Combination of alkalinity and porosity enhances formaldehyde adsorption on pig manure -derived composite adsorbents

Highlights

• The adsorbents tested were synthesized from pig manure chars.

• Breakthrough capacity was higher than those measured on carbons of high porosities.

• Inorganic phase (CaO and MgO)) promoted oxidation of HCHO to formic acid and salt formation.

• High adsorption capacity was linked to combined effect of chemistry and porosity.

Abstract

Carbonaceous porous adsorbents of a high content of an inorganic phase (40–80%) were prepared by the zinc chloride activation of waste bio-char from the liquefication of pig manure at 900 °C. To increase both the content of the carbon phase and conductivity 10% nanographite was added to bio-char before the carbonization/activation process. Even though no strong effect of a char/activation agent ratio on the materials' porosity was found, they slightly differ in the surface areas and in the contents of the inorganic phase, which was found to be rich in calcium and magnesium. The synthesized materials were used as formaldehyde adsorbents from its low concentration of ∼2 ppm. The measured breakthrough capacity reached 0.78 mg/g and was higher than those measured on commercial carbons of high porosities. Since upon exposure to formaldehyde a marked surface acidification was found, the good performance of the composite adsorbents was linked not only to the porosity of the carbon phase but also to the effect of an alkaline environment in mesopores and to that of transition metals oxides on formaldehyde oxidation to formic acid. That acid likely formed salts with alkaline and alkaline earth metals present in our adsorbents. There was an indication that nanographite increased the extent of the formaldehyde oxidation reaction, which was demonstrated in a marked decrease in the surface pH of the spent adsorbent with the moderate amount of formaldehyde adsorbed.

Introduction

Formaldehyde (HCHO) is considered as one of the main indoor pollutants. Its sources are in decorating materials, paints, furniture, carpets, computers and printers and in other household materials. A HCHO concentration above 0.1 ppm causes an irritation of eyes and throat, difficulty of breathing, and nausea. Formaldehyde is known as having a carcinogenic effect on humans and a prolonged contact with this pollutant can lead to a nasopharyngeal cancer and can damage a liver and kidney [1,2]. The exposure limit to HCHO established by the World Health Organization (WHO) is 30 min at 0.08 ppm [3].

A continuous release of a low concentration of HCHO to an indoor environment motivates a search for effective methods to capture this pollutant. One of the approaches widely tested is adsorption or adsorption combined with photochemical degradation [4]. As adsorbents, either carbon-based or oxide-based materials have been investigated. Activated carbons or carbon fibers have been extensively tested either as as-received [[5], [6], [7], [8]], after an introduction of heteroatoms to carbon matrices [9], or after impregnation with various organic [[10], [11], [12]] or inorganic phases [[13], [14], [15], [16]]. A detailed discussion of the performance of carbon-based materials as the HCHO adsorbents is included in a recent review paper [4]. The problem is challenging since formaldehyde is a small molecule of a high boiling point so its physical adsorption at ambient conditions is very weak. Moreover, besides a hydrocarbon moiety with an affinity to a hydrophobic carbon surface, formaldehyde also contains a polar oxygen group with an affinity to a hydrophilic surface. Thus, porous carbon materials, to be effective HCHO removal media, should have both very small pores-to increase dispersive interactions, and some amount of polar adsorption centers/functional groups-to increase specific polar interactions. The latter, even though they can increase the amount of HCHO adsorbed, have also a high affinity to adsorb water. Since some amount of moisture is always preset in ambient air, water competes with formaldehyde for adsorption centers, diminishing the ability of an adsorbent to retain HCHO [5,9,17,18]. Even though water at ambient conditions and under a humidity less than 60% will rather not adsorb in hydrophobic ultramicropores [10], its adsorption in larger pores might prevent HCHO to enter small pores, which are its high-energy adsorption centers.

So far, it has been reported that besides an extended surface area and microporosity of the carbon phase, which are beneficial for the HCHO removal, there are also other important features and they include: 1) polar heteroatom-based groups [5,9,17,18] and basic nitrogen groups, which increase specific interactions [10]; 2) sulfur/nitrogen/oxygen that can be engaged in the reactive adsorption of formaldehyde [9,18]; 3) oxidants deposited on the surface to enhance formaldehyde decomposition/mineralization [19]; 4) specific hydrophobic entities repelling water [20]; 5) specific metals promoting a charge transfer/chemisorption of formaldehyde [21,22]. Even though combining some of these features is expected to lead to the most efficient HCHO removal media at ambient conditions, some combinations can sacrifice such an important quality for the HCHO adsorption as the porosity, which might be the case when metals/metal oxides/oxidants are deposited on a carbon surface [19]. It is important to mention that the specific effect of alkalinity caused by the specific composition of an inorganic phase on the HCHO removal in the composite adsorbents containing a carbon phase has not been addressed so far.

Even though the positive effect of the carbon surface basic nature has been indicated in the recent studies, no further explanation of the enhancing effect had been provided. Thus Carter et al., based on their study of HCHO adsorption on ACF, BPL carbon and Formasorb, concluded that at a low HCHO concentration the amount of formaldehyde adsorbed depended stronger on the density of basic groups than on that of acidic ones [5]. Surface basicity originating from intrinsic inorganic K and Ca compounds was also linked to a good performance of rice husk derived carbons [6]. That effect might have also governed a high HCHO adsorption on a carbonaceous adsorbent derived from sewage sludge [23], which showed a comparable performance to that of activated carbon in spite of its 40% smaller surface area. In spite of the fact that sewage sludge-based adsorbents are known of a high content of alkali and alkaline earth metal oxides [24], the effect of an inorganic phase content and its speciation on the performance as HCHO adsorbents has not been discussed. Another indication of an important role of an inorganic phase was a high HCHO adsorption capacity of bone char of a relatively low surface area [25].

On the other hand, extensive studies have been carried out on adsorption of formaldehyde on metal oxides. Examples include, but are not limited to, ZnO [26], MgO [27], Cr₂O₃ [28], TiO₂ [29] silica [30], alumina [31], and manganese oxide [32], It was found that the formaldehyde decomposition was greatly dependent on the nature of oxides. For example, on the (0001)-O polar face of ZnO formaldehyde decomposed to CO, CO₂ and H₂ via a formate intermediate [26]. On both a MgO film and a MgO(100) single crystal surface, simultaneous formation of methoxy and formate was observed upon HCHO adsorption [27]. Formate-like structures were also indicated as the products of surface reactions by Kakkar and coworkers in their theoretical studies of the HCHO adsorption on magnesium oxide nanomaterials [33]. On mesoporous aluminum oxyhydroxides (AlOOH) materials the amounts of HCHO adsorbed from the concentration of 150 ppm reached 1008 ppm [31]. Bellat et al. [34] studied adsorption–desorption of formaldehyde in the concentration range 0.01–2000 ppm with a relative precision of around ±0.01%. They tested zeolites (LTA and FAU cationic forms), mesoporous silica (SBA15), activated carbon (AC NORIT RB3) and metal organic framework (MOF, Ga-MIL-53) as formaldehyde adsorbents. The adsorption capacity of MOF, NaX zeolite, 3A zeolite, SBR15 and AC were 0.02%, 12%, 0.7%, 0.33%, 0.2%, respectively. On Ga-MIL-53 MOF type IV isotherm was measured and the adsorbed amount was the same as for that on AC.

Therefore, based on the recent experimental and theoretical advances in the development of HCHO adsorbents [4], the focus of this study is on pointing out the role of an inorganic phase, and specifically alkaline earth metals in carbon/inorganic phase composite adsorbents as an enhancing factor for the HCHO removal from ambient air. The composites were obtained from bio-char that was a by-product of a pig manure liquification process [35]. The porosity was developed using activation with ZnCl_2, which is a well-known chemical activation agent [36]. To increase the content of the carbon phase, to introduce more heterogeneity to the porous structure and to increase a conductivity and electron transfer, which might benefit HCHO oxidation, 10% nanographite was added as a surface modifier. To elucidate the adsorption mechanism, the surface chemistry and porosity of these materials were extensively characterized. By using the biochar derived from pig manure/liquefication waste as a precursor for formaldehyde adsorbents, we demonstrate the possibility of an environmentally beneficial waste recycling combined with air cleaning. Even though activated carbons are generally obtained by pyrolysis and physical (H₂O) or chemical activation (H₃PO₄, KOH, ZnCl₂) of the high-carbon content precursors such as a coal, peat, pitch or wood [37], the thermal conversion of waste biomass into adsorbents has recently gained increased attention owing to environmental concerns and the intrinsic properties of these resources [38].