Biphasic apatite-carbon materials derived from pyrolysed fish bones for effective adsorption of persistent pollutants and heavy metals

Highlights

• Biochar-like materials were prepared through pyrolysis of cod fish bones.

• They were tested to adsorb heavy metal and pharmaceutical persistent pollutants.

• Results showed different efficiency according to the pyrolysis temperature.

• Different kinetic mechanisms were observed for heavy metal (Pb(II)) and organics.

• Apatite and graphite were determinant in metal or organics adsorption respectively.

Abstract

Biphasic apatite-carbon biochar-type materials were prepared from pyrolysed cod fish bones and were assessed for the adsorption of persistent organic pollutants (pharmaceuticals diclofenac and fluoxetine), and heavy metals (Pb(II)).

The materials, prepared with a simple pyrolysis process at different temperatures (200–1000 °C), were characterised with XRD, FTIR, Raman and SEM. Results showed that the pyrolysis temperature had a significant effect on the features/composition of the materials: up to 800 °C, carbonate apatite Ca₁₀(PO₄)₆(CO₃) was the main component, while for higher temperatures oxyapatite Ca₁₀(PO₄)₆O was the dominant phase. Graphitic carbon was also detected.

The mixed apatite-carbon products (bone char) exhibited high adsorption efficiency.

Graphite carbon was the main adsorber for the pharmaceuticals, the best performing material being that pyrolysed at 1000 °C. X_m values of 43.29 and 55.87 mg/g were observed (Langmuir fitting), while K_F values of 5.40 and 12.53 (mg/g)(L/mg)ⁿ_F were obtained with the Freundhlich model (diclofenac and fluoxetine respectively). This is the first time that a biochar-like material has been used for fluoxetine adsorption. For Pb (II), the powder pyrolysed at 600 °C was the most effective, with the apatite playing a key role (X_m = 714.24 mg/g).

This work shows that a by-product of the fish industry could be converted into efficient materials for environmental remediation; according to the pyrolysis conditions, powders effective in the removal of either organics or heavy metals can be obtained. Moreover, with pyrolysis at intermediate temperatures, materials capable of adsorbing both kinds of pollutants can be produced, even if less efficient.

Introduction

The pollution of the environment is a problem causing always more concern, both in developed and developing countries. Due to increased industrial activities and an ever-larger global population, many pollutants are accumulating in the environment, especially soils and waters.

Heavy metals are associated with various industrial activities, including electroplating, chemical processing and leather finishing [1], [2]. When present at high concentrations, they can cause major problems to both human health and the environment. Ions such as Pb, Cd or Zn, for instance, can accumulate in marine species, eventually reaching the human food chain [3].

Pharmaceuticals are another class of compounds which pose an increasing threat to the environment, and they are accumulating in wastewaters due to their increased use, especially in developed countries [4], [5]. Moreover, many of them are very stable molecules, not degradable by the standard treatments or photocatalysis; because of this, they have been labelled as emerging persistent pollutants [6].

Fluoxetine (FXT) is a pharmaceutical widely used to treat depression (better known by the trade name Prozac) − as it is only partly metabolised by human body, much of it eventually ends up in wastewaters [7], and its harmful effect on some aquatic organisms is well documented [8]. Diclofenac (DCF) is a non-steroidal anti-inflammatory drug, and one of the most common pharmaceuticals. Literature reports that DCF can negatively affect the aquatic environment [9], [10], and due to its long term possible toxicity, DCF is currently on the pollutants “watch list” compiled by the European Union. Of course, if these persistent pollutants are in ground water and reservoirs, they can also end up in the human drinking water supply, forcing entire populations to also consume these pharmaceuticals.

Because of the increasing levels of such contaminants, it is crucial to develop remediation strategies, and suitable methods should be implemented to remove the pollutants from wastewaters, before they are released into the environment. Adsorption is an effective methodology for wastewater purification, and organic pollutants can be adsorbed by porous materials with high surface area [11]. Biochar-based materials, in particular, have been shown to be a promising solution, with high absorption efficiency. Biochar, such as charcoal, is made from biomass via pyrolysis, and is a stable solid, rich in carbon, which can endure in soil for thousands of years. As charcoal, biochar has been made by mankind for at least 2 millennia; the most common preparation method is a simple pyrolysis of the material, i.e. heating in inert oxygen-free atmosphere, generally in N₂ or Ar [12]. During the pyrolysis, the organic carbon is converted into inorganic amorphous carbon, the extent of the conversion depending on the pyrolysis conditions [13]. The resulting material has a high surface area and a porous structure, which makes it particularly suitable for adsorption. Indeed, literature reports biochar derived from many different sources, including agricultural/food wastes, soil sediments and plants [14], [15], [16], which have all been used to adsorb pollutants.

Although biochar itself has excellent properties, some studies show that enhanced efficiency or materials with additional functionalities could be achieved by combining it with other compounds. A composite of biochar-attapulgite, for instance, showed a higher adsorption rate than simple biochar towards the antibiotic norfloxacin [17]. The combination of biochar with CoFe₂O₄, on the other hand, resulted in a magnetic material, easier to separate at the end of the adsorption process [12]. However, biochars only adsorb organic compounds, and show much lower efficiency in the adsorption of heavy metals.

Hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, HAp) is known for its excellent adsorption properties, and it can adsorb bivalent heavy metals such as Pb (II), Zn (II), Cd (II), etc., with very high efficiency [18] due to ionic exchange between the metal and the Ca ion. HAp can also adsorb organic molecules, such as proteins; because of this, HAp-based columns are used in chromatography [19]. Moreover, HAp is also a very biocompatible material [20]. For all these characteristics, HAp would be the ideal material to use with carbon/biochar in a composite to adsorb both metals and organic compounds

Other than being synthesised from precursors/reagents, HAp can also be extracted from animal and fish bones, as it is their main component [21], [22]. As bones also contain a significant amount of organic matter such as collagen, which is mainly proteins, HAp-biochar composites have been obtained by bone pyrolysis, and there are reports of animal bone-derived biochar being used for environmental remediation, specifically for heavy metal or fluoride ion removal [23], [24], [25]. Only a couple of studies were published on the use of bone char for organic removal adsorption [26], [27], but the promising results confirmed the potential of these kinds of materials, and that they are worth further investigation.

In this work, we report biochar-type (bone char) materials produced by the pyrolysis of Atlantic cod fish (Gadus morhua) bones, a by-product of the food industry. Previous work performed with cod fish bones showed that it is possible to obtain HAp-based materials effective for environment remediation. Indeed, cod fish bone derived samples were shown to be effective for heavy metal removal [28], as well as for photocatalytic degradation of pollutants [29], [30]. Dried Atlantic cod fish bones have been show to contain 52.6 wt% minerals (of which 36.1 wt% Ca, 21.5 wt% P), 35.8 wt% protein (mostly as collagen), 7.8 wt% water and 2.3 wt% lipids) [31]. In the present work the pyrolysed materials were fully characterised, to determine their composition as well as their morphology. They were tested for the absorption of FXT, DCF and Pb (II), and desorption experiments were also performed. Moreover, experimental data were fitted with different mathematical models, to better understand the mechanisms of the process.