Elsevier

Journal of Cleaner Production

Volume 285, 20 February 2021, 124848
Journal of Cleaner Production

Ecofriendly renewable hydrogels based on whey protein and for slow release of fertilizers and soil conditioning

https://doi.org/10.1016/j.jclepro.2020.124848Get rights and content

Highlights

  • Renewable hydrogels based on alginate and whey protein were prepared.

  • Correlations between alginic acid content and hydrogel properties were evaluated.

  • pH responsive swelling behavior.

  • Slow release of urea and high water holding capacity.

  • Promising alternative to commercial hydrogels based on synthetic polymers.

Abstract

Hydrogels show potential in agriculture for overcoming issues associated with conventional fertilizers and irrigation. A combination of hydrogels and fertilizers would limit the loss of fertilizer and curb environmental impact, especially for nitrogen-based compounds, in addition to diminishing the frequency of irrigation. A set of renewable hydrogels, based on a mixture of whey proteins and alginic acid were developed as a soil conditioner and for sustained release of the urea fertilizer. Four separate formulations were prepared from different proteins at a polysaccharide ratio of 1%–10% w/w (with respect to protein content). The hydrogels were prepared by a heat-set process, applying calcium chloride as the cross-linking agent. The fertilizer was loaded into the hydrogel during the preparation stage to heighten loading efficiency. Investigation was made into the impact of alginic acid content on morphology, swelling behaviour - encompassing repeated swelling-drying cycles, and water retention in soil under different pH conditions. The loading capacity and release of urea from the hydrogels were studied, and the data processed in accordance with a mathematical model to discern any correlation between the structure of the hydrogel, the presence of alginic acid and the release mechanism. The results demonstrate how adding alginic acid promotes possible utilization of the whey protein hydrogel in agriculture.

Introduction

Recent years have witnessed the development of innovative systems able to control the release of agrochemicals in soil. Such systems are based on various materials, either synthetic, semi-synthetic or natural, and able to yield a large amount of fertilizer while also offering protection against rapid release (Rabat et al., 2016). Hydrogels as materials have attracted a great deal of attention due to their capability for absorbing and retaining a large amount of water within their structure (up to hundreds of times their weight) without dissolving (Calo et al., 2016). An additional advantage stems from their capacity to release up to 95% of the adsorbed water into the surrounding environment; once dried out, they can be re-hydrated through exposure to water.

Hydrogels are primarily used as soil conditioners in an agricultural context, whereby they control the moisture of the soil and supply water to plants, as well as serving for encapsulation and sustained release of agrochemicals (Song et al., 2020). Although the advantages of employing hydrogels in agriculture have been demonstrated, most commercially available products are based on polyacrylamide (PAAm) and acrylate derivatives (Lv et al., 2019). Since these are not fully biodegradable, they are considered potential soil contaminants. Even though PAAm is not actually designated as toxic, commercial formulations contain residual amounts of acrylamide – a neurotoxic and carcinogen, raising concerns over possible contamination of soil and food. Hence, biopolymers have been rising in popularity because they are environmentally friendly and biodegradable, while also usually being less expensive to produce than synthetic materials (Mishra et al., 2018; McClements et al., 2017).

Of the numerous natural polymers available, the preference is for those with functional groups that make them easy to modify and sensible to variation of the surrounding environment, e.g. pH or temperature (Tang et al., 2020; Thombare et al., 2018). Hydrogels that are pH-sensitive and based on biopolymers are primarily obtained from polysaccharides, such as chitosan, alginates and cellulose, or from protein which has undergone heat-induced or cold gelation.

Whey proteins make for interesting raw materials in the formulation of hydrogels. They consist of the globular proteins beta-lactoglobulin (ca 65% w/w) and alpha-lactoglobulin (ca 25% w/w), primarily responsible for the capacities of gelling, emulsifying, foaming and hydration (Gunasekaran et al., 2007). Gelling is chiefly facilitated through an aggregation process induced by increase in temperature, which unfolds the native protein and exposes polar residues, thereby allowing formation of the gel network (Kharlamova et al., 2018A, Kharlamova et al., 2018). Mixed hydrogels based on a combination of whey proteins and polysaccharides have attracted great interest in recent years, as synergistic interactions between the two components lend additional functional properties to the subsequent hydrogels, compared to those obtained from single components (Ozel et al., 2017). The repulsive and/or attractive forces between the two biopolymers give rise to different behaviour, including associative phase separation, while co-solubility instigates soluble/insoluble complexes, which possess functional properties beyond those found in polysaccharides or proteins alone (Devi et al., 2017; Ates et al., 2020; Chaudary et al., 2020). Moreover, mixed gels benefit from enhanced structural features alongside the ability to carry active agents. The resultant properties of the gels stem directly from the nature and concentration of the components, the given weight ratio and the ionic strength of the surrounding environment (Klein et al., 2020). Herein alginic acid sodium salt has been chosen as component to add to the whey protein concentrate due to its recognised and well-known properties suitable for environmental application (Sourbh et al., 2018; Thakur et al., 2018).

This study set out to develop a set of mixed hydrogels based on a combination of whey protein concentrate and alginic acid sodium salt, deemed applicable for simultaneous or alternate use as a soil conditioner (by releasing water) and an agent for the controlled release of fertilizers. Soil conditioners aid retention of water in the soil by creating favourable conditions for plant roots to grow; while agents reduce any loss of the agrochemicals they contain and inhibit unwanted conversion - especially for nitrogen-based fertilizers - into derivatives with the potential to harm the environment. In this context, urea was selected as a model to assess the loading and release capabilities of the hydrogels, since it is extensively deployed as a fertilizer in agriculture. The hydrogels were prepared by a heat-set process, after which the effect of alginic acid (1%, 5% and 10% of the weight of the whey protein) was investigated on morphology, swelling, water retention in soil, encapsulation efficiency and urea release kinetics.

Section snippets

Materials

Alginic acid sodium salt, urea and 4-(Dimethylamino)benzaldehyde were purchased from Sigma Aldrich. Nutri Whey™ 800F (a pure whey protein product, 80% obtained from acid whey) was sourced from FrieslandCampina Ingredients, NL, while acetic acid and hydrochloric acid came from Chromservis, Prague, Czech Republic.

Preparation of protein-polysaccharide hydrogels

Sets of hydrogels based on whey protein concentrate (WPC) in combination with alginic acid sodium salt (ALG) were prepared by a heat-set process. A 20% w/w solution of WPC in distilled

Hydrogel morphology

Investigation was made of the surface morphology (Fig. 2) and cross-sections (Fig. 3) of the dried hydrogels without urea by scanning electron microscopy. The micrographs presented in Fig. 1 demonstrate how the presence of ALG influenced surface morphology and the surface properties of the material as a consequence. The surface of every sample is nonhomogeneous, with variation in the pattern of the structure. Comparing the two extremes - the surfaces of WPC (Fig. 2 B, C) to WPC+10% ALG (Fig. 2

Conclusions

A set of hydrogels based on natural sources were prepared as a soil conditioner and as carriers for the release of urea. The hydrogels were obtained by a heat-set process, applying whey protein concentrate in combination with alginic acid in addition to CaCl2 as the cross-linking agent. The amount of whey protein was maintained as a constant in the given formulations, while the variable in the experiment constituted the amount of ALG incorporated; this was expressed as the weight ratio between

CRediT authorship contribution statement

Antonio Di Martino: Conceptualization, Methodology. Yelena A. Khan: Methodology, Formal analysis, Analysis and interpretation of data, Writing - original draft, preparation. Silvie Durpekova: Methodology. Vladimir Sedlarik: Supervision, Funding acquisition, Writing - review & editing. Ondrej Elich: Writing - review & editing. Jarmila Cechmankova: Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was funded by the Ministry of Agriculture of the Czech Republic (Project no. QK1910392).

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