Colloids and Surfaces A: Physicochemical and Engineering Aspects

Colloids and Surfaces A: Physicochemical and Engineering Aspects Zn induced surface modification of stable goethite nanoparticles for improved regenerative phosphate adsorption

ABSTRACT

Iron oxide-based adsorbents showed potential to reach ultra-low phosphorus concentrations to prevent eutrophication and recover phosphorus. High affinity, high capacity at low phosphorus concentrations (less than one milligram per liter), good stability, and reusability of the adsorbent are key factors for economic viability. In this study, nanoparticles of goethite (alpha-FeOOH), a highly stable phase, have been synthesized with increasing Zn two+-doping, zero to twenty percent at. Zn/Fe, to manipulate the surface properties, following the results of a previous work. Mössbauer spectroscopy showed preserved goethite phase and increased point of zero charge at low Zn-doping percentages, while at higher percentages (greater than five percent at.) co-existing phases with increased specific surface area formed. Low concentrations (zero point one to ten milligrams per liter) batch adsorption tests showed increased phosphorus removal per unit mass with increasing doping. However, the highest point of zero charge, affinity and phosphorus removal per unit area were observed for the five percent at. doped sample, suggesting this dopant concentration to provide the most effective surface. A regeneration test, performed at a lower pH than usual, showed preserved, even improved phosphorus desorption with increasing doping. Mössbauer spectroscopy showed that the nanoparticle phase and composition, up to five percent at., doping was preserved throughout the process. These results are promising to develop a stable effective Zn-doped goethite-based adsorbent for phosphorus recovery at ultra-low concentrations.

One. Introduction

Phosphorus recovery is fundamental for three main reasons. First, it is an irreplaceable and vital nutrient, essential to the world food production sustainability, and its demand will further increase due to population growth, having increased already by seven point zero percent between twenty nineteen and twenty twenty-one and predicted to increase by fifty percent or more by twenty fifty. Second, phosphorus is a finite and non-renewable resource which comes from phosphate rock mines, with reserves available in only a few countries, with Morocco alone estimated to possess seventy percent of the worldwide reserves, making Europe almost completely dependent on its import. This led the European Commission to include phosphorus in the Critical Raw Materials list, asking for a more circular nutrients and resources management. Third, through agricultural runoff and wastewater treatment-plant effluents, phosphorus reaches surface water-bodies where it accumulates, becoming a pollutant. Phosphorus in water can be found both in particulate and solute state. The latter comprises phosphate, which is the bioavailable phosphorus fraction causing eutrophication, promoting algae bloom entailing several-related issues. Environmental damages, causing the death of aquatic life; health risk, as some algae are toxic; and socio-economic damages, estimated in million to billions of euros (up to two billion euros in the United States) of losses in tourism, fishing activities, property value, and so on. To prevent eutrophication, phosphorus concentrations in freshwater bodies need to be limited to ultra-low concentrations, below zero point zero two milligrams per liter, which is hundred times lower than current regulations for wastewater treatment-plant effluents (less than one to two milligrams per liter). Moreover, the Water Framework Directive requires all European surface waters to reach a good ecological status by twenty twenty-seven, and the latest report from twenty eighteen of the European Environment Agency highlighted that still sixty percent of the surface waters failed to meet this requirement. This could result in high fines if European countries do not comply with the Water Framework Directive by twenty twenty-seven. Therefore, it is important to remove phosphorus from water, as well as to recover it to be reused. Physical, biological and chemical methods have been widely investigated for phosphorus removal, but few of them display potential for phosphorus recovery, even less when targeting ultra-low phosphorus concentrations.

Among the chemical phosphorus recovery methods, adsorption showed promising results at concentrations below one milligram per liter, especially to target the ultra-low phosphorus concentrations and to recover phosphorus, since the process can be reversed. This makes reversible phosphorus adsorption a promising technique as a water polishing step, especially in the context of eutrophication prevention and the Water Framework Directive. A lot of work has been done on adsorption, often under laboratory conditions and either with single use or with expensive and sophisticated adsorbents, mainly focusing on the "maximum" adsorption capacity. However, at the ultra-low phosphorus concentrations of interest, affinity is the key parameter, since it describes how good the adsorbent is in removing phosphorus even when there is little of it left in water (a sort of adsorbent capacity at the low concentrations). It was shown that there is no correlation between the adsorption capacities and affinities of adsorbents reported in literature. Moreover, little efforts have been spent on phosphorus-recovery and adsorbent regeneration, the latter being a key factor to make the phosphorus-removal process economically viable. Studies showed that reusing the adsorbent fifty to one hundred times, would make the process economically convenient.

In this regard, iron oxide-based adsorbents constitute a promising option, being cheap due to their high abundance, and showing good properties for phosphorus removal, such as good affinity and selectivity. Also, by means of an alkaline wash they allow the recovery of phosphorus and the regeneration of the adsorbent, allowing for further reuse of the adsorbent, but also of the regeneration solution itself, by recovering phosphorus from it.

There are several commercially available iron oxide-based adsorbents but the mainly employed ones are porous granular adsorbents and hybrid anion exchange adsorbents. The former type, usually industrial by-products, is cheaper, relatively stable, and good performing thanks to their high specific surface area. However, this high specific surface area mainly comes from micropores, in which diffusion is very slow, resulting in slow kinetics. The latter type is a more expensive engineered adsorbent, consisting of iron oxide nanoparticles embedded in macroporous resin beads, and showed good phosphorus removal performances and faster kinetics. However, Kumar et al., twenty eighteen, observed a consistent phase transformation of the iron oxide nanoparticles already after few phosphorus adsorption/desorption cycles, which highly lowered the performances. In fact, these nanoparticles mainly consist of ferrihydrite, an amorphous and highly reactive species, which offers high specific surface area and hence high capacity. Nevertheless, ferrihydrite is also the most unstable iron oxide species, likely to transform over a wide pH range into more stable and less reactive phases, such as goethite and hematite.

Goethite, alpha-FeOOH, is one of the most abundant and most stable phases, which showed good affinity for phosphate. On the one hand, the stability of goethite makes it an interesting candidate from the regeneration point of view, implying a longer lifespan of the adsorbent. On the other hand, it might limit its reactivity, and thus its phosphate adsorption potential. Many studies suggested ferrihydrite as a promising adsorbent for phosphate recovery, due to its high capacity. Nevertheless, these studies were often performed at phosphate concentrations fifty to one hundred times higher than those of wastewater treatment plant effluents and surface water bodies, giving little insight into the potential for application, and often neglecting ferrihydrite affinity for phosphate. In this regard, Wang et al. in their comparison study between ferrihydrite, goethite and hematite phosphate removal performances, suggested ferrihydrite to be the most promising species, mainly based on its high capacity. In fact, they showed that ferrihydrite had the highest phosphate removal per mass capacity, more than ten and twenty times higher than that of goethite and hematite, respectively. However, at phosphate equilibrium concentrations below approximately seventy-seven milligrams per liter, goethite showed significantly higher phosphate removal compared to the others. This is also supported by its higher estimated affinity, about ten and twenty times higher than that of hematite and ferrihydrite, respectively. Moreover, ferrihydrite dissolution was observed throughout the experiments. These results support the ideas that goethite is the most promising species for targeting the ultra-low phosphate concentrations of interest, and that using adsorption capacity at high phosphate concentrations is a misleading parameter. Nevertheless, improving goethite properties for phosphate adsorption would be beneficial.

In this perspective, the current work aims at developing an efficient goethite-based adsorbent. To exploit goethite stability while increasing its phosphate recovery performances, doping constitutes a promising option. Doping is a technique widely employed in semiconductors and catalysis, in which an elemental metal, M, impurity, i.e., the dopant, is introduced in a hosting material to alter its properties. Doping has often been erroneously referred to when dealing with coating, assembling, loading or impregnation of metal and/or nanoparticles in composite materials. Pure and M-substituted goethite has been widely investigated, both as naturally occurring goethite rock or as synthetic goethite. The effects of impurities in goethite, mainly aluminum and manganese, have been investigated for many different applications, some including phosphate, arsenate, and divalent cations removal alone. Zinc for iron substitution in goethite has been investigated mainly from the crystallization point of view, and it has been proposed to promote goethite protonation as a charge compensation mechanism, and zinc ferrite precipitation above approximately ten percent.

substitution. However, the effect of zinc-doping on surface charge and its application to phosphate adsorption and desorption have never been investigated before. A previous work form our research group showed zinc-doped goethite to be promising for the proposed application, improving surface properties and adsorption performances. The current study aims at systematically investigating the effect of increasing doping on the goethite properties and phosphate adsorption and desorption performances.

In this context, a fine characterization and intrinsic properties of the doped goethite samples has been obtained using Mössbauer spectrometry as the main characterization technique. Mössbauer spectrometry is a high-resolution nuclear gamma-ray based technique mainly used to investigate iron-based materials, providing information on the sample properties from the "iron-nuclei point of view". Employed as a fingerprint technique, it is possible to retrieve mainly three parameters from the spectral analysis: the isomer shift, which provides information such as the oxidation state and character of ligands of iron atoms; the quadrupole splitting, which provides further information on the oxidation state as well as the charge distribution asymmetry around the iron nuclei; and the hyperfine magnetic field, which provides information on the magnetic ordering of the sample. Mössbauer spectrometry offers very high-resolution spectral features, and compared to other techniques, such as X-ray diffraction, it has the advantage to be sensitive even to very fine and amorphous nanoparticles and to be more specific in iron phase identification and quantitative speciation. Especially when performing measurements at different temperatures, since low temperatures are necessary to obtain the Zeeman split in the Mössbauer spectra, allowing the identification of superparamagnetic phases. Previous work in our group showed how Mössbauer spectrometry could provide a more thorough phase identification and detect phase transformation of iron oxide-based adsorbents, with respect to other studies in literature. Moreover, Mössbauer spectrometry can help in confirming a successful and homogeneous metal for iron substitution in doped samples, while providing information on the effect of the dopants on the structural, chemical and magnetic properties. This work stresses the importance of applying Mössbauer spectrometry to iron-based materials development and stability monitoring.

This study presents an investigation on the effect of increasing zinc for iron substitution in goethite nanoparticles, and its influence on the surface charge and the P adsorption/desorption performances, enabling the development of a stable and high performing zinc-doped goethite-based adsorbent for P recovery.