For blended DAP and KNO3 , both reverse fluxes of potassium and nitrate were similar regardless of the FS tested, but smaller than with KNO3 alone. Therefore, it is hypothesized that the Js of K+ and NO3− are more dependent on DS than FS composition. However, it is worth mentioning that when using blended salts, nitrate reverse fluxes were even lower with Na+ in FS than when using only DI water due to lower ion exchange. Thus, the presence more than the type of salt in the FS influenced the Js of nitrate ions. The obtained results show the complexity of ion interactions because the tested DS ions behaved differently and were influenced, to different degrees, by both the presence of salts in FS and the DS composition. Forward fluxes were minimal for all ions except for Na+ in tests with DAP and blended salts in DS, with Jsf ranging from 2.1 to 2.9 mmol m− 2 .h− 1 . As indicated by Hancock and Cath , feed solutes with larger hydrated radii , had better FO membrane rejection than monovalent ions . High sodium fluxes are commonly reported , and in this study its presence influenced ammonium reverse fluxes. This is because Na + has higher diffusivity than Mg2+ and therefore its transport to the DS facilitates the ammonium transport to FS and vice versa . The percentage of mass of ion passage from FS to DS showed higher Na+ passages with the pair of monovalent ions , because of the retarded sodium diffusion due to the divalent SO42− presence in the case of Na2SO4 in the FS . Similarly, lower anion passages were experienced in presence of the divalent Mg2+ . These results point out the importance of FS composition for the performance of the FO process. Concerning draw solutes, divalent ions from FS showed lower forward fluxes, and thus the final FS and DS were less contaminated with ions from the opposite solution when divalent ions were present on both sides of the membrane. Fig. 6 indicates the distribution of solutes in FS and DS at the end of the tests.
A high percentage of draw solutes passed to the feed side, resulting in almost 100% fertilizer losses to FS in some cases . Such observations are of utmost importance as they jeopardize the interest of the FDFO concept. This reinforces the fact that KNO3 cannot be used alone as DS for fertigation. Comparatively,hydroponic bucket using DAP or blended fertilizers reduced the losses, which however were still very high for ammonium, nitrate and potassium in most cases as soon as salts were present in the feed solution. Ideally, osmotic equilibrium should be achieved by an equal EC between original FS and DS solutes without nutrient losses, and not because of large reverse flux of DS solutes to FS, limiting the DS dilution. However, the obtained Js and nutrient losses were very high, osmotic equilibrium was reached, and water fluxes decreased as a consequence of the salinity buildup in FS caused by the reverse fluxes of DS ions. Existing studies are controversial since some of them point out that the solute fluxes from DS to FS are not influenced by the presence of salts in FS , while others indicate the opposite . In this study, while the presence of salts in FS did not influence PO43− behavior, it did clearly influence both Js and losses of NH4+. Although the influence of salts in FS was not clear in terms of reverse fluxes of K+ and NO3− , Fig. 6 shows that it strongly influenced the passage of K+ and NO3− ions to the FS. Adequate nitrogen and potassium dilution for direct hydroponics application was achieved in some tests with magnesium ions in FS . For the rest of the cases, due to the high reverse fluxes, nitrogen and potassium concentrations were below the target ranges. In contrast, phosphate reverse fluxes, as well as mass dilution were minimal, and phosphate concentrations in the final DS were well above desired concentrations and toxicity levels. Therefore, the presence of salts in FS plays a fundamental role in the final concentrations of NPK in DS. Additionally, one of the main problems of sodium diffusion in FDFO is its final concentration in the DS, since the DS is intended to be used as a nutrient solution for direct application in hydroponics. Sodium concentrations over 50 mg.L− 1 are toxic for the plants .
Average sodium concentrations in the final DS of tests with NaCl and Na2SO4 in FS were above the level of toxicity . These results indicate that Na+ forward fluxes may compromise the quality of the final DS and the general efficiency of the FDFO process. Nanotechnology is now considered one of the most promising applied disciplines and involves the manipulations of materials at the atomic level and the manufacture and assembly of metal, semimetal, and non-metal nanoparticles and their various industrial and agricultural applications . However, all NPs manufactured are ultimately released into the environment and eventually reach cultivable soils. Accordingly, NPs are considered an emerging class of contaminants . ZnO-NPs and CuO-NPs are among the most studied nano metal-oxides in terms of their environmental impacts, but their effects on cereal crops like maize have not been well researched. For example, Cu-based pesticides account for a sizeable proportion of the global demand , and research interest had shifted towards the use of CuO-NPs rather than Cu bulk or salt-based pesticides , due to their nano-specific properties such as higher S/V ratios, surface energies, and bioreactivities. CuO-NPs are also used as catalysts and in sensors, batteries, solar energy converters, textiles, optics, and microelectronics . Similarly, ZnO-NPs have potential applications in the manufacture of solar cells, sensors, piezoelectric devices, cosmetics, and medical devices . Given the potential usage of CuO-NPs, it is inevitable that as a result of discharge, soil and water become major nanoparticle sinks. Therefore, NPs pose potential issues concerning the metabolic activities, structures, and functions of plants and for human health due to the consumption of edible plants and the contamination of water . Plants have large surface areas and are potential ecological receptors for NPs, and thus, NPs pose the risk of adversely affecting crop production .
Recent studies have shown that CuO and ZnO NPs are accumulated by some terrestrial plants at high levels and cause phytotoxic effects, which may be due to the uptakes of Cu2+ or CuO-NPs. Furthermore, the addition of maize root exudates during the co-cultivation of maize seedlings in the presence of 25 mg L− 1 of CuO NPs for 7 days increased CuO-NP accumulation and hampered seedling growth . On the other hand, ZnO-NPs at 100 mg L− 1 alleviated Zn deficiency in maize seedlings after adsorption on seed endosperm and internalization . The majority of studies on maize/ZnO-NP interactions have been performed on agar media up to the seedling stage, and in one such study, ZnO-NPs were found to reduce seedling root growth by 51% at 1000 mg L− 1 . Nonetheless, we considered that further comparative studies were required to investigate in more detail the response of maize plants to different concentrations of ZnO-NPs and CuO-NPs in different growth media. Metal-oxide based NPs penetrate plant cells by endocytosis or other passive/facilitated transport routes and are then either accumulated at locations near entry points or are transported to other plant organs, and their accumulations and interactions with cellular components damage plant anatomies and physiologies . Furthermore, it has been reported the uptakes, translocations, and bioaccumulations of NPs in plants depend on physicochemical features, genotypes, and plant anatomies . On the other hand, free metal ions such as Cu2+ and Zn2+ are known to adversely impact crop plants, and due to their solubilities in water and deposition in soils to pose an increasing environmental challenge.Maize was selected for the present study due to its economic importance. Maize is an edible flowering plant of the family Poaceae and is the third most-produced cereal crop after rice and wheat .
It is considered a human staple food and animal feed in most developing countries and is rich in proteins, carbohydrates, essential minerals, fat, iron, and vitamins. Furthermore, maize has also been reported to interact with some metal-based NPs such as TiO2- and Fe3O4-NPs . Little is known about the comparative impacts of nano and bulk ZnO or CuO or Zn2+ and Cu2+ ions on maize cultivation in different media, in which the effects of nano, bulk, and ionic species may be differentially influenced by their different mobilities and interactions with maize plants. Although the use of soil-less media is desirable for the toxicity assessment of NPs on crop plants, stackable planters soil-based approaches are more relevant because soil effects can alter material activities. Thus, the uncontrolled disposal and improper recycling of CuO-NPs and ZnO-NPs in the environment is likely to increase plant exposure, which in turn is likely to affect their accumulation kinetics and other ecosystem trophic levels. Therefore, the objective of the study was to assess the phytotoxic impacts of nano and bulk ZnO and CuO and Zn2+ and Cu2+ on maize using plate assay, hydroponic conditions and soil environment. The depths of ZnO- and CuO-NP penetrations were detected and visualized by SEM, EDX, and elemental mapping analyses, and NPs aggregates and ultrastructures in maize root and leaf cells were examined by TEM. Accumulations of Zn and Cu from NPs, bulk materials, and ions in maize roots and metal translocations to upper ground parts were investigated by ICP-MS. The presence of chemical functional groups and possible alterations were observed by FT-IR of lipids, proteins, and carbohydrates in roots and shoots. Root cells were also analyzed by flow cytometric analysis to detect the apoptotic growth phase . In addition, we assessed the impacts of NPs, bulk materials, and ions on phosphorus accumulation, soluble protein and chlorophyll production, and on the inductions of proline, oxidative radicals, and antioxidant enzymes. Few reports have been issued on the adverse impacts of metal-oxide NPs on the growths and biochemical attributes of maize plants , but few are available on the toxic effects of ZnO-NPs and CuO-NPs on physiological and morphological parameters of maize grown in different media or compared these effects with those of bulk ZnO or CuO or Zn2+ or Cu2+. Similar to the current study, ZnO-NPs at 400 mg l− 1 reduced maize seed germination maximally by 53%, but Zn2+ had no negative impact .
In an earlier study, CuO-NPs at 100 mg L− 1 significantly inhibited root elongation of maize seedlings but CuO-bulk did not . Because of their increased uptakes and consequent accumulations, soil adsorbed ZnO- and CuO-NPs interact with many cellular components, and thus, interfere with the metabolic activities of maize and cause cellular damage that might reduce crop production. To check the adsorption of NPs to soil constituents, SEM-EDX scanning of soil before and after adding CuO-NPs showed the presence of a prominent Cu signal in the EDX spectrum that was not detected in non-treated control soil . Reduced water flow and limited root hydraulic conductivity caused by the physical adsorption of NPs of ZnO or CuO into the nano-sized pores of maize roots restrict growth , and this reduction in water availability inhibits maize leaf growth and transpiration by physically blocking apoplastic flow. Frequency of occurrence of smaller sized particle aggregates after addition of ZnO and CuO NPs in soil was higher than that of non-treated soil which suggests the availability of added NPs and their role in physical blockage of maize root pores . Furthermore, resulting reductions in the supplies of water/nutrients and the higher intracellular concentrations of Zn and Cu could disturb root and shoot transcriptomes and suppress the expressions of genes that participate in primary, cellular, and secondary metabolic processes . At lower concentrations, Zn and Cu species improved the biological parameters . This could be due the fact that Zn when present at below threshold levels in plants, acts as a co-factor for many essential enzymes of plants.