Introduction

Numerous studies describe the utilization of low-cost materials of biological origin in various biosorption processes (e.g., Febrianto et al. 2009; Salman et al. 2015). Besides various kinds of algae (Kratochvil and Volesky 1998; Montazer-Rahmati et al. 2011; Lam et al. 2016), numerous other biomasses, such as bacteria, fungi, peats, agricultural waste, or byproducts (Nguyen et al. 2013; Ali et al. 2016; Jain et al. 2016; Wu et al. 2016), are still tested as biosorbents for the removal of various metal ions from aqueous media through biosorption. At least theoretically, biosorption processes are useful in the treatment of aqueous effluents contaminated with metal ions, mainly due to their advantages such as economic and technological viability, quantitative recovery of retained metal ions, minimization of secondary waste, ease of operating, high efficiency, etc. (Wang and Chen 2009; Romera et al. 2007).

Although biosorption studies have been performed continuously from the middle of last century (Tuhy et al. 2014), this method is not applied at large scale. Even now, conventional methods (such as chemical precipitation, ion exchange, osmosis, electrochemical techniques, etc.) (Dabrowski et al. 2004; Llanos et al. 2010) are still used for the removal of metal ions from contaminated water effluents, even if each of these has significant disadvantages. This fact is determined by the difficulties arising in the adaptation of biosorption processes for industrial applications. Because most biosorbents have a short life of utilization due to their fragile structure, their regeneration and reuse is sometimes difficult to achieve, and the easiest technological operations can destroy them, thus drastically limiting the applicability of biosorption processes at large scale. In most of cases, the biosorbents are used in a single step of biosorption, and after exhaustion, they are often incinerated or deposited. These disposal ways of exhausted biosorbents also have a negative impact on the environment, which sometimes is more important than that of the contaminated industrial effluents (Chojnacka 2010). Therefore, in order to highlight the feasibility of biosorption processes for practical applications, all these drawbacks must be solved.

A possibility that could maximize the advantages of biosorption processes and at the same time minimize their disadvantages could be the utilization of biosorbents loaded with metal ions, via biosorption, as fertilizers for the improvement of the quality of soils. In this way, the capacity of various biomasses to retain metal ions and to release them by desorption in soil can be exploited, under controlled conditions, without the necessary regeneration of the biosorbents.

The utilization of algal biomass as compost for the improvement of soil quality for agricultural crop production has been intensively studied in recent years (Cuomo et al. 1995; Wosnitza and Barrantes 2006; Cole et al. 2016). In most of these studies, the raw algae biomass is used for the compost preparation and their benefits on soil are mainly determined by the high content of macronutrients.

The utilization of algal biomass, first as biosorbent and then as fertilizers for soil, besides being able to least partially solve the problem of biomasses loaded with metal ions will also ensure the reliability and upgrading of soils depleted by microelements by an in situ method. In addition, because most biomasses used as biosorbents have large contents of nitrogen and phosphorus, they can become an important source of macronutrients, as a result of their biodegradation in soil. In order to make such an application possible, in which the biosorption processes can be helpful in the improvement of quality of soils, two conditions must be met: (i) the retained metal ions on the biosorbent must be essential for soils, and (ii) the biomass used as biosorbent should to be relatively readily biodegradable, and its biodegradation products should not have a pollutant potential for soil.

The first condition is fulfilled by Zn(II) ions, which are considered essential microelements for soil, especially those with agricultural uses. It is well known that Zn(II) ions are important for plant growth, due to their involvement in the metabolism of proteins and carbohydrates as an enzyme activator (Alloway 2009; Gurmani et al. 2012; Cabrita et al. 2016). Thus, because the Zn(II) ions stabilize the protein structure of plants, their deficiency mainly determines the discoloration and size reduction of plants, drastically affecting the efficiency of agricultural production. The requirement of Zn(II) for plant growth is taken from soil and where these ions are missing, the necessary amount is provided by fertilizers.

On the other hand, Zn(II) ions can be retained in defined experimental conditions on various biomasses, and the biosorption process can be designed so that its efficiency is high. The Zn(II) ions are present in relatively large concentrations in wastewaters or water processes of some industrial activities (for example, the wastewater from galvanization activities may contain up to 200 mg Zn(II) L−1) (Hojati and Landi 2015), and their removal from such effluents benefits the environment.

The second condition implies the use of adequate biomass. Thus, marine algal biomass was selected for this study, because it can be included in the category of low-cost materials, is available in large quantities in many regions, and requires only few easy steps of preparation (Montazer-Rahmati et al. 2011; Michalak et al. 2013). However, the most important advantage is that the algal biomass is a readily biodegradable material, which has a high content of organic matter and other macronutrients (such as N, P, Ca, K, etc.) (Tabarsa and Rezaei 2012), and these characteristics are also useful for the improvement of the quality of soils.

Therefore, the binding of Zn(II) ions on functional groups of marine algal biomass surface, through biosorption, will allow their removal from aqueous effluents (Parker et al. 2000; Ji et al. 2012), and then the embedment of Zn(II)-loaded biomass in soil may ensure the Zn(II) requirement for plant growth, acting as a fertilizer. Such alternative utilization of biosorption process is in agreement with the principles of sustainable development.

Starting from these considerations, we examined the alternative utilization of marine algal biomass first as biosorbent for the removal of Zn(II) ions from aqueous solution and then as fertilizer for the improvement of soil quality, by its supplementation with Zn(II) as essential microelement. The experiments were performed in batch systems, at room temperature (22 ± 0.5 °C), and targeted both the establishment of experimental conditions for the maximum biosorption efficiency of Zn(II) ions onto marine algal biomass and those required for desorption of Zn(II) from loaded algal biomass. In the first case, the influence of most important operating parameters (initial solution pH, biosorbent dose, contact time, and initial Zn(II) concentration) on biosorption process efficiency was examined, in order to obtain optimal conditions for Zn(II) biosorption onto marine algal biomass. The biosorption data were fitted using various isotherm and kinetics models. In case of desorption experiments, the nature and concentration of some common inorganic salt solutions were considered in order to attain a quantitative release of Zn(II) ions from the loaded biomass. The results obtained from the desorption experiments have suggested the possibility of using Zn(II)-loaded biomass as fertilizers for soil. This possibility was tested using a sample of aric antrosol, which was incorporated with a certain quantity of Zn(II)-loaded algal biomass, and the variation of Zn(II) content together with other quality parameters of soil was analyzed before and after 8 weeks of treatment.

Materials and methods

Materials

The marine alga Ulva sp. was collected from the Romanian Coast of the Black Sea in August 2014. The biomass was washed several times with tap water to remove foreign materials and then with distilled water to remove the salts from the algae. After washing, the biomass was dried in air at 60 °C for 8 h, crushed and sieved until the particle granulation was lower than 1.0 mm.

The soil sample used for experiments was collected from the peripheral zone of Iaşi City (Romania) from upper horizons (0–40 cm). Some pedo-geochemical characteristics of soil, used for its framing as a suitable soil class, are summarized in Table 1. The soil sample was dried in air at room temperature (22 ± 0.5 °C) for 72 h in order to remove only the excess water, without affecting the permanent humidity of the soil. After drying, this was crushed and homogenized (maintaining constant its mineralogical composition) and placed into plastic cuvettes (with volume of 100 cm3).

Table 1 Some pedo-geochemical characteristics of soil sample used in experiments
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Reagents

All chemical reagents used in this study were of analytical grade and were used without further purifications. Distilled water obtained from a commercial distillation system was used for the preparation and dilution of all solutions.

The stock solution of zinc ions (680 mg Zn(II)·L−1) was prepared by dissolving zinc nitrate in distilled water. The working solutions were obtained by diluting the stock solution with distilled water. Solutions of 0.1 mol·L−1 of common inorganic salts (NaCl, NaNO3, CaCl2 and Ca(NO3)2) were prepared by dissolving respective salts in distilled water and from these were then obtained by dilution the solutions used as desorption agents. The initial solution pH was adjusted to the required value using 0.1 mol·L−1 HNO3 or NaOH solution, before adding the biosorbent, and was measured with a MM-374 pH/ion meter equipped with a combined glass electrode.

Experimental methodology

The biosorption experiments were performed in batch systems at room temperature (22 ± 0.5 °C), varying one by one the operating parameters (initial solution pH, biosorbent dose, contact time, and initial Zn(II) concentration) and maintaining the others constant. Therefore, the initial solution pH was varied between 1.0 and 6.0, biosorbent dose between 4.0 and 40.0 g L−1, contact time between 5 and 180 min, and initial Zn(II) concentration between 10 and 220 mg L−1. In each case, the biosorption experiments were carried out in 150-mL conical flasks by mixing a given amount of algal biomass with 25 mL of aqueous solution containing a known concentration of Zn(II) ions, with intermittent stirring for a determined period of time. At the end of the biosorption procedure, the two phases were separated by filtration on quantitative filter paper, and the Zn(II) concentration in the filtrate was analyzed by atomic absorption spectrometry (AAS1 Carl Zeiss Jena spectrometer, air-acetylene flame, 213.9 nm), using a prepared calibration graph.

The biosorptive performances of marine algae biomass for Zn(II) ions was evaluated using biosorption capacity (q, mg·g−1) and removal percent (R, %), calculated from experimental results, according to the following equations:

$$ q=\frac{\left({c}_0-c\right)\cdot V}{m} $$
(1)
$$ \mathrm{R}=\frac{c_0-c}{c_0}\cdot 100 $$
(2)

where c 0 and c are initial and equilibrium concentrations of Zn(II) ions in the solution (mg·L−1), V is volume of solution (L), and m is the biosorbent mass (g).

In case of desorption experiments, 1.0 g of algal biomass was mixed with 100 mL of Zn(II) solution (220 mg·L−1) and intermittently stirred for 2 h. After filtration, the biomass was washed twice with distilled water to remove residual Zn(II) ions from surface and dried in air. Samples of 0.05 g Zn(II)-loaded biomass was then treated with 10 mL of common inorganic salts (NaCl, NaNO3, CaCl2, or Ca(NO3)2), used as desorption agents. Each sample was intermittently stirred for a determined period of time (2 h), then filtered, and the Zn(II) concentration in filtrate was analyzed as mentioned above. The amount of Zn(II) desorbed from the loaded biomass (q d , mg·g−1) was calculated according with the following equation:

$$ {q}_d=\frac{c_d\cdot v}{m\hbox{'}} $$
(3)

where c d is the concentration of Zn(II) in eluate (mg·L−1), v is the volume of desorption agent solution (L), and m′ is the mass of loaded biosorbent.

The possible utilization of Zn(II)-loaded biomass as fertilizer for soil was studied by adding 1 g of Zn(II)-loaded biomass (29.63 mg Zn(II)·g−1 of biomass) on the surface of 100 g of soil, placed into polyethylene cuvettes with volume of 100 cm3, and maintained at room temperature and pressure. Two series of ten samples of loaded biomass and soil were prepared. Each sample was watered daily with 10 mL of 10−2 mol L−1 CaCl2 solution, which was shown to be the most efficient desorption agent. At intervals of 1 week, one by one, the soil samples were out from the cuvettes, homogenized and used for the determination of Zn(II) content after the un-degraded biomass was removed. The other characteristic parameters of the soil samples were determined according with standardized procedures (Sparks et al. 1996; Bulgariu et al. 2014). The un-degraded biomass also was weighed and then treated with 10 mL of 0.1 mol L−1 HCl solution, for quantitative release of retained Zn(II) ions. After filtration, the obtained solution was used for Zn(II) determination.

All experiments were carried out in duplicate and the mean values were used for interpretation.

EDX and IR measurements

The elemental composition of the Ulva biomass was analyzed by scanning electron microscopy (SEM; Hitachi S 3000 N microscope) coupled with an energy dispersive X-ray spectrometer (Bruker EDX spectrometer). The main functional groups from the surface of the algal biomass were highlighted using an FT-IR spectrometer (Bio-Rad model, 400–4000 cm−1 spectral domain, 4 cm−1 resolution, KBr pellet technique).

Results and discussion

Biosorptive characteristics of marine algae biomass for Zn(II) ions

Most studies in the literature indicate that biosorption of metal ions onto various low-cost materials of biological origin occurs with maximum efficiency only in well-defined experimental conditions (Donmez et al. 1999; Febrianto et al. 2009). These conditions are generally established by batch experiments and aim to study the influence of the most important operating parameters on the biosorption process efficiency. Such information is important in the design of an adequate wastewater treatment system.

In the case of Zn(II) biosorption onto marine algal biomass, the operating parameters considered in experimental studies are initial solution pH, biosorbent dose, contact time, and initial Zn(II) concentration, and their influences on the Zn(II) biosorption efficiency are individually illustrated in Fig. 1. As expected, each of these parameters has a significant influence of Zn(II) biosorption efficiency onto the algal biomass, and thus their optimal values should be found.

Fig. 1
figure 1

Influence of operating parameters on Zn(II) biosorption onto Ulva biomass: a initial solution pH (c0 = 54.20 mg L−1; 8.0 g biosorbent·L−1, 24 h contact time), b biosorbent dose (pH = 5.5; c0 = 54.20 mg L−1; 24 h contact time), c contact time (pH = 5.5; 8.0 g biosorbent·L−1; c0 = 54.20 mg L−1), and d initial Zn(II) concentration (pH = 5.5; 8.0 g biosorbent·L−1; 2 h contact time).

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The increase of the initial solution pH from 1.0 to 6.0 (Fig. 1a) showed gradual increase of Zn(II) biosorption, the maximum being obtained at pH 5.5. This is mainly determined by the increase of dissociation degree of functional groups from the biomass surface with the increase of the initial solution pH, facilitating the binding of Zn(II) ions from aqueous solution through ion exchange interactions (Plaza Cazon et al. 2013). The slow decrease of biosorption capacity at initial solution pH higher than 5.5 is probably a consequence of some secondary hydrolysis processes that appear under these conditions and which can change the speciation of zinc in aqueous solution. Accordingly, an initial solution pH of 5.5 was considered to be optimal and was used in all subsequent experiments.

The second operating parameter, biosorbent dose, was varied between 4.0 and 40.0 g L−1, and the results (Fig. 1b) showed that the biosorption capacity (q (mg·g−1)) significantly decreases (from 5.99 to 1.42 mg g−1) over the entire studied interval. This is due to the fact that at a given Zn(II) concentration, if the biosorbent dose is increased, Zn(II) uptake decreases as a lesser number of Zn(II) ions are available for binding on all superficial functional groups of the biosorbent. The overawing effect of biosorbent particles that appears at high biosorbent doses and that determines the overlapping of available biosorption sites (Anastopoulos and Kyzas 2015) also cannot be excluded. Therefore, from practical considerations, a biosorbent dose of 8.0 g L−1 was selected as the optimum value for biosorption of Zn(II) ions onto the algal biomass.

In the case of contact time, its influence on Zn(II) biosorption onto the algal biomass was investigated at different time intervals, ranging between 5 and 180 min (Fig. 1c). The results indicate that Zn(II) biosorption increases rapidly at the beginning (0–30 min) and becomes very slow with longer contact time (30–180 min). This variation is strictly dependent by the number of available biosorption sites on the biosorbent surface (Freitas et al. 2008). Initially, when the number of biosorption sites is high, the biosorption process is fast (in the first 30 min, more than 60% of Zn(II) ions are retained). After this, the rate of biosorption becomes slower, and the equilibrium state is practically attained in 60 min. Under these conditions, a contact time of 120 min (2 h) was considered enough for quantitative biosorption of Zn(II) ions onto the algal biomass, and this value was selected as optimal.

The influence of the initial Zn(II) concentration was studied in the range of 10–220 mg L−1, under the optimal experimental conditions established previously, and the results are presented in Fig. 1d. Over the entire studied interval, the increase of initial Zn(II) concentration (c0 (mg·L−1)) determines the increase of biosorption capacity of the algal biomass (q (mg·g−1)). The highest value of this parameter (29.63 mg g−1) is obtained at an initial Zn(II) concentration of 220 mg L−1 and proves that through biosorption, the marine algal biomass retains a sufficient amount of Zn(II) ions and can be used as potential fertilizer to improve the quality of soils. On the other hand, from an environmental protection point of view, the best efficiency of biosorption is obtained at an initial Zn(II) concentration less than 20 mg L−1 (Fig. 1d), where the metal ion concentration in the effluent solution (after biosorption, c (mg·L−1)) is lower than the maximum permissible limit in Romanian legislation (NTPA 002/2005), so that Ulva biomass can be considered an efficient biosorbent for the treatment of wastewater.

Equilibrium and kinetic modeling of biosorption process

In order to understand the mechanism of Zn(II) biosorption onto the Ulva biomass, the experimental results were modeled using three isotherm models (Langmuir, Freundlich, and Dubinin-Raduskevich models) and three kinetic models (pseudo-first order, pseudo-second order and intraparticle diffusion models). The mathematical equations of all these models are presented in Table 2, and the best fit model was selected based on the values of correlation coefficients (R2) calculated by non-linear regression.

Table 2 Isotherm and kinetic characterization of Zn(II) biosorption onto marine algae biomass
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The choice of these models was done considering their usefulness in the description of biosorption process. Thus, the Langmuir model considers that biosorption occurs on a homogeneous surface until complete monolayer coverage is formed at the outer surface of the biosorbent, while the Freundlich model assumes that biosorption takes place on a heterogeneous surface and it is not restricted to the formation of a monolayer (Chong and Volesky 1995; Rangabhashiyam et al. 2014). In addition, the Dubinin-Raduskevich model is used to estimate the nature of interactions between Zn(II) ions and functional groups of the biomass surface during the biosorption process (Farooq et al. 2010).

From a kinetic point of view, the pseudo-first order and pseudo-second order models assume that the rate-limiting step in the biosorption process involves chemical interactions (ion exchange and/or sharing of electrons) between metal ions and superficial functional groups of the biosorbent, and that the Zn(II) ions bind on the biomass surface through one binding site, or by two binding sites, respectively (Ho and McKay 1999; Chojnacka 2010). The intraparticle diffusion model was also considered in kinetic modeling to estimate the role of the diffusion process in the biosorption of Zn(II) ions onto the algal biomass (Cheung et al. 2007; Wu et al. 2008).

The comparison between these models with experimental data is presented in Fig. 2, and the isotherm and kinetic parameters, obtained in the case of Zn(II) biosorption onto the Ulva biomass under established experimental conditions, are also summarized in Table 2. As can be observed from Fig. 2 and Table 2, the highest correlation coefficients (R2) are obtained in the Langmuir isotherm and pseudo-second kinetic models. This indicates that the biosorption of Zn(II) onto the algal biomass occurs until the formation of monolayer coverage on the outer surface of the biosorbent, and the maximum biosorption capacity (qmax, mg·g−1) required for the monolayer formation is 31.0559 mg g−1. Also, the high value of Langmuir constant (KL, L·g−1) (Table 2), which is related to the biosorption energy (Kumar et al. 2007), suggests that strong interactions, most probable of the ion exchange type, occur between the superficial functional groups of the algal biomass and Zn(II) ions from aqueous solution.

Fig. 2
figure 2

Isotherm a and kinetics b plots for Zn(II) biosorption onto Ulva biomass

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This hypothesis is sustained also by kinetic modeling, which shows that the experimental results are better described by the pseudo-second-order kinetic model (Fig. 2b and Table 2). This means that Zn(II) biosorption onto the biomass predominantly involves chemical interactions between metal ions from aqueous solution and functional groups on the biosorbent surface, and the binding of Zn(II) ions onto biomass surface requires two binding sites. A similar behavior has been reported for the biosorption of Zn(II) ions onto various types of algal biomass (Kumar et al. 2007; Ji et al., 2012; Zhou et al. 2012; Pozdniakova et al. 2016), but in this study, such information is important from the perspective of using Zn(II)-loaded biomass as fertilizer.

Mechanism of Zn(II) ion biosorption

The elemental composition of the biosorbent, obtained by EDX analysis before and after Zn(II) uptake (Table 3), shows that after biosorption, the percent values of some elements, such as Ca, Mg, Al, Na, and K, decrease, which indicates that these play the role of mobile ions in the Zn(II) uptake from aqueous solution.

Table 3 Elemental composition of marine algae biomass before and after Zn(II) biosorption
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The possible interactions between Zn(II) ions and functional groups from biosorbent surface were investigated by FT-IR spectrometry, and the spectra of the Ulva biomass, recorded before and after Zn(II) biosorption, are presented in Fig. 3. The characteristic peaks from both spectra are significantly similar and indicate the importance of electrostatic interactions in the biosorption mechanism of Zn(II) ions onto algae biosorbent. Comparison of the spectra indicates that after Zn(II) biosorption, the peaks corresponding to hydroxyl (3278 cm−1), carbonyl (1630 cm−1), and carboxyl (1542 cm−1) groups are slighlty shifted to high wave numbers, suggesting that such functional groups could interact with Zn(II) ions during the biosorption process.

Fig. 3
figure 3

IR spectra of Ulva biomass before (a) and after (b) Zn(II) biosorption

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All these experimental observations show that the biosorption of Zn(II) ions onto Ulva biomass occurs gradually, mainly through an ion exchange mechanism, whereby mobile ions are released from biomass composition, until a complete monolayer coverage is formed at the outer surface of the biosorbent particles. Such a biosorption mechanism has important consequences from a practical point of view and represents the starting point in the understanding of experimental desorption studies.

Desorption of Zn(II) from loaded marine green algae biomass

An overview of the literature indicates that mineral acid solutions are the most efficient desorption agents of metal ions retained on various biomasses through biosorption (Jayakumar et al. 2015; Cechine et al. 2016). The treatment of loaded biosorbents with small volumes of mineral acids (such as HCl, HNO3, etc.) determines the quantitative release of retained metal ions and regeneration of biosorbents, which then can be used for another biosorption cycle. Unfortunately, treatment with mineral acids solutions cannot be used for the release of metal ions from loaded biosorbents in soil, due to the severe negative consequences on the soil, determined by strong acid media. Therefore, the utilization of salt solutions as desorption agents is better, because they do not significantly change the acidity of soil due to neutral pH.

We used four different common inorganic salt (NaCl, NaNO3, CaCl2, and Ca(NO3)2) solutions at four different concentrations (10−4 to 10−1 mol L−1) for the release of Zn(II) ions from loaded Ulva biomass, and the results are shown in Fig. 4. To exclude the possibility that the desorption of Zn(II) ions from the loaded biomass is a consequence of physical elementary interactions (determined by its concentration gradient), distilled water was also used in the desorption experiments for comparison.

Fig. 4
figure 4

Influence of nature and concentration of desorption agents in the case of Zn(II) retained onto Ulva biomass

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Figure 4 clearly shows that distilled water has an insignificant effect on Zn(II) desorption and only the inorganic salts are capable of releasing Zn(II) ions from loaded biomass. In addition, from the inorganic salts used as desorption agents, CaCl2 solutions were more effective in the release of Zn(II) ions, compared with NaCl, NaNO3, and Ca(NO3)2, although the differences between desorption percentages are lower than 10% in all cases. On the other hand, the Zn(II) release efficiency depends on the concentration of the desorption agent solutions (Fig. 4), with the highest desorption of Zn(II) ions from loaded biomass (>80%) being obtained at the highest desorption agent concentration (10−1 mol L−1) in all cases.

All these observations indicate that the desorption of Zn(II) ions from loaded-marine algal biomass mainly occurs through an ion-exchange process, between retained Zn(II) ions and Ca(II) ions from aqueous solution, according with the elementary process:

$$ \mathrm{R}\hbox{--} Zn(II)+{Ca}^{2+}\to \mathrm{R}\hbox{--} Ca(II)+{Zn}^{2+} $$
(4)

where R is the marine skeleton of algae biomass. The efficiency depends on the Ca(II) ion concentration in the desorption agent solution, being little influenced by the nature of the anion of calcium salt.

Such a desorption process is desirable from the perspective of the utilization of Zn(II)-loaded algal biomass for improving the quality of soils. This is because according to the obtained experimental results, the release of Zn(II) ions from the loaded biomass takes place only when the humidity of biomass is high and in the presence of some mobile ions (such as Ca(II)) that can exchange the Zn(II) from the biomass surface. These conditions can be achieved by watering the Zn(II)-loaded biomass through irrigation which is a common operation during agricultural soil exploitation.

Utilization of Zn(II)-loaded biomass in soil treatment

In order to check the possibility to use Zn(II)-loaded Ulva biomass to improve the quality of soil, 1 g of loaded biomass was mixed into 100 g of soil. Daily, the samples were bathed with a constant volume (10 mL) of 10−2 mol L−1 CaCl2 solution, which was found to be the most efficient desorption agent. After 1 week, the samples were removed from plastic cuvettes, and the two components (loaded algal biomass and soil) were separated under a microscope and analyzed for the Zn(II) content. It can be observed from Fig. 5 that during the 8-week experimental period, the Zn(II) content of the soil increased 4.29 times, which represents an acceptable improvement of the content of this essential microelement in the soil sample. Furthermore, only 42.09% of the total content of Zn(II) ions retained on Ulva biomass is released during the 8 weeks of treatment, with an average desorption rate of 0.3662 mg Zn(II)·day−1.

Fig. 5
figure 5

Variation of Zn(II) content in soil sample during 8 weeks of treatment with loaded-algal biomass

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The increase of Zn(II) content in soil is exclusively determined by its desorption from the loaded marine algal biomass, as a result of the action of the desorption agent solution. After the Zn(II) ions are released from the loaded biomass, they are retained by the soil components, which causes their accumulation (Fig. 5—integral variation). The average Zn(II) accumulation rate in soil was 3.6591 mg Zn(II)·day−1 kg−1 soil. If there are no plants to consume Zn(II) ions (as in this case), these ions are accumulated until saturation is attained. In the saturation region, the rate of Zn(II) accumulation in soil dramatically decreases, which represents an advantage, because it avoids pollution of soil due to high Zn(II) concentration (Fig. 5—differential variation). This means that the desorption of Zn(II) ions occurs gradually, and therefore, the Zn(II)-loaded marine algal biomass can be used to ensure the Zn(II) requirement in soil for a long period of time.

Simultaneously with the increase of Zn(II) content in soil, other characteristics of the soil are also changed (Table 4). Over the 8-week treatment period, the soil pH decreases by 1.35 units, from pH 8.22 (alkaline media with weak agrochemical characteristics) to 6.87 (very close to the optimal value required for plant growth). Therefore, the Ulva biomass is behaving as a pH moderator which has the ability to correct the extreme values of pH in soil. The significant increase in the concentrations of some microelements and macroelements in soil after its treatment with Zn(II)-loaded Ulva biomass (see Table 4) indicates that during the studied time period, other ions are released from the algal biomass together with Zn(II) ions. These ions, together with the compounds resulting after degradation of the Ulva biomass during the treatment period, are also retained onto soil components contributing to the increase of its quality. Therefore, the treatment of soil with Ulva biomass loaded with essential microelements, such as Zn(II) ions, is a viable procedure that can be successfully used to improve the quality of soils, in agreement with the principles of sustainable development.

Table 4 Some characteristic parameters of soil before and after 8 weeks of treatment with Zn(II)-loaded marine algae biomass
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Conclusions

In this study, we investigated the alternative utilization of marine algal (Ulva sp.) biomass first as a biosorbent for the removal of Zn(II) ions from aqueous solution, and then as a fertilizer for the improvement of soil quality by its supplementation with Zn(II) as an essential microelement. The procedure, designed in agreement with the principles of sustainable development, involves two stages: (i) retention of Zn(II) from aqueous solution on the algal biomass and (ii) release of Zn(II) ions from the loaded biomass in soil. The biosorption/desorption experiments were performed in a batch system, at room temperature. The results showed that the maximum biosorption efficiency of Zn(II) ions is obtained at an initial solution pH of 5.5, 8.0 g L−1 biosorbent dose, and at least 2 h of contact time. Under these conditions, the biosorption capacity of the Ulva biomass is 29.63 mg Zn(II)·g−1, which is close to the value of maximum biosorption capacity calculated from the Langmuir isotherm model (qmax = 31.05 mg Zn(II)·g−1). The isotherm and kinetic modeling of experimental results indicated that the retention of Zn(II) ions onto Ulva biomass is best described by the Langmuir isotherm model and pseudo-second kinetic model. This means that the biosorption of Zn(II) onto the algal biomass occurs gradually, mainly through an ion exchange mechanism, until a complete monolayer coverage is formed at the outer surface of the biosorbent particles. A CaCl2 solution (10−1 mol L−1) was most effective in the release of Zn(II) ions from the loaded algae biomass. Such a desorption process is desirable from the perspective of soil treatment because the release of Zn(II) ions will occur only when the humidity of biomass is high and in the presence of mobile ions (such as Ca2+) that can exchange the Zn(II) ions from the biomass surface. The possibility to use the loaded biomass to improve the quality of soil was tested using an aric antrosol sample, which was treated with Zn(II)-loaded Ulva biomass and daily bathed with a constant volume of CaCl2 (10−2 mol L−1) solution. After 8 weeks of treatment, the Zn(II) content in the soil increased over four times. Other important characteristic parameters of the soil change, indicating that other ions were released from the algal biomass. This study shows that marine algal biomass loaded with Zn(II) ions can be a viable alternative to fertilizers and can be used successfully to improve soil quality by supplementation with essential microelements.