Research papersVariation of the hydraulic properties in sandy soils induced by the addition of graphene and classical soil improvers
Graphical abstract
Introduction
The hydraulic properties of a soil are strongly influenced by its texture and structure, which also determine the soil porosity (Lai and Ren, 2016). Soil porosity, and pore size distribution, is very dynamic and can be altered by anthropogenic activities such as the addition of organic and inorganic improvers to the soil (Badorreck et al., 2012). Indeed, the application of these materials to soils can have a large impact upon their hydraulic and solute transport parameters (Igalavithana et al., 2017, Abel et al., 2013). The application of Biochar on sandy soils, for example, can improve pore size distribution increasing the number of meso- and micro-pores and decreasing the number of macropores (Dokoohaki et al., 2017), which in turn change the soil hydraulic conductivity (Ibrahim and Horton 2021). On the other hand, compost application in sandy soils characterized by low organic matter content can have a positive impact on soil fertility and on soil properties (Shiralipour et al., 1992). Compost addition can also alter soil hydro-physical properties changing size, form, number, and distribution of soil pores (Fischer and Glaser, 2012), and can improve soil water retention (Gonzalez and Cooperband, 2002). Likewise, the addition of zeolites in sandy soils can improve the soil water content and change the pore size distribution reducing the number of large pores and increasing the number of small pores (Ibrahim and Alghamdi, 2021). Given the ability of these materials to alter soil hydro-physical properties it is essential to test new materials, such as the engineered carbonaceous material (ECM), in controlled laboratory conditions to foresee improvements or drawbacks of their potential application to the soil in the open field. Graphene, for example, is an ECM designed as bidimensional carbon atoms monolayers (Novoselov, 2011), with peculiar and very promising properties (Facure et al., 2020) which has found application in different fields, like medicine with drug delivery and tumor therapy (Song et al., 2020), or electronics within optoelectronic devices and transistors (Kusmartsev et al., 2015) and desalination membranes (Mortazavi et al., 2020), among many other emerging fields. The graphene manufacturing industry has seen a dramatic expansion over the last decade (Tabish et al., 2018) and it is expected to further increase via non-expensive production methods (Rabia et al., 2020). The massive use of graphene in various compartments of the hi-tech global economy will likely create large graphene-based wastes (da Costa and Hussain, 2020). Thus, there is a strong push to find new ways to introduce such new wastes into the circular and sustainable economy (Korhonen et al., 2018). To assess graphene’s potential to be introduced in a circular and green economy chain, graphene was tested as soil improver due to its good adsorption capacity and thus its potential to retain nutrients (Goodwin et al., 2018). In fact, graphene-based materials like hybrid foam of graphene and carbon nanotubes, have been proved to be effective in organic compounds removal from water (Dong et al., 2012), as well as reduced graphene oxide coated cotton (Sun et al., 2014), or functionalized graphene aerogels (Wang et al., 2020). Moreover, graphene has remarkable hydrophobic and oleophilic properties so that it is used in treatment plants to remove organic pollutants from wastewater (Thakur and Kandasubramanian, 2019). While the use of graphene mixed in sandy layers or walls in agricultural landscapes to retain organic agrochemicals (like pesticides) was demonstrated in laboratory batch experiments (Gupta et al., 2012). Still, little is known on possible variations of the hydraulic properties induced by graphene incorporation into soils and subsoils (He et al., 2017).
For instance, sandy soils are known for their low water holding capacity associated with a low fertility (Reichert et al., 2016), being usually poor in organic matter and nutrients (El-Naggar et al., 2015), thus the application of soil improvers, organic and inorganic, can increase their water retention capacity (Abel et al., 2013, Nakhli et al., 2017) and alter their physical properties like hydraulic conductivity, dispersivity, or effective porosity (Fetter et al., 2018, Sudicky, 1986). Although many studies have focused on the effects of compost, biochar, and zeolite on the physical properties of soils (Basso et al., 2013, Colombani et al., 2014, Siedt et al., 2020), studies on graphene effects on porous media are rare and often focused on the graphene oxide mobility rather than on the changes induced in the porous medium properties (Chen et al., 2018, Wang et al., 2021, Zhou et al., 2016). Beyond the peculiar characteristics of graphene listed above, what distinguishes graphene from the other materials of this study, is the extreme difference in bulk density. Graphene has an extremely low bulk density (Iqbal and Abdala, 2013) and this could guarantee similar performance to other materials but with the use of a significantly lower amount. In addition, the decrease in the dry bulk density of a soil can change the shape and the distribution of the soil pores, increasing the number of small pores and decreasing that of large pores, resulting in a reduction of the hydraulic conductivity (Assouline, 2006, Assouline, 2005).
In view of the lack of studies on graphene impact on soil hydraulic and transport parameters, in this study traditional soil improvers (compost, biochar, and natural zeolite) and an innovative one (graphene) were tested and compared using both a calcareous sandy soil and a siliciclastic riverine soil. Specifically, the aim of the study was to quantify the variations of the hydraulic properties of sandy soils induced by the addition of graphene in saturated soil column experiments.
Section snippets
Study materials
A calcareous sandy soil (hereinafter referred to as “α”) and a siliciclastic riverine soil (hereinafter referred to as “β”) were selected for this study (Table 1). The grain size was determined via wet and dry sieving methods. The calcareous soil “α” was a coarse sandy soil, while the siliciclastic soil “β” was a medium-fine sandy soil. Both soils had low soil organic carbon (SOC) percentage (α = 0.55; β = 0.48).
The soil improvers were purchased from commercial manufacturers and their raw
Changes in soil properties induced by graphene and other soil improvers
Hydraulic conductivity (K) did not vary appreciably among different treatments (Fig. 2) for both soils α and β. K values showed relatively large standard deviation values, since K is a parameter easily influenced by the arrangement of the pore space, which can be relatively different among columns replicates filled via dry packing technique (Lewis and Sjöstrom, 2010). Despite this limitation, K values showed an average value of approximately 4 × 10−4 m/s which is typical for sandy soils (
Discussion
In the two sandy soils α and β tested in this study, there were no significant changes in parameters driving the water leaching rate, like K and θ, due to the addition of the different soil improvers. The only exception was represented by Biochar addition that increased K values for both soils, however, due to the large standard deviation the increase was not statistically significant (Fig. 2). The recent literature points to a decrease of K when biochar is added to sandy soils (Esmaeelnejad et
Conclusions
In this study, graphene addition to calcareous and siliciclastic sandy soils was monitored, modelled, and compared to the addition of other improvers (compost, biochar, and zeolite) via saturated column experiments, to assess the eventual changes in the hydraulic parameters. While hydraulic conductivity, porosity, and effective porosity were relatively unaffected by graphene and classical soil improvers, given the large proportion of pristine soils in the experimental set up, specific
CRediT authorship contribution statement
Luigi Alessandrino: Formal analysis, Investigation, Methodology, Writing – original draft. Anna Laura Eusebi: Resources, Supervision, Writing – review & editing. Vassilis Aschonitis: Formal analysis, Validation, Writing – review & editing. Micòl Mastrocicco: Conceptualization, Data curation, Methodology, Project administration, Supervision, Writing – review & editing. Nicolò Colombani: Conceptualization, Data curation, Methodology, Software, Visualization, 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.
Acknowledgments
The authors would like to thank Mirco Marcellini for helping during the laboratory activities and for the ion chromatography analyses. The research activities of Luigi Alessandrino were part of the Environment, Design and Innova-tion Ph.D. Program funded by the V:ALERE Program (VAnviteLli pEr la RicErca) of the Univer-sity of Campania “Luigi Vanvitelli”.
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