1. Introduction
The contamination of soil with wastewater has become a global problem over the last few decades, along with increases in the demands on the food and water supply [
1]. Natural processes such as weathering and volcanic eruptions are the main source of heavy metal accumulation in the environment [
2]. However, exposure to synthetic fertilizers and heavy metal discharge via anthropogenic activities have become a major threat to the environment [
3]. Wastewater pollution has increased at an alarming rate due to the suboptimal discharge of effluents from rapidly increasing global industrialization [
4]. Different units and industrial pools (chemical, cement, leather, textile steel, petrochemical, food processing, construction, rubber manufacturing, crockery, publishing, and paper printing) produce pollutants that are released into the wastewater [
5]. However, the discharged wastewater of different industries is the major source of soil metal pollution such as iron (Fe2+), cobalt (Co2+), nickel (Ni2+), cadmium (Cd2+), zinc (Zn2+), lead (Pb2+), copper (Cu2+), chromium (Cr2+), mercury (Hg2+), molybedenum (Mo2+), and selenium (Se2+) [
6]. Discharged wastewater is directly ruining agricultural fields, as the harmful chemicals as well as pollutants released by industrial units into nearby drains have caused drastic effects that alter the sustainability of the ecosystem. Heavy metals are the elements with an atomic weight between 63 and 200 and a specific gravity greater than 4. The specific gravity of these metal ions is five times greater than those of water bodies. Trace amounts of some metals are required by living organisms; however, any excess amount of these metals can be detrimental to organisms [
7]; hence, pollution monitoring and trace metal eradication are essentially needed. Moreover, the application of nanomaterials is of great importance for phytoremediation, improve crop production and ecosystem restoration [
8], as NPs may interact with pollutants in water and absorb them due to their special physicochemical properties: being nanosized, having a large specific surface area, and being potentially efficient carriers for many pollutants including heavy metals and organic pollutants [
9].
It is estimated that the well-being of humans is affected due to heavy metal intrusion in water and vegetables grown under wastewater. In Karachi alone, >6000 industrial units discharge about 300 million gallons of industrial waste/day, which are dumped in the coastal areas affecting living beings in the ocean [
10], as the accumulation of heavy metals in fish (iron clogging in fish gills), marine birds, and seaweeds has resulted in their increased mortality and, ultimately, has disrupted the process of the food chain [
9]. Almost 10% of the total dyes used in textile industries are disposed as wastewater and are one of the major causes of water pollution [
11]. The effluents from industries are complex, containing a wide variety of dye products (such as dispersants, acids, bases, salts, detergents, and oxidants), and the discharge of these colored effluents goes into rivers and lakes [
12]. The excessive exposure of heavy metals (arsenic (As), lead (Pb), cadmium (Cd), zinc (Zn), and copper (Cu)) in discharge areas and agricultural land also causes a negative impact on marine organisms, cultivated crops, and ultimately human beings through the food chain [
13]. The above-mentioned problems warrant proper treatment of wastewater effluents prior to their final discharge into the environment, particularly for large water bodies. Not a single industrial sector has paid any attention toward the predischarge treatment of wastewater. Industrial as well as sewage effluents are usually discharged into agricultural areas, where wastewater is used for the purpose of irrigation. It not only poses a harmful effect on the soil surface and its characteristics but also affects the quality of crops. About 20% of the irrigation in Pakistan uses untreated wastewater [
14]. Crops that are being irrigated by wastewater accumulate large concentrations of heavy metals. Metal ions are attached to the negatively charged soil particles, and when cations detach from soil particles, they are freely available and adsorbed by plants via water uptake [
15]. A variety of contaminated crops are consumed by people, which becomes a major route for heavy metals to enter the human body via the food chain, causing serious health hazards [
16]. A variety of toxins present in industrial waste alters soil properties and the crop yield, besides affecting the human immune system [
4]. Toxic pollutants also negatively affect aquatic life. The accumulation of heavy metals and other toxins in fish (iron clogging in fish gills), marine birds, and seaweeds has increased mortality and has ultimately disrupted the process of the food chain [
17].
Nanotechnology is a promising field to treat wastewater contaminants in a reliable and cost-effective manner. Bioremediation process is carried out with the help of different types of NPs, which can adsorb effluents (heavy metals) from contaminated water [
18]. In the fields of environmental science and technology, nanomaterials are identified as effective substances due to their large surface area, high thermal resistance and capacity, very small size, self-assemblage, and high chemical reactivity [
19]. The basic metallic NMs are metal oxides, quantum dots and noble nanomaterials. Due to their small size, they show a significant change in their atomic structure and magnetic characteristics. Different nanoscale materials have a large surface area, a size within 1–100 nm (means one-billionth of a material) [
20], and physicochemical characteristics with a notable impact in lowering the concentration of pollutants, especially heavy metals from wastewater [
21]. Nanoparticles synthesized by algae are a very simple, cost-effective, and time-saving method [
22]. Algae are known to have potential as a suitable biosorbent because of their fast and easy growth as well as their widespread availability [
23]. Algal extract contains a variety of pigments, polysaccharides, proteins, and peptides along with a functional group [
24] that acts as a reducing agent and reduces the metals’ ions at optimal conditions, without producing any toxic byproducts [
25]. The sorption capability of algae has been attributed to their cell walls, which are often porous and allow for the passage of molecules and ions in aqueous solutions. Nanoparticles synthesized by microalgae have gained lot of interest, since they can bioremediate toxic metals, further converting them to more amenable forms [
26].
The treatment of polluted water is the leading application of nanotechnology. Different nanomaterials including nanotubes, magnetic nanoparticles, a variety of nanofilter membranes, metal oxide NPs, and nanosensors help to isolate heavy metals in polluted water [
27]. Some studies on nanoparticles have been conducted on wastewater treatment in conventional activated sludge (CAS) processes [
28]. The activated charged functional groups on the surface area of NPs have a great affinity to adsorb metallic ions. With the passage of time, the demand for NPs due to their superb adsorption ability has increased [
29]. They are currently applied in different industries including the biomedical field (acting as antimicrobial, drug delivery, biosensing, biomarker mapping, molecular imaging, targeted therapy, and antitumor agents) [
30]. The green synthesis of ZnO nanoparticles has increased the demand for these nanomaterials because of their ecofriendly nature, cost-effectivity, and time- and energy-saving process compared to other conventional physicochemical processes [
31]. An agricultural area that is continuously irrigated with wastewater bears a consistently higher concentration of toxic pollutants. There are 16–68.4% protein content decreases due to the toxicity of cadmium in maize [
32]. It also influences the nitrogen concentration in different tissues of the plant. These contaminants affect the microbial community in soil very badly, disturb ecological processes, and ultimately result in stunted growth and decreased crop yields [
33]. In view of the different advantages of NPs, the aim of the present study was to assess the ability of ZnO-algal-synthesized nanoparticles to reduce heavy metal content and to lower down the concentration of other pollutants in wastewater. We hypothesized that the removal efficiency of toxic waste using an algal nanoparticle would increase with time and dosage. The following questions were addressed in this study: (1) What concentration of algal-synthesized nanoparticles would be effective in removing trace metals from wastewater? (2) Does the removal efficiency of toxic waste increase with the passage of time?
4. Discussion
Nanotechnology is a promising and advanced field of science particularly for treating wastewater effluents. Green synthesis of nanoparticles from algal resources is cost-effective due to their high surface area, high reactivity, and strong mechanical properties that are highly efficient and effective for wastewater treatment related to tannery effluents [
30].
The synthesis of nanoparticles was observed by the initial color change of
Oedogonium sp., from light green to pale yellow. Physicochemical parameters such as pH, EC, TDS, chlorides, BOD, COD, and heavy metals (Cr, Cd, and Pb) were also monitored, along with different treatments. The calculated values in pretreatment effluents were as follows: pH 8.21, EC 23.08 mS cm
−1, TDS 11.54 mg/L, BOD 420 mg/L, COD 1123 mg/L, Cl- 6750 mg/L, Cd 210.5 mg/L, Cr 310.1 mg/L, and Pb 75.5 mg/L. Some macrophytes and algal species effectively reduced pollutants from industry effluents with a substantial reduction in pH (6.97, 6.97, and 6.42 in the case of macrophytes and 6.46, 6.59, and 6.34 in the case of algal species) with the passage of time (i.e., after 1, 2, and 3 d of incubation) [
49]. Although in this study, the pH levels began to lower down with time, the lowest values were recorded after 45 d of incubation at 5% and under the treatment of 1 mg NPs. It is also suggested that increases in chemical treatment may differentially influence physiochemical parameters such as pH, EC, and COD [
50]. Using different doses of FeCl
3 as the coagulant showed that increases in treatment dose did not reduce the EC levels from tannery effluents. Contrary to previous findings, the present study showed an effective reduction in EC burden from different concentration of effluents. Precisely, the application of 1 mg NPs at 15%, reduced the EC value from 17.58 mg/L to 13.09, 8.71, and 5.37 mg/L after 15, 30, and 45 d, respectively [
51]. Our results suggest that biosynthesized ZnO nanoparticles efficiently reduced the EC content compared to the chemical approach. The bioremediation potential of ZnO nanoparticles toward the reduction in TDS content was good enough compared to that of a revolving algal bioreactor [
31]. In another study, researchers used such reactors to lower the burden of TDS from wastewater, and the technique proved to be 27% efficient [
52]. In the present study, 53% of removal efficiency for TDS content was found at 100% concentration for the leather tanning industry, with a significant reduction in values from 11.54 mg/L to 8.72, 6.73, and 6.17 mg/L after 15, 30, and 45 d, respectively, at 1 mg nanoparticle treatments. A further 5% reduction in TDS content was observed before and after the experiment. Moreover, a significant reduction in chloride content was achieved at 5% effluent concentration and 1 mg of ZnO nanoparticles. At 10%, the effluent load of the chloride reduction was within permissible limits (about 1000 mg/L Cl
−1) but at 15% and 100% the values were higher than the permissible limits. The precipitation method was applied at a pH range of 6–11 and a temperature of 30 °C. The amount of biochemical oxygen demand (BOD) was reduced at the 1 mg treatment and 5% effluent concentration (65, 53, and 41 mg/L) after 15, 30, and 45 d of treatment, respectively, and a further reduction in BOD was recorded at 10% and 15%. In the current study, the reduced COD content at the 1 mg treatment and 100% was 801, 612, and 521 mg/L after 15, 30, and 45 d of treatment, respectively. Similar reduction in chemical oxygen demand was observed in other concentrations of leather industrial effluents [
50].
The interaction of heavy metals with nanoparticles resulted in the variation in adsorption of heavy metal content in a study conducted by [
53]. The results showed the potentially higher removal efficiency of Cu
2+, Ag
1+, and Pb
2+ and the lower removal efficiency of Cr
4+, Mn
2+, Cd
2+, and Ni
2+ in the presence of nanoparticles. In the current research, green synthesized ZnO nanoparticles showed maximum adsorption for Cr, Cd, and Pb in a similar sequence. Chromium was reduced to 60.9, 52.5, and 26.1 mg/L after 15, 30, and 45 d, respectively, at 5% effluent concentration. About 48% Cr reduction was observed after 7 d of treatment in
Phragmites australis [
54]. In the present investigation, the reduction in Cd content was ~42, 40.5, and 23.5 mg/L at 5%; 98.4, 83.3, and 42.4 mg/L at 10%; and 127.1, 113.2, and 57.4 mg/L at 15% after 15, 30, and 45 d, respectively. A similar reduction in cadmium levels by ZnO-NPs was reported by [
51]. Pb concentration was reduced from 25.3 mg/L to 17.8, 16.4, and 9.9 mg/L at a concentration of 5%. At 100%, the content was reduced to 57.3, 51.2, and 25.7 mg/L [
51]. The bio-sorption of Pb was aided by using indigenous microorganisms under applied stress for about 20 d, which significantly reduced the lead content in the sample [
55]. Iron oxide NPs remove 90% of Cr from tannery effluent [
56]. MgO-NPs have the potential to reduce TSS (98%), TDS (98%), BOD (89%), COD (97%), and Cr (from 835 mg/L to 21 mg/L; 97%) in tanning effluent [
57]. Similarly, a study reported that the green synthesis of MgO-NPs from
Rhizopus oryaze reduced TSS (97%), TDS (98%), BOD (87%), COD (95%), and Cr (98%) in tanning effluent [
58]. These findings indicate that the biologically mediated synthesis of nanoparticles could become a useful resource in removing pollutants. The phytochemicals present in algae (as well as plant extracts) may also increase the antibacterial, antifungal, and anticancer properties of green-synthesized nanostructures, by generating a synergetic nanostructure that combines the antimicrobial properties of both the plant extract and the NPs themselves [
59].
In the present study, heavy metals were significantly reduced in all industrial effluent concentrations, and the removal efficiency for all parameters was optimized after 45 d of treatment with 1 mg nanoparticles. The green synthesis of zinc oxide nanoparticles showed effective bioremediation against leather industry effluents. The removal percentages of tannery effluents may be attributed to the adsorption of heavy metals onto the adsorption sites present on the NPs surface [
60]. Nanomaterials are utilized as adsorbents that depend upon structural qualities including good specificity for the use of eliminating heavy metal ions from wastewater at low concentrations. Agglomeration of nanoparticles with the passage of time (such as in this study) may be ideally helpful for the adsorption of toxic substances or other pollutants, as they could elevate the surface-region to volume proportion [
61]. The adsorption processes are governed by the diffusion of metals to the pores fixed on the adsorbent surface, which reacts with the surface active sites. The high adsorption of ZnO-NPs could also be attributed to the liberation of OH
− from Zn(OH)
2, which forms as a result of the hydration process of ZnO [
62].
Adsorption is a simple physicochemical method used to purify harmful wastewaters from heavy metals. In this specific case, surface adsorption onto solid sorbents takes place through electrostatic forces. These can be caused, for example, by hydroxyl groups and/or other functional groups, resulting in a positively or negatively charged sorbent surface. Depending on the charge of the contaminants to be removed, oppositely charged adsorbents are applied. The efficiency of the adsorption is characterized by chemical interactions on the surface of the adsorbents. The main parameters influencing adsorption are pH, temperature, stirring duration (i.e., contact time), initial concentration of the substance to be adsorbed, and the adsorbent dosage. In the case of heavy metal adsorption, absorbents such zinc oxide nanoparticles (ZnO-NPs) have been investigated, which selectively interact with Cr. In batch experiments, ZnO-NPs exhibited very high affinity toward Cr3+, with an optimum pH range of 3–7 and a very short contact time of 20 min. It is worth mentioning that the author also showed that ZnO-NPs selectively adsorbed Cr3+ from wastewater, which contained a mixture of heavy metals, including Ni2+, Pb2+, Cu2+, and Cr3+. Other important heavy metals to be removed by adsorption from wastewater are cadmium (Cd2+) and lead. The initial pH is a driving factor for successful adsorption, as initial pH influences the deprotonation of the adsorbents, which favors enhanced adsorption in a suitable pH range by reducing the repulsion of metal cations (i.e., electrostatic interactions). This mechanism is based not only on electrostatic interaction but also on metal coordination and complexation, which interact with and provide synergetic effects that result in enhanced adsorption and finally increased removal efficiency. Metal coordination and especially complexation strongly depend on the pH. The pH also influences the precipitation of metals. The authors stated that organic matter did not negatively affect the adsorption capacity, but the nitrogen content of the wastewater significantly reduced the capacity of the adsorbent.
Our findings showed that application of the ZnO nanoparticle could help with reducing toxic substances in the wastewater effluents of the leather industry to safe levels and at a reasonable cost, as the conventional methods of remediation may cost from USD 10 to 1000 per cubic meter. The costs for remediation, especially for groundwater, are very high. The American Environmental Protection Agency (EPA) estimated, in 2004, costs of about USD 250 billion for remediation, including construction and post-construction activities, by several cleanup programs only in the United States. The production of ZnO nanoparticles was very prospective. The technical analysis of producing 250 kg of ZnO nanoparticles per day shows the total cost of the equipment to be USD 21,450.00. Such an investment will be profitable after more than three years. This project can compete with payback period (PBP) capital market standards because of the short-term investment returns. To ensure the feasibility of such a project, the project should be estimated from the ideal conditions to the worst case for production, including labor, sales, raw materials, utilities, and external conditions. In the synthesis of ZnO-NPs, zinc sulphate, algal extract, and distal water are used with an algal extract and zinc sulphate ratio of 7:3. Moreover, the conversion rate for the zinc oxide formation process is 100%. Nevertheless, the ZnO-NPs material are successfully applied to real wastewater and show proof for routine application.