APPLICATION OF POWDERED BUILDING WASTES IN ADSORPTION TECHNOLOGIES

Two powdered wastes originating in building materials production (waste brick dust (WBD) from the production of ceramic blocks and concrete slurry waste (CSW) from prestressed poles production used also in Femodified form (CSWFe)) were studied as potential adsorbents of ecologically risk cations (Cd2+, Pb2+) and anions (AsV as AsO43and CrVI as CrO42-) from contaminated waters. The WBD indicated very good adsorption selectivity for Cd2+, Pb2+ and AsV at a high adsorption efficiency (90 %), and worse parameters for CrVI adsorption (30 % adsorption efficiency). The sorbent consumption was low (70 100 g per 1 g of contaminant) for cations (Cd2+, Pb2+) and more than four times higher for AsV. The adsorption of CrVI on WBD was ineffective. The adsorption of cations (Pb2+ and Cd2+) on CSW and anions (AsO43and CrO42-) on CSWFe ran in the same manner as on WBD. Fe-modification of CSW to CSWFe increased its adsorption efficiency to anions by 15. 20 %. The cations (Pb2+, Cd2+) were adsorbed almost quantitatively (≈100%) on the initial CSW at the sorbent consumption of 10 120 g per 1 g of contaminant. The adsorption of anions (AsV, CrVI) on CSWFe reached ˃95% for AsV and 40 60% for CrVI, at the sorbent consumption of 80 200 g per 1 g of contaminant.


INTRODUCTION
A secondary use of building waste has got into increased professional concern due to the growing development and production of building materials. The powdered waste brick dust (WBD), which arised at the production of ceramic blocks, is usually recycled in concrete production [1], or, in specific cases, it can be also applied as pozzolanic component of cement based materials to reducing the Portland cement consumption [2]. The annual production of Portland cement exceeds 4000 Mt [3,4] and is still rising. According to e.g. Damptoft er al. [5], the energy-intensive production of Portland cement .is responsible for about 5 % of global anthropogenic emissions of CO2, which initiated the development of new technologies primarily focused on recycling and secondary use of waste building products [6,7]. Fortunately, the powdered building wastes including WBD and CSW represent promising materials due to availability, low cost and appropriate properties. The chemical stability, environmental safety issues and silicate-like behavior determine their potential application as selective adsorbents in decontamination technologies [6,8].
In natural adsorption systems the sorbent selectivity has been primarily controlled by the pH of zero point of charge (pHZPC) [9]. The solids with a low pHZPC (aluminosilicates, quartz) attract mostly cations, while a high pHZPC is typical for anion-active sorbents (Fe/Al oxides, gibbsite) [10,11]. The surface of mixed materials (soils, sediments, brick dust, slurries) mostly consists of diverse active sites, and the pHZPC corresponds more or less to the median of all particular components. Therefore, they can behave as both cation-active and anion-active adsorbents with respect to current conditions. The aim of this work was to study the sorption properties of WBD and CSW in initial and Fe-modified form on heavy metal cations (Pb 2+ and Cd 2+ ) and toxic oxyanions (As V as AsO4 3and Cr VI as CrO4 2-). The optimal https://doi.org/10.37904/nanocon.2019.8772 adsorption parameters were investigated in model water systems and calculated according to the Langmuir model.

Used sorbents
The WBD is generated as a waste (grinding dust) during the production of vertically perforated ceramic blocks intended for thin joint masonry, while a sedimented CSW containing cement, mineral additives, fine fillers, admixtures and water, remains after the partial recycling of fresh concrete waste (1-4 wt.% of total concrete production). The elementary chemical composition, mineralogy and surface properties are given in Table 1.

Model solutions
Model solutions of Pb 2+ , Cd 2+ , H2AsO4and CrO4 2were prepared from inorganic salts (PbCl2, Cd(NO3)2, KH2AsO4 and (NH4)2CrO4 of analytical grade and distilled water, in the concentrations of 0.1 and 0.5 mmol.L −1 and the natural pH (i.e. pH ≈ 3.5 for cationic solutions and pH 5-6 for anionic solutions). The concentration range was selected as appropriate for the simulation of a slightly increased amount of the contaminant in a water system to a heavily contaminated solution.

Fe-modification
The suspension of CSW (20 g) in 0.6M FeSO4·7H2O (1 L) was shaken in a sealed polyethylene bottle at laboratory temperature (20 °C) for 24 h. Then the solid phase was filtered off, washed with distilled water, dried at 60 °C, and homogenized [12]. Only a tiny surface layer of available Fe ions in reactive form is sufficient for the adsorption of oxyanions on the active surface sites of bulk oxi(hydroxides) [13].

Adsorption process
In adsorption experiments the cations (Cd 2+ , Pb 2+ ) were adsorbed on the WBD and CSW, whereas the anions (As V , Cr VI ) were adsorbed on the WBD and Fe-modified CSWFe.
The suspension of model solution (50 mL) and defined dosage (0.5-15 g L −1 of WBD and 1-20 g L -1 of CSW or CSWFe) was agitated in a batch manner at laboratory temperature (20 °C) for 24 hours [13]. The product was filtered off, and the filtrate was analysed for residual concentration of cations or anions. All adsorption data were fitted to the Langmuir isotherm [14,15] as the suitable and widely used adsorption model for natural sorbents, including oxides, aluminosilicates and soils.
X-ray fluorescence (XRF) analyses of the solid phase were determined with an ARL 9400 XP+ spectrometer with a voltage of 20 -60 kV, probe current of 40-80 mA and effective area of 490.6 mm 2 . UniQuant software was used for data evaluation.
The specific surface area (SBET) was measured on a Micromeritics ASAP 2020 (accelerated surface area and porosimetry) analyzer using the gas sorption technique. The ASAP 2020 model assesses single and multipoint BET surface area, Langmuir surface area, Temkin and Freundlich isotherm analysis, pore volume and pore area distributions in the micro-and macro-pore ranges by the BJH method. The micro-pore option used the Horvath-Kavazoe method, with N2 as the analysis adsorptive and an analysis bath temperature of -195.8 °C.
The samples were degassed at 313 K for 1000 minutes.
The concentration of Pb and Cd in aqueous solutions was determined by atomic absorption spectrometry (AAS) using a SpectrAA-880 VGA 77 unit (Varian) in flame mode. An accuracy of AAS analyses was guaranteed by the Laboratory of Atomic Absorption Spectrometry of UCT Prague, CR, with the detection limit of 0.5 gL -1 , with a standard deviation ranging from 5 -10 % of the mean.

The concentration of As in aqueous solutions was determined by Hydride Generation Atomic Fluorescence
Spectrometry (HG-AFS) using a PSA 10.055 Millennium Excalibur apparatus. The samples were pre-treated with a solution of HCl (As-free, 36 % v/w) and KI (50 %) with ascorbic acid (10 %). The instrumental parameters included ppm and ppb modes, HCl (12 %) with KI + ascorbic acid solution as the reagent blank and 7% NaBH4 in 0.1 mol.L -1 NaOH as the reductant. The declared detection limit was 0.05 ppm and the standard deviation was experimentally determined as 2.5 %.
The concentration of Cr as Cr2O7 2in aqueous solutions was measured with a UV/VIS spectrophotometer (Evolution 220, Thermo Fisher Scientific) at 350 nm following acidification with H2SO4 (10%wt.) [16]. The verified detection limit was 50 µgL -1 and the experimentally determined standard deviation was less than 5 %.

Adsorption capacities (Langmuir model)
The theoretical adsorption capacities Qt calculated according to the Langmuir model (Figure 1) indicated a higher adsorption affinity of cations (Cd 2+ >Pb 2+ ) to WBD, and significantly better adsorption of oxyanions (AsO4 3->>CrO4 2-) to CSWFe. These results corresponded well to the previous studies of Doušová et al. [6,13] supporting the theory of inner-sphere surface complexation for Cd 2+ , Pb 2+ and AsO4 3adsorption, and both outer-and inner-sphere complexation with prevailed weaker monodentate forms for CrO4 2- [17]. Relatively different Qt values resulted from the distinct structural and binging properties (size, charge distribution, binding energy etc.) of tested ions.

Adsorption efficiency
The efficiency of adsorption process represents very important characteristic of adsorbent-adsorbate system. According to the percentage adsorption efficiencies (Figure 2), Pb 2+ and As V were selectively adsorbed (75-99%) on all sorbents. The Cd 2+ was almost quantitatively adsorbed on cation-active adsorbents (WBD and CSW), at the ineffective adsorption on anion-active CSWFe. This phenomenon was in agreement with the expected mechanism of cation adsorption [6,9]. A generally worse adsorption of Cr VI associated with low adsorption efficiency resulted from a lower adsorption energy of Cr VI oxyanions and their tendency to the electrostatic binding via outer-sphere surface complexes [18].
The highly alkaline WBD and CSW adsorption environment provided the formation of insoluble surface precipitates and polynuclear complexes, which could also improve the adsorption yield [19].

Sorbent consumption
According to obtained data, a hypothetical sorbent consumption for the removal of 1 g of toxic element was calculated for better estimation of its possible use in decontamination technologies (Figure 3). As shown in the scheme (Figure 3), all adsorbents were perspective for Pb 2+ removal at a low sorbent consumption (in tens of g per 1 g of Pb). In the case of Pb 2+ the participation of predicted poorly soluble Pb(OH)2 clusters, which substantially increased the adsorption yields, should be considered [8]. The both cation-active adsorbents (WBD and CSW) were also promising for Cd 2+ adsorption, particularly CSW.
According to the excellent adsorption properties of arsenates associated with the inner-sphere surface complexation [13,17], As V was selectively adsorbed on anion-active CSWFe at a low sorbent consumption (in tens of g per 1 g of As), and also effectively (75 %) but at a higher sorbent consumption (in hundreds of g per 1 g of As) to the both WBD and CSW. From the sorbents tested, only CSWFe appeared to be perspective for Cr VI removal at a sorbent consumption in hundreds of g per 1 g of Cr. For tested ions, the potential use of bulding wastes in decontamination technologies decreased in the order: Pb 2+ ˃ Cd 2+ ≈ As V ˃ Cr VI .

CONCLUSION
Powdered building wastes (WBD and CSW) can be used as selective sorbents of cationic and anionic contaminans at the adsorption efficiency more than 75 %. The pHZPC values of sorbents indicate the predicted affinity of adsorbed cations or anions, but a highly alkaline system promoting the formation of surface precipitations and clusters can cause the deviation from usual adsorption behaviour. The CSW appears to be even better adsorbent compare to the WBD and its surface modification with Fe 2+ improved the selectivity to anionic contaminants. The utilization of tested sorbents to remove toxic ions from contaminated water decreased in the order: Pb 2+ ˃ Cd 2+ ≈ As V ˃ Cr VI . Lead as Pb 2+ was selectively adsorbed on all tested sorbents at the sorbent consumption in tens of g per 1 g of Pb. In this case, the formation of insoluble surface precipitation Pb(OH)2 and clusters Pb4(OH)4 4+ /Pb6(OH)8 4+ promoting the adsorption yield, could be considered. Cadmium as Cd 2+ was similarly adsorbed on the CSW, and with a higher sorbent consumption (in hundreds g per 1 g of Cd) on the WBD. For As V and Cr VI removal, CSWFe was the most appropriate adsorbent with the sorbent consumption in tens of g per 1 g of As and hundreds of g per 1 g of Cr, respectively. Regardless of the sorbent consumption, the adsorption of Cd 2+ on CSWFe and Cr VI on WBD was completely ineffective.