The main focus of this work was to study developed novel granular iron-based adsorbents named IRON-1 and IRON-2 and compare these products to commercially already available granular iron-based adsorbents. The production of granular adsorbents was scaled from laboratory scale to semi-production scale so that the tested new iron-based adsorbents would be fully comparable to existing commercial products. A simplified flowsheet is presented in Figure 6.1. The process steps are as follows; neutralization of ferric sulphate to precipitate ferric oxide/hydroxide, filtering, washing, and drying the precipitate. The final step was the granulation of the dried precipitate.
Figure 6.1. Illustrative flowsheet of the manufacturing of granular iron-based adsorbent.
Totally seven iron-based materials (IRON-1 to IRON-7) were analysed and tested.
Samples IRON-1 and IRON-2 were the developmental products. Adsorbents IRON-3 to IRON-7 are commercial readily available iron-based granular adsorbents. Samples IRON-4, IRON-5, and IRON-6 are marketed for phosphate adsorption. Typical phosphate removal applications for these products are aquariums, lake restorations, and the phosphate polishing of wastewaters. In addition, adsorbents IRON-3 and IRON-7 are specifically developed and marketed for arsenic removal. A typical arsenic removal application for these adsorbents is the drinking water treatment.
To better understand how these two developmental products IRON-1 and IRON-2, perform, additional comparison testing was done with gypsum, Al2O3, and TiO2 –based adsorbents as described below.
Reactor 1 Reactor 2 Filter & wash
pH control Ferric sulphate
Drying pH control
Granulation
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GYPSUM-1 adsorbent was produced explicitly in this work by granulating gypsum and iron-containing solid waste from the titanium dioxide industry with the same granulation process as was used for producing IRON-1 and IRON-2 adsorbents. It has quite high iron content, so it might have been more suitable to include it in iron-based adsorbents, however, its composition varies considerably from iron-based adsorbents, so in this work, it was categorized as “gypsum-based adsorbent”. The GYPSUM-2 and the GYPSUM-3 adsorbents are commercial adsorbents targeted for phosphate removal. The GYPSUM-2 product is based on a natural mineral, and GYPSUM-3 is manufactured through chemical precipitation.
Two aluminium oxide-based products and two titanium dioxide-based products were also tested. “Al2O3-1” is a commercial granular Al2O3 adsorption product explicitly aimed at phosphate removal. Adsorbent “Al2O3-2” is based on solid waste material from the aluminium industry. This adsorbent was produced by the same granulation process used to produce IRON-1 and IRON-2 granular adsorbents. No binder was used in the “Al2O3-2” adsorbents granulation process. The idea with these very low-cost “waste granules”
was that it would be interesting to see how they compare to adsorbents produced from virgin materials.
The third comparison group was titanium dioxide-based adsorbents. “TIO2-1” is a commercial titanium dioxide-based granular adsorbent aimed for arsenic removal from drinking water. “TIO2-2” adsorbent was produced by granulating widely available commercial TiO2 food-grade anatase pigment with CaCO3 as a binder again with the same granulation process used for producing IRON-1 IRON-2 granular adsorbents.
All the adsorbents mentioned above were not used in every experiment, but the focus was on IRON-1 and IRON-2 adsorbents, and suitable comparisons samples were selected depending on the test under evaluation.
Physical analyses of adsorbents are shown in table 6.3, and Figure 6.2 shows photographs of some of the granular adsorbents.
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Table 6.3: Physical analyses of adsorbents Sample name
Specific Surface Area
[m2/g]
Sieve Size D50 [mm]
Apparent Density
[kg/L]
IRON-1 81 1.50 1.26
IRON-2 110 0.90 1.20
IRON-3 190 not analysed not analysed
IRON-4 290 1.51 0.81
IRON-5 310 1.26 0.71
IRON-6 130 not analysed not analysed
IRON-7 120 0.76 0.51
GYPSUM-1 23 not analysed 0.89
GYPSUM-2 not analysed not analysed not analysed
GYPSUM-3 200 0.98 0.60
AL2O3-1 320 1.9 0.83
AL2O3-2 220 not analysed 0.52
TIO2-1 240 not analysed 0.84
TIO2-2 160 not analysed 0.70
Figure 6.2. Photograph of some of the adsorbents. All tested products were granulated products.
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Examples of particle size distributions of the adsorbents are presented below. Particle size was analysed by sieve series.
Figure 6.3. Examples of particle size distributions from the sieve analyses of adsorbents. All tested products were granulated products.
Chemical analyses of adsorbents are shown in table 6.4. Three products, namely GYPSUM-1, GYPSUM-2, and GYPSUM-3, differentiate from the rest of the materials.
These three products contain high concentrations of different elements. In the case of the granulated waste product, GYPSUM-3, iron is a significant part of the product's composition.
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Table 6.4: Chemical analyses of adsorbents, nd=concentration lower than XRF’s detection limit,
“-“= not measured.
Titanium was analysed from the TIO2-1 and TIO-2; TIO2-1 had 51.8%, and TIO2-2 had 52.1% of titanium. Titanium was not detected on other samples.
XRD analyses (see appendix 1) show the significant difference between products. Some of the materials have a very crystalline structure, and some are very amorphous. For example, iron-based adsorbents IRON-3 to IRON-7 have crystalline structures, but the IRON-1 and IRON-2 samples have an amorphous structure. The table also presents the estimation of the adsorbent composition (significant components) based on chemical analyses and the XRD findings.
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Table 6.5: XRD analyses of adsorbents
Sample name Crystallinity XRD – main components
IRON-1 amorphous ~70% Goethite α-FeOOH, ~30%
Hydronium jarosite (H3O)Fe3(SO4)2(OH)6
IRON-2 amorphous ~80% Goethite α-FeOOH, ~20%
Hydronium jarosite (H3O)Fe3(SO4)2(OH)6
IRON-3 crystalline ~50% Iron Oxide Fe2O3, ~50% Goethite α-FeOOH
IRON-4 crystalline ~100% Goethite α-FeOOH
IRON-5 crystalline ~100% Goethite α-FeOOH
IRON-6 highly crystalline ~100% Goethite α-FeOOH
IRON-7 highly crystalline ~100% Goethite α-FeOOH
GYPSUM-1 crystalline ~75% Gypsum CaSO4*2H2O, ~25%
Goethite α-FeOOH
GYPSUM-2 crystalline
~30% Calcite CaCO3, ~20% Gypsum CaSO4*2H2O, ~10% Quartz SiO2, ~40%
Ettringite Ca6Al2(SO4)3(OH)12*26H2O
GYPSUM-3 amorphous
~10% Calcium Oxide CaO, ~2%
Manganese Oxide MnO, ~10% SiO2, ~70%
Goethite α-FeOOH
AL2O3-1 crystalline ~100% Aluminium Oxide Al2O3
AL2O3-2 crystalline ~100% Aluminium Oxide Al2O3
TIO2-1 crystalline ~85% Anatase TiO2, ~10% Gypsum
CaSO4*2H2O
TIO2-2 crystalline
~85% Anatase TiO2, ~2% Calcium Carbonate CaCO3, ~12% Gypsum
CaSO4*2H2O
Examples of the Scanning Electron Microscope (SEM) pictures are shown in the appendix. Products differ clearly from each other; IRON-1 and IRON-2 products have a large pore structure and are quite amorphous. On the other hand, IRON-6 and IRON-7 products are more crystalline, having a needle-shaped crystal structure typical to α-FeOOH.
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7 Phosphate removal - Equilibrium studies
7.1 Introduction
Adsorption isotherms are an essential tool to describe how adsorbates will interact with adsorbents and are critical in optimizing the use of adsorbents. The adsorption mechanism in aqueous solutions is complicated, and therefore the correlation of equilibrium data with theoretical isotherm equations can explain possible mechanisms in the adsorption process.