1. Introduction
The FAO gives a central role to agriculture in order to achieve the Millennium objectives. By improving agricultural productivity and promoting better nutrition, the targeted reductions in worldwide poverty and hunger could be met [
1]. However intensive agriculture does generate a large negative impact due to high levels of agriculture inputs, such as chemical fertilizers. These produce the highest yield of crops at the lowest possible cost [
2], thus maximizing food production. This is an efficient use of resources; however, the high levels of contamination forces the consideration of other sustainable alternatives for agriculture.
According to this, the real objective is to replace the linear process—based on production, consumption and disposal—with a circular economy in which the materials are reincorporated into the production process in order to create new products. In this approach, waste management plays a key role, since many residues contain beneficial nutrients for agriculture [
3]. In this way, up to 30% of inorganic fertilizers could be substituted [
4]. Nevertheless, it must be ensured that these fertilizers from waste do not cause negative effects on the soil and the environment.
Sewage sludge is the solid waste produced in urban wastewater treatment plants. It is one of the most abundant types of organic wastes from the tertiary sector and its production is expected to increase due to expanding urbanization and industrialization [
5]. Sewage sludge composition varies with treatment technology, but in general, it contains valuable nutrients (it is particularly rich in C, P and N) and therefore its recycling as a soil amendment is promoted in agriculture [
6,
7,
8]. However, the application of sewage sludge as a fertilizer for food production is often limited by its high concentrations of potentially toxic elements [
9]. In addition, the presence of pathogens and salinity in its composition jeopardize soil-plant systems and may further threaten human health [
10,
11,
12]. For this reason, extensive research has been developed during recent decades to guarantee the safe application of sewage sludge to agricultural soils [
13]. One technological treatment that has gained attention is the pyrolysis of sewage sludge to produce biochar [
14]. If the carbon content in the biochar remains below 50%, the product is called “pyrolyzed material” instead [
15]. The high treatment temperatures during pyrolysis ensure the sanitization of the sludge by decreasing the availability of potentially toxic elements [
16]. Before the large-scale implementation of biochar and other pyrolyzed biomasses can be seriously contemplated and developed into policy, it is essential to have scientific evidence of its agronomic and agro-environmental footprint. Some progress has been made in this regard [
17,
18,
19], but more work needs to be done. Experimental results, when available, are often inconsistent and largely dependent on the experimental conditions and design, while causative mechanisms remain unclear [
20].
With respect to fuel savings, nutrient recovery and control of potentially toxic elements, pyrolytic conversion of sewage sludge into biochar is an improvement over the conventional incineration processes [
21,
22]. For this reason, a growing body of research is examining the pros and cons of pyrolyzing sewage sludge for its use as fertilizer in agricultural soils [
23,
24,
25]. Among the main pros are: (i) the recycling of relevant nutrients, such as P [
26,
27] or N [
28], (ii) the destruction of potential pathogens [
29] and (iii) the reduction of toxic organic contaminants [
30]. The cons might include: (i) biochars toxicity ([
31]) and (ii) the release and accumulation of potentially toxic elements in soil [
32]. These disadvantages can be minimized by mixing the sludge with other lignocellulosic residues for their co-pyrolysis. For instance, Wang et al (2018) [
33] added cotton stalk to sewage sludge and observed the migration and transformation of potentially toxic elements from bioavailable to stable fractions. This significantly reduced the potential environmental risks of using biosolids.
Increased crop production is the most commonly anticipated effect of the application of pyrolyzed material to soils. This reflects its potential to partially substitute chemical fertilizers [
34]. For instance, the results from field trials have shown significant increases in N agronomic use efficiency with biochar amendment in rice paddies [
35]. Qiao et al. (2014) [
36] showed a crop performance response comparable to those seen when using a chemical fertilizer. In contrast, in the experiments developed by Zhang et al. (2010) [
37], rice yields were not significantly different between plots with or without nitrogenous fertilization under a single treatment with biochar. In a meta-analysis conducted by Jeffery et al. (2011) [
38] and by Liu et al. (2013) [
39], results showed that the crop yield increase was significant in acidic soils and poorly structured sand and clay soils. This was speculated to be due to a liming effect and an aggregating effect. It is also possible to find research, where biochar is combined with other materials in order to improve its own characteristics and promote sustainable agriculture [
40]. It has been observed that the combination of biochar with fertilizers maximizes the positive impacts of biochar application to soil [
41,
42]. Nevertheless, more research is needed to critically evaluate biochar application to other soil types to improve plant yield and avoid negative environmental effects.
Numerous previous projects failed to fully investigate either nutrient supply versus contamination, or uptake versus leaching. Conversely, the purpose of this study was to produce different pyrolyzed materials based on mixtures of grape seeds and sewage sludge waste—both of which are abundant in Mediterranean agricultural areas—and to examine their agronomic potential and environmental impacts. This was carried out firstly by characterizing the pyrolyzed materials—as in previous studies in Spain [
43]. Then, secondly, by testing them in pots with one typical Mediterranean agricultural soil with corn plants grown in the greenhouse. We hypothesized that increasing the ratio of grape seeds: sludge mixture would decrease the fertilization potential of the biochars but also reduce their environmental risks. These differences would impact nutrient availability to support crop growth and to minimize contaminant leaching.
2. Materials and Methods
2.1. Feedstocks
Two abundant organic wastes were selected for the production of pyrolyzed materials. Sewage sludge was provided by Ferrovial (Madrid, Spain). Its origin was from municipal wastewater treatment plants and was mixed with pruning waste compost used in agriculture and land restoration. Grape seeds, a residue from wine making, were supplied by the Department of Chemical Engineering of UAM (Madrid). Their main chemico-physical characteristics are shown in
Table 1 and
Table 2. Before pyrolysis, feedstocks were air dried. While grape seeds conserved their original particle size (approximately 10 mm) the sewage sludge was ground to <2 mm.
2.2. Pyrolyzed Materials
Five pyrolyzed materials were produced by mixing different proportions of sewage sludge and grape seeds (
Table S1). The air-dried feedstocks were first homogeneously mixed and introduced in a rotary oven (CARBOLITE CB HTR11/150P8, Spain) under a nitrogen atmosphere (flow rate 1 L/min). The pyrolysis temperature ramp was programmed as follows: (1) linear heating at a rate of 10 °C/min from room temperature 22 °C to 400 °C; (2) isotherm for 1 h at the highest treatment temperature (HTT) of 400 °C (3) cooling down to ambient temperature (4 h in total).
An additional treatment, where raw sewage sludge was incubated with 100% grape seeds pyrolyzed material was also tested. This treatment will be referred to hereafter as the
Incubated mixture. It consisted of a 50%–50% mixture (dry weight basis) of raw sewage sludge with pyrolyzed grape seeds. Deionized water was added to the mixture to reach 80% moisture (by 172 g water/kg of mixture) and pre-incubated for 15 days before its use as soil amendment. Pyrolyzed material elemental composition (C, H, N) was determined by automatic elemental analysis (LECO CHNS-932, Model 601-800-500, Isomass scientific Inc, Calgary, AB, USA). Dissolved organic C (DOC) was determined in 1:10 (w: v) water extracts (shaken for 4 h, centrifuged for 10min at 180 rpm and filtered (1238 Filter-Lab, 20–25 μm filters) with a TOC analyzer (Shimadzu Total Carbon Analyzer, TOC-V CSH, Kanagawa Japan). The pH and electrical conductivity were measured in 1:2.5 and 1:5 (w: v) suspensions, respectively. Soluble cation determination (Ca
2+ Mg
2+, Na
+) was carried out following the procedure of Rayment et al., (2011) [
44] and measured by Inductively Coupled Plasma spectroscopy ICP-OES (ICAP 6500 DUO/IRIS INTREPID II XDL, Thermo Scientific, Waltham, USA). Extractable micronutrients (Fe, Cu) and potentially toxic elements (As, Cd) in soil were measured after their extraction in a 1:5 (w: v) ratio with a solution of diethylenetriaminepentaacetic acid (DTPA) (0.02M) and CaCl
2.2H
2O (0.1M) [
45] and analyzed by ICP-OES. For the determination of soluble P, 1 g of the pyrolyzed material was sequentially washed with twelve portions of water measuring 10 mL each. The volume—previously filtered (1238 Filter-Lab, 20–25 μm filters)—was later brought up to 250 mL [
46] and determined using an ion chromatograph Dionex ICS-900. The concentration of total elements was determined by an acid digestion with HNO
3 (13.7 M) and H
2O
2 (33% w: v) in autoclave at 125 °C and 24 kPa for 30 min [
47,
48]. This was then measured with ICP-OES. The detection limits of the ICP-OES for As, Cd and P ranged from 1–10 μg/L.
2.3. Pot Experiment with Calcareous Soil and Corn
Pot experiments with corn (Zea mays L.—LG ambitious) were carried out at the greenhouse facility of the Department of Agricultural and Food Chemistry in the UAM. The greenhouse kept temperatures between 10 °C and 26 °C, relative humidity (37.8% to 89.7%), and a photoperiod of 12 h for the duration of the experiments (6 weeks during March–April).
Pots (methacrylate cylinders, size 3.2 cm diameter, 17 cm height with a mesh at the bottom) were filled with 600 grams (dry weight) of a 50%–50% mixture of Chromic Luvisol soil (
Table S2) [
49], which had been previously cultivated with vineyards, and limestone. This was done in order to create aeration in the mixture. The different materials were added and homogeneously mixed with the soil at a 2% rate (dry weight basis) as individual treatments, equivalent to an agronomic dose of 40 ton/ha. A treatment without fertilization was included as a control. This made up a total of seven treatments (five pyrolyzed materials, the
Incubated mixture and the control). Each treatment was replicated four times. The pots were spatially distributed in a randomized block design and moved to a new random location once a week. During the incubation, the pots were irrigated with 105 mL of deionized water to reach 80% of their field capacity. Regarding the seeds, these were previously germinated in controlled conditions (28 °C, 48 h). Secondly, two germinated plants of maize were sown in each pot. After a week, the germinated plants were later thinned to one plant per pot. The plants were watered regularly (every two days) with deionized water to maintain the soil at a moisture content of 80% of its field capacity (172 g water per kg of dry soil). No leaching was observed after each regular irrigation event.
A leaching experiment was conducted to evaluate the movement of potentially toxic elements present in the treatments. For this, 100 mL of extra-water (156% of its field capacity) was added to the pots after five and six weeks. The leachate was collected after each leaching event. Then, after being filtered, (1238 Filter-Lab, 20–25 μm filters) the corresponding samples were combined to make one sample to measure the cumulative leached elements (As, Cd, Cu, Fe and P) with ICP-OES.
After six weeks of growth, both the above and belowground corn biomass were harvested. The biomass was washed with deionized water and oven-dried at 70 °C in paper bags for 24 h. The dry matter yield was recorded and the dried samples were sieved to a fine powder using a mechanical grinder. The total concentration of elements in corn biomass was extracted by acid digestion with HNO3 (13.7 M) and H2O2 (33% w: v) in an autoclave at 125 °C and 24 kPa for 30 min, and then measured with ICP-OES.
Once the corn was harvested, the soil was oven-dried for 48 h at 70 °C, sieved to 2 mm and stored until further analysis. Soils were analyzed for pH, and soluble nutrients were extracted by the Ammonium Bicarbonate-DTPA method [
50], and measured by ICP-OES.
2.4. Data Processing and Statistical Analysis
The translocation of metals As, Cd and Pb to the aerial part was calculated according to the following equation:
The percentage of fertilizer efficiency as the amount of nutrients initially applied in the pyrolyzed material that was taken up in shoots at harvesting was calculated with the following formula:
The percentage of an element that was leached from the materials which are being analyzed was calculated as:
In order to select the treatments which could be used most appropriately for agricultural applications, we compared the corresponding percentages of efficiency and leaching obtained by each treatment. Color codes were then used to illustrate the differing efficiencies among the treatments.
Firstly, in order to identify the results for “total efficiency” and “total leaching”, a ranking scale was created. The scale consists of numbers 1 to 4. These were assigned to the different treatments according to the results from % Efficiency (
Table S3) and % Leaching (
Table S4). Number 4 refers to the treatment that obtained the highest percentage. Number three was given to the one that produced the second highest percentage and so on until number one was designated to the treatment with the lowest percentage.
The treatments were ranked by efficiency and leaching for the five different elements: Cu, Fe, P, As and Cd. Once these were applied, the rankings were then added up. These totals provided an overall score for each treatment for their total leaching and total efficiency.
Whereas, a higher number for total leaching indicates a greater environmental risk irrespective of the element, the evaluation of total efficiency differs depending on whether it is a contaminant or a nutrient.
The color code system was then employed to illustrate the relative total efficiency and the relative total leaching for each treatment. Green represents the treatment which produced the most favorable results for the crop and the environment. Yellow for those that produced moderate results. Then, finally, red was given to the treatment that provided the least favorable results.
Nutrients: Efficiency: 1–4 (red), 5–8 (yellow), 9–12 (green)
Leaching: 1–4 (green), 5–8 (yellow), 9–12 (red)
Contaminants: Efficiency: 1–3 (red), 4–6 (yellow), 7–8 (green)
Leaching: 1–3 (green), 4–6 (yellow), 7–8 (red)
2.5. Statistics
All data was analyzed using a 1-way analysis of the variance (ANOVA) followed by a Tukey post-hoc test (p < 0.05) using SPSS. For non-normal data, a nonparametric test, such as the Games-Howell, was used.
5. Conclusions
Until now, there have been a great number of studies which have described how biochars, or incubated biochars, are a good source of nutrients for crops, following the idea of waste management. Nevertheless, these projects lack a global vision, in which nutrient supply versus contamination, as well as uptake versus leaching, are fully investigated. In this study, the positives and negatives of the application of biochars on agronomic potential and environmental risk have been addressed, taking into account P, Zn, Cu, Fe, As and Cd. Different materials have been produced based on total or partial pyrolysis of mixtures of grape seeds and sewage sludge wastes. Some products have shown different characteristics regarding their potential use as macro fertilizers (P) and micronutrients (Cu, Fe) sources. On the basis of the presented results, the recommended ratio of sludge:grape seeds treatment is 50% grape seeds-50% sewage sludge due to its intermediate behavior of leaching and efficiency. Nevertheless, in the agronomic trial, the Incubated mixture showed the highest corn biomass production as well as providing the plants with macronutrients and micronutrients. However, the environmental impact from the Incubated mixture was greater than the rest of the treatments. Therefore, the optimization of this material is considered a suitable option as a soil amendment for the future. This could be done by improving the ratio of pyrolyzed material:sludge to decrease leaching while maintaining fertility. This would provide a compromise between maximizing the elements that are translocated to the plant, along with reducing those that are leached. A reduction in the dosage of sludge compared to the pyrolyzed material in the incubation is expected in future investigations.