Ceramic bricks containing Ni ions from contaminated biomass used as adsorbent

This article shows how pine sawdust residues can be used to adsorb nickel ions from synthetic solutions and then to produce porous bricks for civil construction using a mixture of natural clay and biomass containing the adsorbed metals. The adsorption tests were performed by mixing NiCl 2 solutions with pine sawdust during a xed stirring period of 24 h. The set was ltered and the ltrate was analysed. Highest eciency adsorbate/adsorbent ratio was 1 mol L -1 of NiCl 2 solution and 20 g L -1 of pine sawdust. This was the contaminated biomass sample used in the manufacture of the bricks. This paper analyses the properties of the bricks achieved and compares them with bricks without added biomass, porous bricks containing zinc and commercial bricks. The obtained values of apD: bulk density, aspW: apparent specic weight, apP: apparent porosity, H 2 O Abs: water absorption, apV: apparent volume, LOI: weight loss on ignition, σstr: compression fracture stress, MOR: exural modulus of rupture and RE: retention eciency, demonstrate that the ceramic pieces obtained are optimal for construction.


Introduction
Heavy metals are the main inorganic micro pollutants, and their discharge into aquatic bodies affects the ecosystem and health, due to their toxicity and possible carcinogenic effect. The lack of biodegradability of these pollutants produces their bioaccumulation and persistence in the environment, and their danger is given both by the concentrations in which they may be present, as well as by their chemical behaviour.
The anthropogenic sources of heavy metals are very varied, with the metal nishing industry being the most contributor due to the large number of companies that integrate and its geographical dispersion [1,2].
Nickel is a dangerous heavy metal. The World Health Organization (WHO) states that 0.07 mg L -1 is the allowed limit of this contaminant in water for consumption. This contaminant can cause adverse effects on the blood, kidneys, bronchi, lung and stomach. The main sources of nickel are the stainless steel and alloy, electroplating, catalysts and Ni/Cd industries. The reduction of nickel concentration in water and wastewater is of great interest due to the current increase in demand of such emitting industries [3,4].
The most commonly used conventional methods for removing Ni(II) and other toxic metals from wastewater are chemical precipitation, solvent extraction, ion exchange, otation, adsorption with activated carbon, among others [4,5]. However, to date, there is no low-cost technology that allows to reach very low levels of these pollutants. In this situation, the removal of heavy metal ions using low-cost adsorbents has recently gained relevance [6][7][8]. Research has shown that materials of biological origin, such as biomass of algae, fungi and bacteria, as well as residues and by-products from the agroindustrial sector, can retain heavy metal ions [9]. Bioadsorption of pollutants from cellulosic biomass residues has attracted attention because it not only allows the removal of heavy metals in industrial e uents, but also treats agricultural waste that is produced today in huge tons and does not have a speci c use. In this sense they have been studied as adsorbents, rice husk [10,11], sawdust [12,13], fruit peels [14,15] and others [16,17].
Heavy metals are attracted to the adsorbent through a complex process involving mechanisms such as surface adsorption (by Van Der Waals forces, dipole interactions, or hydrogen bonding), interstitial adsorption (through the pores of the material), ion exchange adsorption (between metal ions and adsorbent ions, facilitated by carboxyl groups and hydroxyl), by electrostatic forces (related to the pH of the solution and the zero charge point of the adsorbent), complexation (through carboxylate groups, amides, phosphate, thiols and hydroxide), precipitation, among others. The availability of the active biomass sites is in uenced by the kind of biomass, the metallic solution, the pH, the temperature, the contact time and the stirring speed of the mixture. [8,18].
Currently, there are researches on the production of porous ceramic bricks from the incorporation of agroindustrial biomass residues. These residues act as pore-forming agents because at the sintering temperatures their combustion produces gases and ashes. For this purpose, residue of olive [19], sun ower and wheat [20], fruit pits [21], vine [22] and rice [23] have been investigated.
The aim of this article is to adsorb Ni(II) ions on wood industry residues (pine sawdust) and then use these new contaminated residues as an aggregate in clay to generate optimal porous ceramic matrices for civil construction, which will contain retained metals. The metals will be immobilized in the ceramic pieces, stabilizing them and reducing their release, solubility and toxicity. In this way, the negative effects on the environment are minimized [24,25].

Adsorbate
The metal solutions used in this study (0.125 to 1 mol L -1 ) were prepared by dissolving appropriately measured amounts of NiCl 2 x 6 H 2 O salt (analytical grade, Cicarelli) in distilled water.

Bioadsorbent
The adsorbent used was the residue obtained from the processing of Pinus elliottii wood in a sawmill from the north east area of Argentina. This residue was sieved to obtain particles smaller than 1 mm. It was then boiled with distilled water for 90 min, ltered and washed repeatedly. Finally, the material was dried in an oven at 50 °C.

Batch adsorption experiments
A qualitative study of the adsorption at different times of Ni(II) ions of solutions 0.25 mol L -1 of NiCl 2 in contact with 10 g L -1 of pine sawdust was carried out. 50 mL metallic solutions prepared at pH 5-6 were put in contact with pine sawdust by constant stirring at room temperature of 200 rpm (SK-0330-Pro shaker). Then, each mixture was ltered and the solid residues were dried at 50 °C. The pH was chosen according to what is reported in the bibliography [8,26], at higher pH values Ni(II) forms a complex with hydroxide ions, precipitating and decreasing adsorption, and at a pH less than this value, H + can compete with ions of heavy metals by the active sites of the adsorbent and also, functional groups of the adsorbent that intervene in the adsorption process can be protonated (general positive charge on the adsorbent), generating an electrostatic repulsion on the positively charged metal ions leading to adsorption. This test was performed with the aim of estimating an optimal time for contact between heavy metal ions and pine sawdust biomass by means of X-ray uorescence (XRF) measurements on the solids obtained from the ltrate. The tests were done in triplicate.
Subsequently, adsorption tests were performed at different concentrations of NiCl 2 from 0.125 to 1 mol L -1 and varying from 10 to 40 g L -1 the biomass residue content to determine in this way the most e cient adsorbate/adsorbent ratio used in the manufacture of ceramic pieces. For this, the amount of residual metal that was not adsorbed on the biomass and remained in the solution after the ltration process was quanti ed by UV-Vis Spectrophotometry at λ = 722 nm (this λ was chosen to reduce the signal obtained by the organic matter, in addition to making blank samples). The tests were done in triplicate. The adsorption e ciency R (%) of Ni(II) was calculated following Eq. (1) and the adsorption capacity qe (mg g -1 ) was de ned by Eq.

Bricks production and characterization
The bricks used to carry out the mechanical tests were made by mixing 100 g of clay from a natural quarry in eastern Buenos Aires and the biomass containing the adsorbed Ni(II) in a proportion of 20 % by volume.
The adsorption sample used for the manufacture of bricks was the one representing the highest e ciency adsorbate/adsorbent ratio determined by the adsorption experiments.
For formation, a uniaxial pressure of 25 MPa was used, with the addition of 8 mL of water, in molds of 70 mm x 40 mm x 18 mm. After a drying period of 24 h in the environment, the green bricks were calcined at 950 °C for 3 h. The heating rate was 1 °C min -1 and the cooling rate was 5 °C min -1 .
The sintered ceramic pieces were physically characterized and their mechanical properties were evaluated in triplicate.
The apparent porosity (apP), together with the apparent density (apD), the apparent speci c weight (aspW), water absorption (H 2 OAbs) and apparent volume (apV), were determined following the Archimedes principle and based on the IRAM standard 1554.
The modulus of rupture (MOR) and the compressive strength (σstr) were evaluated in test specimens in accordance with the provisions of the IRAM 12,587 standard and the IRAM 12,586: 2004 standard, respectively. The MOR was determined on recti ed samples of 70 mm x 40 mm x 18 mm, submitted to a three-point bending test. The σstr was determined on recti ed samples of 35 mm x 40 mm x 18 mm.
Leaching tests on 10 g of the bricks were carried out following the procedure described in EPA 1311 TCLP (Toxicity Characteristic Leaching Procedure) and in triplicate. The brick fragments were contacted with 20 times the volume of extraction solvent (leaching solution of pH 4.93 ± 0.05, prepared from 5.7 mL of acetic acid in 500 mL of distilled water, adding 64.3 mL of 1 N sodium hydroxide and diluting up to 1 L).
The mixture was stirred at 50 rpm for 18 h. The system was ltered and the concentration of Ni(II) was determined on the ltered liquid by Atomic Absorption Spectrophotometry (AA).

Equipment
Thermogravimetric-differential thermal analysis (TGA-DTA): Shimadzu TGA-50 and Shimadzu DTA-50 equipment, with TA-50 WSI analyser. Thermal characterization of pine sawdust residues. Conditions: air ow, heating up to 1000 °C at a rate of 10 °C min -1 and approximately 20 mg of residue.
Scanning electron microscopy (SEM) -energy dispersive analysis of X-ray (EDS): SEM Philips 515 equipment, with energy dispersive analyser (EDAX-Phoenix). Microstructural characterization of pine sawdust residues. Determination of Ni(II) in the biomass after the adsorption process and in the manufactured bricks.
Fourier-transform infrared spectroscopy (FTIR) in attenuated total re ectance -ATR mode: Nicolet 6700, Thermo Electron Corp. equipment. Characterization of the functional groups present in the biomass of pine sawdust before and after the Ni(II) adsorption process.

Bioadsorbent characterization
Thermal analysis by TGA-DTA for pine sawdust is presented in Figure 1. It is very important to know the behaviour of biomass as the temperature rises, since this is closely related to the properties that the brick will reach when sintered.
The DTA diagram shows an endothermic peak at 52 °C and a small exothermic peak at 263 °C assigned to the loss of water from the sample and the combustion of volatile components, respectively. In addition, in this diagram it is also possible to recognize two exothermic peaks at 327 °C and 488 °C, attributed to the decomposition of hemicellulose (roasted in wood called active pyrolysis), and decomposition of cellulose (passive pyrolysis) and lignin (active and passive pyrolysis), respectively.
It is possible to observe three major mass losses in the ATG diagram, a rst loss up 230 °C corresponding to the loss of moisture and the decomposition of volatile components, a second loss up to 311 °C assigned to hemicellulose (to give lower weight compounds molecular, mainly acetic acid), and, nally, a loss of mass up to 502 °C assigned to cellulose and lignin (to nally give CO 2 , H 2 O and ash).
Hemicellulose combustion occurs at lower temperatures due to its linear structure with short side chains. Cellulose and lignin have more complex and stronger structures, with associated dispositions and aromatic compounds, so they have greater resistance to heat.
These results demonstrate that, during the sintering of the ceramic pieces, the biomass added to the clay mix the whole will gradually calcine as the temperature of the oven increases. This will allow the gases formed to diffuse slowly and the red bricks will not crack. From the data obtained from the TGA analysis, it is possible to obtain an estimated composition for pine sawdust residues. This composition is detailed in Table 1.  show that pine sawdust has a brous and irregular structure with pores smaller than 50 microns and elongated particles. These surface characteristics could make the adsorption of heavy metals possible. Figure 3 shows the FTIR spectrum of the biomass. FTIR was used for the qualitative analysis of the biomass residues of pine sawdust and to identify the functional groups that could participate in the adsorption of contaminating metals. It is possible to observe the presence of bands corresponding to    Figure 5 shows that Ni(II) adsorption on pine sawdust increases with contact time because, initially, there is a greater availability of active biomass sites. After a certain time, a plateau is reached which represents a stability in the adsorption of nickel ions. This shows a successive occupation of the active sites of pine sawdust by the pollutant, which causes, after a time, the adsorption to decrease. According to the results obtained, the stirring time for the adsorption tests was 24 h to ensure arrival at the plateau.

Effect of the initial concentration metal and dose of bioadsorbent
Ni(II) adsorption tests were carried out by modifying the initial concentration of the metallic solutions, but maintaining the volume of the solution, and studying adsorption on 3 different amounts of biomass, 10, 20 and 40 g L -1 . After a 24 h stirring time, the mixtures were ltered and the remaining solutions were analysed to determine their residual nickel concentration. Figure 6a shows the relationship between adsorbent dose, initial adsorbate concentration, and Ni(II) adsorption e ciency. R values are higher for low NiCl 2 concentrations because there are many binding sites available in pine sawdust biomass but then decrease for all residue doses studied. In Figure 6b it is possible to observe how at higher initial concentrations of NiCl 2 greater adsorption capacity. As the concentration of metal ions increased, the collisions between these and the adsorbent increased. Finally, it seems that at high metal concentrations there is a tendency to reach a plateau. According to Figure 6b, in the concentration range studied, at the concentration of 1 mol L -1 NiCl 2 (5.87 x 10 4 mg L -1 Ni + 2 ) the adsorption capacity is greater. This value increases when the adsorbent dose of 20 g L -1 is used. Furthermore, it is observed in Figure 6a that when the amount of biomass doubles from 10 g L -1 to 20 g L -1 , the R increases 2.5 times, but when the sawdust dose is doubled again, the R increases only 1.5 times. Therefore, the adsorbate/adsorbent ratio of 1 mol L -1 NiCl 2 and 20 g L -1 sawdust was considered the most e cient and was then used in the construction of clay bricks. The Figure 9 shows the XRD of sawdust after the sorption process with 1 mol L -1 NiCl 2 . It is from this concentration of NiCl 2 that it can be seen, in addition to the peaks corresponding to sawdust, peaks belonging to NiCl 2 (H 2 O) 2 in concordance with what was observed in Figure 8.
The biomass electron micrographs after adsorption are shown in Figure 10. Agglomerates of pine sawdust particles exceed 25 microns. From EDS, the homogeneous presence of adsorbed Ni can be identi ed. The percent composition of the residue after adsorption is presented in Table 2 (C not shown). 3.4. Characterization and evaluation of the bricks Figure 11 shows the physical appearance of the manufactured sintered ceramic pieces, (a) without added residue (ARC), and (b) with added 20 % by volume of sawdust containing nickel adsorbed (AN20). Both pieces are compact, but ARC has a reddish color due to the Fe present in the clay, while AN20 has a darker color that could be contributed by high levels Ni(II) retained on the biomass and then immobilized in the cooked brick. Figure 12 shows the DRX obtained for AN20 after cooking. It is possible to observe the presence of SiO 2 together with peaks corresponding to numerous secondary phases because the clay used was obtained from natural quarries. Figure 13 shows the XRF diagrams of the AZ20 and ARC bricks together with a commercial brick (COM). The presence of lines corresponding to nickel corroborates that the metal remains in the clay matrix after the sintering process.
The EDS images of powder AN20 are shown in Figure 14. These images allow to corroborate the presence and immobilization of Ni after the sintering process. The percentage composition of the powder brick is presented in Table 3 (C not shown).  Table 4 shows the average values obtained from LOI and the parameters determined from the apparent porosity test of the manufactured bricks. The LOI ignition weight loss was greater for AN20 than for ARC due to the combustion, during sintering, of the aggregate biomass. This combustion of sawdust waste generates a greater apP observed in ceramic pieces made from clay and the addition of 20 % biomass. Higher values of apP and H 2 OAbs determine lower values of apD and aspW for AN20. In addition, the regulates the use in construction, and sets a lower limit of 5 MPa. These σstr values were lower for AN20 than for ARC due to aggregate biomass. This same behavior is observed in exural strength. According to the literature [28], the exural strength varies between 10 and 30% of the compressive strength, so AN20 still has a MOR value within the appropriate parameters of the market (MOR = 23 % σrot). Table 4 Properties of sintered ceramic pieces and a commercial brick.
Very similar values of these properties were obtained for the brick manufactured from sawdust with Zn(II) adsorbed (AZ20) [29]. Also, in Table 4, the values for a commercial brick (COM) are presented.
AN20 has superior physical and mechanical characteristics compared to commercial brick.
National Law 24,051 and its regulatory decree establish that leachates in dangerous products must be analyzed in accordance with TCLP. The measurements were obtained in triplicate following the protocol where, ER = e ciency retention of metal in the brick (%), MMIB = mass of metal incorporated in AN20 and MML = mass of metal extracted in the leachate. M MIB is estimated from the nickel mass adsorbed by pine sawdust in the ratio of highest e ciency selected and corrected for the mass of sawdust added as poreforming agent in AN20. M ML is estimated from the nickel concentration in the leachate corrected for the mass of the entire brick. ER for AN20 bricks is 99.99%.

Conclusions
Pine sawdust generated by the timber industry was evaluated as an adsorbent of synthetic Ni(II) solutions. This residue, without a speci c use at present, showed the ability to retain the ions of the heavy metal studied. In this way, pine sawdust constitutes a simple and low-cost system for wastewater treatment. The adsorption tests show that the most e cient adsorbate/adsorbent combination was found to be 20 g L -1 of sawdust and an initial NiCl 2 concentration of 1 mol L -1 .
The novelty is to use this new contaminated biomass in the manufacture of porous clay bricks, immobilizing heavy metals and minimizing the impact on the environment.
The sintered bricks presented values of apparent porosity and mechanical characteristics within the market requirements. In addition, TCLP tests show a retention e ciency close to 100 %, which determines an excellent immobilization of Ni(II) within the ceramic pieces.
With these results, it would be possible to respond to the needs of the industry that generates contaminated e uents and of the agribusiness that generates biomass waste without a speci c use and considerably reducing the negative implications on the environment.

Declarations
Availability of data and materials All data generated or analyzed during this study are included in this published article or request the corresponding author.

Competing interests
The authors declare they have no competing interests.  Figure 1 TGA-DTA of biomass residues from pine sawdust.         Macroscopic appearance (a) ARC y (b) AN20.

Supplementary Files
This is a list of supplementary les associated with this preprint. Click to download. Equations.pdf