zeolite composites and zeolite-like GPs are two different categories of adsorptive materials, which have recently attracted increased interest . GP-zeolite composites are hybrid materials, unite the advantages of both constituents. The GP here serves as a durable support, while the zeolite provides a high surface area, porosity, and adsorption capacity. For instance, metakaolinite–zeolitic tuff GPs have been proposed in . The report clearly showed the beneficial influence of the zeolitic tuff addition into a starting mixture on the microstructure and the adsorption potential of GPs. Andrejkovičová et al.  prepared metakaolin-based GPs blended with by 25, 50 and 75% of Nižný Hrabovec zeolite. It was shown that the zeolite particles are responsible for the higher amount of crystalline phases, producing a more compact and firm microstructure of blended GPs. The amount of blender has significant influence on the order of adsorbed metals and on the adsorption capacities of the formulations. Hayashi et al.  incorporated clinoptilolite into GPs though sol–gel protocol in order to further use of the resulting coatings for heavy metal ion adsorption.
It should be noted that zeolitic phase could be incorporated into GPs’ structures not only externally. Zeolite-like crystalline phases could be derived from synthesis routes through fusion method or even at moderate temperatures leading to zeolite-like GP structures. Javadian et al.  converted FA into a mesoporous aluminosilicate adsorbent through a fusion method at 600°C. Deng et al. showed that a hydrothermal synthesis of zeolite-like materials from IBA with higher crystallinity than through a fusion method is possible . Similarly, Visa  converted FA into zeolite through a hydrothermal process. Rios et al. synthesized zeolite-like GPs from metakaoline at 100°C through the hydrothermal procedure . Studies reported indicate that such materials have higher surface area and porosity than GPs/AAMs obtained through simple alkaline activation. Although the ultimate set of preferable conditions to form a GP instead of a zeolite are still under discussion, ratios Si:Al > 1.5 have been empirically established as providing more amorphous structures .
Not infrequently, industrial side streams cannot be used alone for geopolymerisation due to disharmonious Si/Al molar ratios. Therefore, by-products are commonly used as mixtures of aluminosilicate sources . Table 1 summarizes the studies on different compositions of GPs/AAM that have been proposed for water and wastewater treatment applications. An afford was made to collect and match the precursors, synthetic protocol specificity, and distinctive characteristics resulting materials.
|GP/AAM||Precursor/additives||Preparation method, prime oxide ratios||Surface Area/Pore Volume/Pore size||Type/form of GP||Ref.|
|MK-GP with TiO2||MK||HT||27.21 m2/g 0.207cm3/g|
|Porous/Spheres, 2–4 mm|||
Si/Al = 1.7
SiO2/Al2O3 = 1
SDS 0.06 wt%
SiO2/Al2O3 = 1.6
|Porous/spheres 2–4 mm|||
SiO2/Al2O3 = 5
|̶||Foam/Powder <100 μm|||
|AA + SSM|
SiO2/Al2O3 = 1.6
0.05 mL/g 11.5 nm
|Bulk/Spheres 2–4 mm|||
SiO2/Al2O3 = 3.2
|39.24 m2/g||Bulk/Powder 150 μm|||
|MK-GP functionalized with CTAB||MK|
|Bulk/rubbles, 1.5 mm|||
SiO2/Al2O3 = 1.5
SiO2/Al2O3 = 3.2
|39.24 m2/g||Bulk/Powder, 150 μm|||
|MK-GP activated with hull ash||MK|
SiO2/Al2O3 = 3.18*
|̶||Porous/Spheres 2–3 mm|||
|MK/FA-GP||MK:FA 50:50 wt%||SSM|
0.5 wt% H2O2,
1.5 wt% SDS
FA class C
SiO2/Al2O3 = 2.7
SiO2/Al2O3 = 3.1
SiO2/Al2O3 = 2.31**
|̶||Bulk/Powder, 200 μm|||
SiO2/Al2O3 = 4.0
SiO2/Al2O3 = 2–8
|MK-GP||Waste MK Aluminum scrap recycling waste|
SiO2/Al2O3 = 1.25**
|15.95 m2/g||Bulk/Granules 4–11.2 mm|||
Granules 0.5 mm
functionalized with HDTMABr
|̶||Bulk/Powder, 53 μm|||
SiO2/Al2O3 = 3.96
SiO2/Al2O3 = 2.13
SiO2/Al2O3 = 7.01
SiO2/Al2O3 = 3.60
26.24 nm MK/aloxid GP
23.18 nm ANA-GP
21.69 nm ANA/aloxid-GP
|Bulk/Powder, 63–125 μm|||
SiO2/Al2O3 = 1
|MK-GP functionalized CTAB||Calcinated halloysite clay||Precipitation|
SiO2/Al2O3 = 2.91
functionalized with CTAB
|Bulk/Powder, 125 μm|||
Magnetite 5 wt%
SiO2/Al2O3 = 4.55* H2O2
functionalized with Cr
silica fume 9:1 (w/w)
SiO2/Al2O3 = 1.90**
SiO2/Al2O3 = 2.14**
|̶||Bulk/Powder, 355 μm|||
SiO2/Al2O3 = 2.45**
|27 m2/g||Bulk/rubbl 1.0–0.3 mm|||
SiO2/Al2O3 = 4.0
SiO2:Al2O3 = 4
SiO2/Al2O3 = 4.55
SiO2/Al2O3 = 1.5
|̶||Bulk/Granules, 4–8 mm|||
functionalized with K4Fe(CN)6
SiO2/Al2O3 = 3.60
|MK-GP/graphene oxide hybrid||MK graphene oxide 10 wt%||AA|
SiO2/Al2O3 = 0.45
< 0.5 mm
SiO2/Al2O3 = 2.03
|FA-GP||Fly ash, 75 μm||FM|
SiO2/Al2O3 = 1.98*
|FA-GP Iron-enriched||Calcinated FA,|
< 70 μm
SiO2/Al2O3 = 1.00
Fe2O3/Al2O3 = 0.151
|FA-GP modified with iron||Coal fly ash||AA|
SiO2/Al2O3 = 1.43*
SiO2/Al2O3 = 4.61
|̶||Bulk/Powder, 71–90 μm|||
|FA/IOT -GP||Fly ash|
|6 nm - 360 μm||Porous/Cubes|||
SiO2/Al2O3 = 1.12**
|Bulk/Powder, 74 μm|||
SiO2/Al2O3 = 0.69**
|Bulk/Powder, 74 μm|||
|FA-GP||Fly ash C||AA|
SiO2/Al2O3 = 3
|FA-GP/LECA||Fly ash C LECA support||AA|
SiO2/Al2O3 = 1.5
|̶||Bulk/Granules, 4–8 mm|||
SiO2/Al2O3 = 5.36
|̶||Bulk/Powder, 71–90 μm|||
SiO2/Al2O3 = 3.49*
iron oxide hybrid
Fe2O3 5 wt%
SiO2/Al2O3 = 3.30**
|60.75 m2/g||Bulk/Powder, 50 μm|||
|FA-GP/Graphene hybrid||Fly ash graphene (1 wt%)||AA|
SiO2/Al2O3 = 3.41**
|FA/BFS-GP||Fly ash BFS||HT|
SiO2/Al2O3 = 3.23*
|FA-GP||Boiler fly ash|
< 80 mesh
SiO2/Al2O3 = 2.75**
SiO2/Al2O3 = 3
|Bulk/Powder, < 74 μm|||
|FA-GP/Polyethersulfone hybrid||Fly ash||AA|
SiO2/Al2O3 = 3.05
|168.3 m2/g||Bulk/Powder, 150 μm|||
|FA/Z-GP||Calcinated fly ash||SiO2/Al2O3 = 1.61**||̶||Bulk/Powder|||
|FA-GP||Fly ash||AA||131.4 m2/g||Bulk/Powder, <105 μm|||
|FA-GP||Coal Fly ash||FM|
SiO2/Al2O3 = 1.25*
Si/Al = 2.2
|Bulk/Powder, 125–212 μm|||
|FA-GP||Fly ash F,|
≤ 177 μm
SiO2/Al2O3 = 2.97**
SiO2/Al2O3 = 2.10**
SiO2/Al2O3 = 5.42
|Bulk/Powder, 150 μm|||
|FA-GP||Rice husk ash, waste alum cans||HT|
SiO2/Al2O3 = 1.82*
|FA-GP||Fly ash class C and F||AA|
SiO2/Al2O3 = 6.6**
SiO2/Al2O3 = 10.9** (class F)
aluminum powder, anionic surfactant
|IBA-GP/Graphene hybrid||Bottom ash graphene||AA|
0.15 wt% Mn2+
19.5 wt% CuO
Particles, 0.180–0.315 mm
SiO2/Al2O3 = 4.40*
|Bulk/Powder, 63–125 μm|||
SiO2/Al2O3 = 3.2
|̶||Bulk/Powder, 63–125 μm|||
|BFS –GP/graphene hybrid||BFS graphene 0.01 wt%||AA SiO2/Al2O3 = 2.61*||146.17 m2/g|
|Bulk/Powder, 250-315 μm|||
|BFS –GP/barium modified||BFS||AA|
SiO2/Al2O3 = 4.00**
|Bulk/Powder, 63–125 μm|||
SiO2/Al2O3 = 11.5**
|Bulk/Particles, ∼0.1 mm||[39, 40]|
SiO2/Al2O3 = 4.02*
0.3 wt% SDS
|Porous/Spheres, d ≈ 100 μm|||
|Silicomanganese slag-GP||Silicomanganese slag|
SiO2/Al2O3 = 1.44**
|Bulk/Particles, 0.16–0.315 mm|||
|BOFS-GP modified with Ni(II) or Zn(II)||BOFS-GP||AA||Zn/LDS-GP|
|Porous/Powder, ∼0.1 mm|||
SiO2/Al2O3 = 3.08*
SiO2/Al2O3 = 70.65**
Fe2O3/Al2O3 = 188
Fe2O3/SiO2 = 2.66
|233.8 m2/g||Bulk/Microspheres, 75–300 μm|||
|Steel slag/fly ash/analcime-GP||Steel slag fly ash||HT|
SiO2/Al2O3 = 2.01**
SiO2/Al2O3 = 5.26
|Bulk/Powder, 150 μm|||
|EAFS-GP||electric arc furnace slag||AA|
SiO2/Al2O3 = 2.02*
|BOFS-GP||Basic Oxygen furnace slag||AA|
SiO2/Al2O3 = 3.31**
|Bulk/Sphere, 75–300 μm|||
SiO2/Al2O3 = 1.88**
|clay/gangue microsphere -GP||Kaolin|
coal gangue 50/50 wt%
SiO2/Al2O3 = 4.0
|Bulk/rubbles, 0.45–0.15 mm|||
|Clay-GP/Fe3O4 hybride||Calcined bentonite clay||AA||2.32 m2/g|
|Clay-GP||Lateritic clay, 58 μm||AA||17.441 m2/g|
|Bulk/Powder, 58 μm|||
|Natural tuff-GP||Volcanic tuff||AA|
SiO2/Al2O3 = 3.74**
|̶||Bulk/Powder, < 200 μm|||
|Alumino silicate-GP||Alumino silicate powder||AA|
SiO2/Al2O3 = 4
|Bulk/Monoliths or granules|||
|Synthetic GP||Chemosynthetic Al2O3-SiO2 powder||SSM|
SiO2/Al2O3 = 2
|Chitosan modified geopolymer||Aluminum salt and silica solution,|
SiO2/Al2O3 = 3.06**
|OTB-GP||Pyrophyllite mine waste samples||AA|
SiO2/Al2O3 = 2.39**
|̶||Bulk/Powder, <45 μm|||
|OTB-GP||Gold mine waste||FM||74.92 m2/g||Bulk|||
|OTB-GP||Gold mine tailings|
|Municipal solid waste-GP||Sludges||FM|
SiO2/Al2O3 = 3.12**
|Municipal solid waste-GP||Municipal solid waste biochar||AA||6.5 m2/g|
|Dolochar ash based geopolymer||Dolochar|
< 100 mesh
SiO2/Al2O3 = 4.97**
|Bulk/Particles, ≈ 0.1 mm|||
2.2 Forms and manufacturing techniques of GPs/AAMs for water treatment
Originally, a basic composition applied for manufacturing GP/AAM adsorbents consisted of an alumosilicate precursor, an alkali, and an additional source of silicate in a form of water glass. Initially, both sodium and potassium forms of alkaline activators were used to induce geopolymerization. In the vast majority of the research reviewed, sodium alkaline and water glass are used in the activation process. It was shown by Bakharev that dissolution rates of the minerals was higher when a sodium form is used . Luukkonen et al.  found that adsorption characteristics of metakaolin-based GP prepared with NaOH is better than with KOH in case of ammonium removal. An in-depth discussion of G chemistry and vivid explanations could be found in the latest reviews [57, 150, 151].
Forms and manufacturing techniques of GPs/AAMs for water treatment application are emerging and evolving constantly. In the first instance,
3. Properties and performance of GPs/AAMs towards various contaminants
Despite the fact that the first identification of GPs as unconventional construction materials was in 1979 , broader applications of GPs/AAMs started in late 90s. Although GPs/AAMs are to be considered by some authors as an economic alternative to zeolites or activated carbons for water purification, the lack of real cases reported is obvious. To urge commercial importance, GP/AAM adsorbents should be readily available, economically feasible, steady in characteristics, and easily regenerated. Several comprehensive reviews on the GP/AAM materials for the water treatment sector have been published just recently [57, 150, 151, 153]. Therefore, in this section the bright and promising works will be highlighted as well as challenges and trends for future studies revealed.
In order to obtain adequate adsorption parameters, an excessive alkaline residue in GP/AAM should be washed out properly (pH 7 ± 0.5 within 24 h required) . Otherwise, the increment of pH of aqueous solutions containing heavy metals will favor the hydroxide precipitation process, leading to wrong result interpretation. For porous GPs, washing away the excessive alkalis resulted in the increment of total porosity , which led to better performance. Moreover, excessive alkalis were used intentionally to neutralize AMD  and remove metal ions. However, a strict protocol must be followed to characterize newly designed materials.
Selective adsorption is relies on several factors such as a metal ion activity, hydration radius and free energy of hydration, and a pore size distribution of GP.
Geopolymerisation by itself could lead to the formation of new ion-exchange sites at the GP surface, but additives in composite formulations could have even higher influence the adsorption characteristics.
An ionic exchange reaction between the heavy metal ions and sodium ions has resulted in heavy metal removal by the metakaolin GP . The adsorption selectivity of heavy metal ions by the GPs at pH 4 in multi-component solution was in the following order: Pb2+ > Cd2+ > Cu2+ > Cr3+, while qe [mg/g]: 100 > 76 > 55 > 10. The order of adsorption was in accordance with the hydrated radius and free energy of hydration for selected ions. However, the free energy of hydration and the activity for Cr3+ are all higher compared to those of other metals, though its adsorption rate does not correspond to the assumed order. The selectivity towards Cr3+ was be explained through its ionic status. When the pH exceeded 4, Cr3+ transforms to Cr(OH)2+, which might lead to its lower adsorption ability. It is also noted that at lower pH, the balancing ions present on the GP surface tend to be replaced by the hydrogen ions instead of the metal ions that lead to lower capacity at acidic pH.
Lopez et al.  investigated the selectivity of metakaolin-based GPs in multicomponent solutions (Pb2+, Cu2+, Cd2+, Ni2+, Zn2+ and Cs+). For a composition with Si/Al ratio 2, the best capacities and selectivity towards Pb2+ and Cs+ were observed. The adsorption selectivity for the mixture of metal ions was in the following order Cs+ > Pb2+ > Cu2+ > Zn2+ > Ni2+ > Cd2+, while qm [mg/g]: 43 > 35 > 15 > 3 > 1 > 2. The adsorption capacity for individual elements were higher: 57 mg Pb2+/g > 52 mg Cs+/g > 46 mg Cu2+/g > 14 mg Cd2+/g > 9 mg Zn2+/g > 4 mg Ni2+/g. Moreover, the effect of solution salinity (NaCl, 5% and 10%, wt) was studied, and no considerable effect on the adsorption order of metal ions or GP capacity in multi-composition solution was found. The authors presumed the existence of at least two types of binding sites with different affinities toward the metal ions to explain such a tolerance.
Selectivity of GP composites with zeolite filler was studied by Andrejkovičová et al. . The highest adsorption was observed for Pb2+ for all the GPs obtained, while an adsorption order was as follows: Pb2+ > Cd2+ > Zn2+ > Cu2+ > Cr3+. The adsorption of Cu2+ and Cr3+ increased as the amount of metakaolin in the GP increased, whereas the composite with 25% zeolite doping had higher adsorption characteristics towards Pb2+, Cd2+ and Zn2+. GPs prepared from zeolitic tuff and kaolinitic soil by El-Eswed et al.  showed totally different order of adsorption: Cu2+ > Pb2+ > Ni2+ > Cd2+ > Zn2+. Moreover, the adsorption order strongly depended on the GP composition, although Cu2+ and Pb2+ adsorption has always prevailed.
The ability of BFS- and metakaolin-based GPs to remove Ni2+ and metalloids (As and Sb) in form of oxyanions was shown in . Both adsorbents completely removed Ni2+ that most likely was associated with precipitation of its hydroxides on the GPs, while both metalloid oxyanions were adsorbed by BFS-GP equally. Another remarkable merit is that the adsorption capacities were obtained with real matrixes (spiked mine effluents), and were 4.42 mg/g, 0.52 mg/g, and 0.34 mg/g for Ni2+, As3+, and Sb3+, respectively. It is specified by the authors that the low capacities could be a result of competition of some matrix ions (Sr, Ca, Mg, Mn) with the target ions for binding sites.
Researches with increasing frequency pay attention to this problem and try to demonstrate the removal efficiencies with real samples. Removal of Ca2+ and Mg2+ from intact groundwater was examined in  on kaolin-based GP. With adsorbent dose of 1 g/L, the removal rate were 37.5% and 16.2% for Ca2+ and Mg2+, respectively. Metakaolin-based GP was tested by Kara et al.  for Mn2+ and Co2+ removal from real wastewater. The removal rates in real wastewater decreased from 97.5% to 53.01% and 94.6% to 39.12% for Co2+ and Mn2+, respectively. The results demonstrated that the adsorption performance affected negatively by the coexistence of some other cations and/or anions in the adsorption medium. Bentonite-based GPs were used for heavy metals removal from synthetic wastewater . Porous biomass FA-based GPs were used in  for simultaneous removal of heavy metals from wastewater samples. Mixed FA/metakaoline-based GPs were used in  for Cu2+ removal from real wastewater. In the showcase, the adsorption capacity of GPs towards Cu2+ decreased by 27% as compared to synthetic samples. Sithole et al. treated acidic industrial effluents by FA/BOFS-based GPs [42, 43]. New GPs containing hollow gangue microsphere were applied for Zn2+ removal from smelting plant wastewater in . At an adsorbent dose of 30 g/L, a complete Zn removal was observed. The distinctive aspect of the reported cases was that a complex composition of treated solutions is likely to decrease substantially capacity of the GP. Thus, the adsorption capacities obtained for the ideal laboratory conditions should be primary used as the guiding not decision-making parameters.
The removal of phosphorus was attempted in  with a pervious FA-based GP. The removal rate increased with the increase of pH. Up to 85% of phosphorus were removed from a treated wastewater. Simultaneous removal of ammonium and phosphate by composite metakaolin/BFS-based GPs was demonstrated in . Phosphate removal was enhanced in presence of ammonium. At slightly alkaline conditions (pH 7–8), the removal rate towards phosphate ions was relatively high (>86%), whereas the ammonium removal up to 35% was also achieved. FA-, BFS- and fiber sludge GPs were investigated as promising adsorbents for phosphorous removal from diluted solutions. The capacities at initial phosphate concentration of 100 mg/L are 26 mg PO4/g for BFS-GP, 36 mg PO4/g for FAF-GP, and 43 mg PO4/g for FSHCa-GP .
Sulfate ions were removed by barium-modified BFS-based GPs . Adsorption capacities were 91.1 and 119.0 mg SO4/g for model solution and mine effluent, respectively. The surface complexation or precipitation of barium sulfate were suggested as probable removal mechanisms.
Removal of halides by GPs/AAMs is an emerging topic. For this end, composite or functionalized materials are designed. Removal of F ̶ was demonstrated by slag-based GP microspheres modified with CeO , Fe2O3 , and bivalent metallic species  with capacities towards the contaminant 127.7 mg/g, 59.8 mg/g, and 60 mg/g (zinc impregnated BOFS-GP), respectively. A metakaolin-based GP functionalized by surfactant was developed for efficient removal of radioactive iodide . High concentrations of competitive anions had limited influence on the adsorption process.
Oxidative degradation or photodegradation after adsorption have been specified by authors as primary mechanisms of organic pollutants’ removal. Although conventional GPs have been reported for these purposes [86, 89, 104, 126, 139], they would rather have had low adsorption/degradation characteristics. Hybrid or composite materials were proposed to improve the removal efficiency of organic pollutants. Thus, graphene [120, 132, 133, 165], TiO2 [88, 98, 105], CdS , various metal oxides [101, 106, 135] were introduced in GP matrix in order to enhance degradation abilities of resulting materials.
4. Regeneration of GPs/AAMs and further resource recovery options
In last a few decades, significant improvements were made in both efficiency and economy in removal of metal(oid)s and other substances by adsorbents. Nevertheless, regeneration and recycling of used adsorbents, or recovery of the removed species from the desorbing agents are still rarely reported. For regeneration and reuse of GPs/AAMs, various possible regenerating agents such as acids, alkalis and chelating agents could be used. Only a few of the reported studies were focused on recovery of adsorbed (from saturated adsorbents) and desorbed (from regenerating agents) metals [11, 87, 96, 131]. However, for industrial application and success completion of new GP/AAM adsorbents on the market, research studies on number of adsorption–desorption cycles are in high demand. Moreover, revenues gathered from resource recovery options will have a decisive role in further technology implementation.
The regeneration of metakaolin-based GP by sodium chloride under alkaline conditions after ammonium adsorption for the first time were demonstrated in . Three adsorption–desorption cycles were carried out with a steady removal efficiency. Sodium chloride and sulfate, potassium sulfate and phosphate were studied in  as regenerating agents for saturated metakaolin-based GPs. Sodium sulfate showed better results during five cycles under continuous sorption–desorption experiment, only 34% of an initial overall capacity of the GP were lost. Sodium chloride regenerant was also efficient, but only 55% of ammonium could be removed after 5th desorption cycle. The same adsorbents were used to test a nitrogen recovery option in a laboratory-scale demonstration setup . The layout consisted of an adsorption/desorption unit and Liqui-Cel® membrane. A liquid phase obtained during adsorbent regeneration was purified in the membrane contactor in order to recover ammonium nitrogen as ammonium sulfate or phosphate. The purified regeneration solution was used repeatedly for further adsorbent regeneration. Several regeneration-purification cycles were conducted to estimate system sustainability and chemical consumption demand. Operational conditions of a membrane process such as shellside and lumenside feed flows, temperature, and pH were adjusted to gain maximal capacity of the setup. One membrane contactor (2.5 × 8-inch Liqui-Cel) was used under following operational conditions: 100 L/h shellside and 60 L/h lumenside feed flows, 40°C working temperature, pH ≥ 10. Technical sulfuric or phosphoric acids, up to 5%, were used as lumenside phases. The concentration of ammonium-content salt in a resulting received phase were 17% and 22% for phosphate and sulfate salt, respectively.
Metal recovery from GPs/AMMs via ion-exchange mechanism can only take place if physical adsorption occurred and the pH was low enough to prevent precipitation of metal hydroxide during adsorption process. Acids of over 0.1 M strength affect the structure of the GPs, and while metals are regenerated by acid washing, the reuse of adsorbents are diminished both in batch  as also in continuous mode [87, 167] experiments. Mild acid washing with 0.01 M H2SO4 or HNO3 removed metals from GPs efficiently in short time (1–2 h). It has also been shown that the adsorption capacity after mild acid washing could increase , which could be explained by exchange of Na+ with easier replaceable H+ cations. Selective desorption of copper has been observed by ammonia. A linear desorption ability with respect to ammonia concentration was observed, and complete desorption being possible by 10% ammonia solution [50, 61].
Sequential desorption tests of Cd2+ have been conducted on a loaded metakaolin GP, establishing the percentages of physically adsorbed, ion-exchangeable, EDTA extractable, and residual forms of metal . The authors showed that physical adsorption is negligible, and ion-exchange with MgCl2 constituted to only 2–8% of adsorbed Cd2+. The bulk amount of Cd2+ adsorbed by the metakaolin GP was EDTA extractable, and the adsorbent remained 85% of its adsorption capacity after EDTA desorption for 5 cycles. Luukkonen  and Naghsh  suggested the efficient metal desorption by 5% NaCl. However, care must be taken since the balancing ions can form a positively charged film on the adsorbent surfaces. El Esweed et al. have achieved ion-exchange based desorption of Cu2+ by 0.1 M NaCl . From all the studies reported, only Cd2+ has been shown to be desorbed at pH > 8 with NaOH solution, achieving 24–84% desorption .
5. Environmental impact and costs of treatment with GPs/AAMs
An efficient use of GPs/AAMs in real wastewater treatment practices including economic evaluation is little investigated. Above all, these adsorbents show rather low selectivity, and therefore the ubiquitous metal ions (Na+, Ca2+, Mg2+, Fe3+) present in wastewater solutions demonstrate either competing interaction with the target ions, or the interaction has not been studied . Additionally, for economic and ecological assessment is essential that the adsorbent would be regenerable . To be economically successful, exhausted adsorbents need to pass the non-hazardous leaching criteria of the adsorbed materials, while the amount of waste regenerated should be as little as possible. This means that the adsorption-regeneration cycle needs to be performed as often as possible. And yet, afterwards the adsorbent needs to find end storage place, e.g. in tailing pond, or further use, e.g. as binder, filler, or soil amendment.
Adsorption capacity of a powdered GP is usually higher, but technical implementation of powdered forms requires precise dosing, contact vessel with stirring, solid–liquid separation step, and transfer of exhausted adsorbent to regeneration vessel. The powder can then be regenerated by addition of suitable regenerant, e.g. mild acid, separated, and dried prior to the next adsorption cycle.
Technically, the use of granular forms is an easier option. However, the size of the column vs. wastewater stream can easily become very large, as granules per se, are larger particles and adsorption is a surface process. This puts additional burden on geopolymer production as the overall capacity should be sufficient, and the granules will need to show suitable compressive strength to withstand the gravimetric pressure in the purification column. Conversely, regeneration is technically easily realized by counter flow of regeneration liquid through the column.
Economic evaluation therefore needs to take these considerations into account during CAPEX estimation. OPEX, in turn, is not only the ongoing replacement of exhausted adsorbent, electricity consumed, maintenance, staff, and regeneration chemicals, but also the transportation costs of adsorbents, which can be high at low adsorption capacity.
As a thought experiment, an example of 55 mg/g adsorption capacity of copper adsorbent, with 85% cycling capacity has a 47 mg/g adsorption capacity after desorption cycle, shall be considered. For a mine effluent or process water with 5 mg/L Cu2+ and a flow of 200 m3/h requires about 21 kg adsorbent per hour. The price of GPs is given as 1–1.5 € per kg , and as such the treatment costs of merely 1 h would be between 10 and 21 €. Regeneration up to 20 times gives more realistic cost factors, of 0.5–1 € per h, only for adsorbent costs. It becomes quickly clear that without regeneration, high efficiency, and selectivity GPs/AAMs will be too expensive for wastewater treatment.
Much work has been done on the adsorption properties of GPs/AAMs towards a wide variety of inorganic pollutants during the last decade. While the effect of competing ions in real water samples remain an issue, the incorporation of new composite materials and the tailoring of reaction conditions have a high potential to increase their selectivity as adsorbents. However, more and more authors have understood the need to regenerate adsorbents and research are being conducted on the recovery of valuable materials, such as metals or nutrients. The recovery of high energy products from side streams utilizing adsorbents made from industrial side streams, will bring circular economy towards the next level. It is also of interest, to cover the costs of water treatment by the revenue of removed materials. While still much work needs to be done, the authors remain confident, that GPs/AAMs will continue to have a prominent place in wastewater treatment.
The research was partially funded by European Regional Development Fund (Leverage from the EU, WaterPro project № A74635; Keski-Pohjanmaan liitto/Kainuun liitto/Pohjois-Pohjanmaan liitto) and by Maa- ja vesitekniikan tuki (№ 13-8271-17).