Environmental Engineering and Management Journal October 2020, Vol. 19, No. 10, 1669-1676

http://www.eemj.icpm.tuiasi.ro/; http://www.eemj.eu

“Gheorghe Asachi” Technical University of Iasi, Romania

DESIGN AND CHARACTERIZATION OF CONTROLLED RELEASE

PK FERTILIZERS FROM AGRO-RESIDUES

Silvia Barbi1, Francesco Barbieri2, Fernanda Andreola3, Isabella Lancellotti3, Carmen Martínez García4, Teresa Cotes Palomino4, Monia Montorsi1,2, Luisa Barbieri3

1Department of Science and Methods for Engineering, University of Modena and Reggio Emilia,

Via Amendola 2, 42122, Reggio Emilia, Italy 2Interdepartmental center for applied research and services in advanced mechanics and motoring, INTERMECH-Mo.Re.,

Via P. Vivarelli 10/1, 41125, University of Modena and Reggio Emilia, Modena, Italy 3Department of Engineering “Enzo Ferrari”, University of Modena

and Reggio Emilia, Via P. Vivarelli 10/1, 41125, Modena, Italy 4Department of Chemical, Environmental and Material Engineering, High Polytechnic School of Linares, University of Jaen,

Linares Scientific and Technological Campus, Cinturon Sur, s/n, 23700 Linares, Spain

Abstract The aim of this study is to design and characterize a new controlled release fertilizers category by using valorization of residues from agri-food, industrial and post consumers activities normally allocated into landfill disposal. The underlying strategy in materials research fully agreed with the circular economy perspective and the more recent European legislation about critical raw materials recovering. This study covers the development of lightweight aggregates, based on a local red clay (km 0 concept) and pores former such as coffee grounds and brewery sludge. Functionalization has been performed with vegetable biomass and cattle bone flour ashes, as received and after vitrification in a tailored fertilizer glass, containing high quantity of potassium and phosphorous. Fully characterization by means of a multidisciplinary approach, including chemical, mineralogical, thermal, physical, and plants growth in soils tests has been performed on starting and derived materials. Finally, growth test on basil’s plant have demonstrated the beneficial effect of the aggregates designed and produced in this study in comparison with standard fertilizer already on the market. Key words: agro-waste, circular economy, core-shell fertilizer, LWAs, PK fertilizers Received: February, 2020; Revised final: June, 2020; Accepted: July, 2020; Published in final edited form: October, 2020

1. Introduction

The exponential growth of the global population forces agriculture to use more efficient fertilizers to increase crop production. Over the next five years the global soil demand for nitrogen, phosphorous and potassium will annually grow on average by 1.5%, 2.2% and 2.4%, respectively, and so for the global demand of fertilizer products (FAO, 2017; Herrera-Estrella and López-Arredondo, 2016; Taddeo et al., 2016). At the same time, hydro-culture and roof garden, that are favorable culture techniques Author to whom all correspondence should be addressed: e-mail: fbarbieri@unimore.it; Phone: +39 059 2056221

to avoid soil exploitation, arise the demand for high-engineered lightweight fertilizers (Kim et al., 2018). Lightweight aggregates (LWAs) may take advantage of their intrinsic characteristic to be highly porous materials promoting the ability to retain nutrients and water for certain times promoting a controlled release (Lubkowski and Grzmil, 2007; Tomaszewska and Jarosiewicz, 2006). Controlled release fertilizers (CRFs) peculiar slow release make them the best choice to reduce toxicity related to a sudden solubilization and loss of ions into the soil (Mikkelsen et al., 1994). On the other hand, they have high costs

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for production and raw materials purchase while their storage is difficult because of premature release of nutrients due to moisture absorption (Davidson and Gu, 2012). Among them, nutrients incorporation based CRFs are easier and cheaper to produce, nevertheless the nutrient’s content is reduced since it has to be mixed with other compounds (Al-Zahrani, 1999; Liang et al., 2007), such as montmorillonite (Rashidzadeh and Olad, 2014), bentonite (Xiaoyu et al., 2013) and zeolites (Hoeung et al., 2011). In this context strong effort should be done in order to reduce CRFs disadvantages and, therefore, research of new raw materials for the fertilizer’s manufacturing became a critical issue. Moreover, it must be considered that the employment of renewable resources for the design and development of innovative fertilizers has been recently recognized as a key point since these nutrients are exhaustible (EEA, 2015). This it is particularly true for phosphorus that was recently included in the list of critical raw materials (CRMs) by European commission.

In the present study, LWAs have been designed and characterized to act as CRFs starting from agri-food industry and post-consumer activities residues, reducing starting material costs, and therefore fit into a context of cleaner products (Farias et al., 2017). In this sense this work can be also considered as innovative example of food residues valorization as CRFs (Du et al., 2018). Ceramic core of the fertilizer has been investigated employing a local red clay, thereafter, employing km 0 concept: saving time and costs for transport, leading to a reduced environmental impact of the overall recycling process. Red clay was obtained from industrial manufacturing processes, as base material. Organic residues such as brewery sludge and spent coffee grounds (SCGs), that are produced in high quantities close to 6 million of tons yr-1, have been employed as expandable materials promoting an alternative way of reuse (ICO, 2017; Murthy and Madhava Naidu, 2012; Tokimoto et al., 2005). In addition, cattle bone flour ashes, vegetable ashes and a tailored fertilizer glass have been studied to include potassium (K) and phosphorus (P) to be further released in soil. Therefore, this study promotes reliable alternative to well-known phosphor-enriched leftovers (e.g. phosphogypsum) characterized by a too high toxic metal content (e.g. As, Cd, Ni, Cr, Se, Pb) that could be potentially transfer in soil (Cánovas et al., 2018).

2. Material and methods 2.1. Materials

All the residues employed to produce LWAs come from brewery fabrication, meat and vegetable transformation or post-consumer activities (coffee consuming, pruning) geographically near to each other (in the North of Italy), with the aim to promote the local re-employment and therefore reducing costs related to transports. The only natural raw material is represented by the red clay used as matrix, which is also of local origin of the same part of Italy, having grain size <1mm. Sludge from wastewater treatment during brewery fabrication and spent coffee grounds were employed as pores former agents for clay. Vegetable biomass and cattle bone flour ashes were introduced in LWAs formulation as received and through the creation of a well-tailored glass (fertilizer glass), based on vegetable biomass, differently from other studies (Andreola et al., 2019). 2.2. Fertilizer glass and LWAs preparation

Fertilizer glass was prepared using Glassy Sand (40 wt%) and ashes (60 wt%). All the compounds had a grain size lower than 100 m (D90). All the starting materials were mixed in a low-velocity rotatory mill for 30 min to enhance the homogenization of the mixture. Subsequently, the mixture was put in a refractory crucible and melted (Lenton, EHF 1700) until 1450 °C employing a heating rate of 10 °C min-1 with four isothermal steps: 900 °C for 40 min, 1000 °C for 60 min, 1300 °C for 30 min and 1450° C for 120 min. At the end of the heating cycle the melted glass was poured in water to obtain a frit that was ground in an agate mill for 45 min and sieved through a vibration laboratory machine (Retsch, AS200, Haan, Germany), to obtain a powder with D99 <100 m. LWAs formulations were reported in Table 1 in wt% showing the presence of two main categories, the first containing PK nutrients sources (ashes) as received, the second including fertilizer glass. To obtain LWAs, raw materials were mixed and then spherical shape were realized manually, dried at 105 ± 5°C and then fired at 1000°C for 60 min into a hot oven, as reported in detail elsewhere (Andreola et al., 2019). Fertilizer glass and LWAs production processes are reported in Fig 1.

Table 1. LWAs formulation in wt%

Clay Brewery sludge

Coffee grounds Vegetable biomass ash

Cattle bone flour ash Fertilizer glass

B-STD 85 15 - - - - B-Ash 64 12 - 12 12 -

B-Glass 57 10 - - - 33 C-STD 85 - 15 - - - C-Ash 64 - 12 12 12 -

C-Glass 57 - 10 - - 33

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Fig. 1. Fertilizer glass and LWAs production process

2.3 Characterizations

Chemical and elemental compositions, respectively measured through X-Ray fluorescence (XRF, Thermo Fisher, ARL ADVANT’XP+) and elemental analyzer (Thermo Fisher, FLASH 2000), of the two ashes and fertilizer glass were reported in Table 2, while the complete characterization of the other raw materials can be founded elsewhere (Andreola et al., 2019). Also X-Ray Diffraction (XRD, Philips, PW3710) was performed on LWA’s base materials. Thermal analysis such as differential scanning calorimetry (DSC) coupled with thermogravimetric analysis (TG) were employed to determine the raw material degradation profile and therefore suitable heating treatment for LWAs sintering (Netzsch, DSC 404). DSC was performed in air at the heating rate of 10 °C min-1 using 30 mg of the powdered sample, whereas TG was done using a ramp temperature of 10 °C min-1 from 0 °C to 1200 °C in inert atmosphere (nitrogen, 50 mL min-1).

Physical tests were mainly focused on the estimation of LWAs porosity. Therefore, environmental scanning electron microscopy (ESEM, Quanta-200) was employed to evaluate LWAs morphology and shape porosity, whereas, static water absorption (for 24 hours) following EN 772-21:2011 and dynamic water absorption (boiling for 6 hours) according to EN 772-7:1999 were employed to evaluate the technological performance of the materials.

Water absorption tests were performed through analytical scale (Bel Engineering M124A), with a full scale of 120g and an accuracy of 0.0001g on the average of 10 measurements. Moreover, apparent density measurement (GeoPyc 1360, Micromeritics), absolute density analysis (AccuPyc II 1330, Micromeritics) and calculation of total porosity were carried out according to EN 1936-2006. For density measurements an accuracy on the average of 5

measurements and a standard deviation of 0.0001 g cm-3 must be considered in order to obtain reliable comparison among data. LWAs total porosity was calculated as densities difference - absolute density ratio. Chemical tests included pH determination (pH meter XS Instruments model pH 6, sensibility: 0.01) according to EN 13037:2012 and specific conductivity measurement (Oakton portable conductivity meter model CON 6 / TDS 6, sensibility: 0.01) following UNI EN 13038: 2012.

Finally, germination and growth tests were performed to investigate the possible phytotoxic effect of the materials and the influence on the growth of a plant species in vivo, respectively. Growth tests were performed only on LWAs containing SCGs, which resulted in higher porosity, leading SCGs as a more effective pores former agent than brewery sludge (as will be discussed ahead in Result and Discussion section). Basil seeds were employed (5 seeds for each test) under LED light (45W) and over a period of 40 days. Basil was selected because its direct seeding is often difficult and seed germination may be limited, especially in cold soils. In particular, four different types of soil for burial were evaluated: 1) commercial control soil (base cultivation substrate containing NPK fertilizer with a composition of N 1.65%, C 39.97%, H 4.65%, P 5.04 %, K 6.10%, CNTR; 2) soil with LWAs containing only SCGs as poring agent, C-STD; 3) soil with LWAs nutrients enriched as ashes, C-Ash; 4) soil with LWAs containing fertilizer glass, C-Glass.

The germination was evaluated by monitoring the number of shoots, while the growth was measured in terms of plant’s weight, highness and number of leaves after 40 days. Plants weight was measured through analytical scale (Bel Engineering M124A), with a full scale of 120g and an accuracy of 0.0001g on the average of 10 measurements. Plants highness was measured with a digital micrometer (Mitutoyo, YY-T1BD-2GYE, sensibility: 0.02 mm).

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3. Results and discussion 3.1. Raw materials characterization

Chemical analysis (Table 2) confirms the high organic and carbonates content of brewery sludge, which show a loss on ignition (L.O.I.) above 50 wt%. These values are similar to those of SCGs reported in Andreola et al., 2019 and prove that brewery sludge can be tested as contribute to gas formation during sinterization of the LWAs. At the same time, Table 2 confirms the not-negligible content of nutrients in the fertilizer glass and in the two ashes: above 40 wt% for phosphor pentoxides into cattle bone flour ash and above 60 wt% for potassium oxide into vegetable biomass ash. At the same time chemical analysis confirms the absence of pollutant due refractory crucible employed for fertilizer glass melting. Not-significant amount of potential toxic compounds, such as heavy metals or sulfur dioxide and chlorides has been detected, therefore the risk of soil contamination is restrained. Clay’s detailed chemical analysis is reported in (Andreola et al., 2019) and shows values expected from a red clay composition (Fiori et al., 1989).

Elemental analysis (Table 2), indicates that the main organic content is due to compounds related to carbon, in brewery sludge. On the other hand, cattle bone flour ash is enriched in nitrogen, whereas vegetable biomass ash presents a negligible amount of carbon, while nitrogen and sulfur are totally absent. A not-negligible content of sulfur is present only in cattle bone flour ash (~2%) but, considering the amount of this compound in the final aggregates following (12 wt%), the formation of possible toxic species such as SO2 is strongly limited.

The mineralogical analysis performed on the red clay was previously reported elsewhere and confirms that it is composed mainly by crystalline materials (Andreola et al., 2019). Selected clay fits the most important requirement necessary to act as LWAs matrix: plastic clay materials favors the processing of the LWAs mixture and promotes mechanical resistance. All the crystalline phases detected suggest that the employed raw materials lead to formation of non-toxic crystalline compounds. Finally, the XRD pattern of glass fertilizer (data not reported) confirms the amorphous nature of this material, with the presence of crystalline silicate-sodium calcium phosphate (Na2Ca4(PO4) 2SiO4, ICDD:00-033-1229). The presence of nutrients in different crystalline phases could explain differences in release behavior depending on the raw materials employed.

Fig. 2a shows DSC and TG curves related to the clay employed as LWAs matrix: around 100 °C there is the loss of free water with an endothermic phenomenon, subsequently, around 250 °C combustion of the organic compounds is shown, around 600 °C dehydroxylation of interlayer water occurs, at about 800 °C decomposition of the carbonates appears, finally, around 1200 °C fusion occurs characterized with an endothermic process. For

each of these peaks the related weight losses (%) are presented in the same figure through a red curve; the main are humidity loss (1.89 wt%), dehydroxylation (4.40 wt%) and carbonates decomposition (4.74 wt%). In Fig. 2b the thermal behavior of coffee grounds has been represented, with the aim to show an example of the overall thermal attitude shared with brewery sludge and vegetable biomass: up to 200 °C a fairly stable situation is evident, with a limited decrease in weight of the sample in proximity to small endothermic peak which implies the sole loss of moisture inherent the material. In the range from 250 to 600 °C several exothermic peaks are detectable on the DSC curve, with particular relevance around 300 °C, due to the combustion of the organic fraction. These peaks are in fact accompanied by an overall considerable weight loss near to 50 wt%. Around 650-700 °C a further endothermic peak occurs due to the decarbonation of the material, in strong similarity of what has been shown previously about clay. Above 700 °C no further peaks are detected, therefore, there are no more significant thermal decomposition. As confirmed by thermal characterization the employment of 1000 °C for LWAs sintering should reach both the objectives: clays sinterization and organic fraction degradation and consequently pore formation. 3.2. LWAs physical characterization

Weight loss % has been calculated through LWAs weight measurements before and after aggregates firing in order to evaluate the content of humidity and volatile compounds into the different LWAs. As shown in Table 3 LWAs containing brewery sludge have a lower weight loss than those containing spent coffee grounds (~ -5%), that is coherent to the higher amount of organic fraction in spent coffee ground with respect to brewery sludge. On the other hand, even if no particular difference can be detected comparing the STD samples with the ones containing ashes a further decreasing of weight loss is observed employing fertilizer glass, in agreement to a content of volatile compounds equal to zero for the fertilizer glass (Table 2). This result is verified for both LWAs series: containing brewery sludge or spent coffee grounds. Therefore, higher porosity and lower density is expected in samples containing ashes and spent coffee grounds.

ESEM analysis (Fig. 3) have been performed on the inner part of the LWA showing, as expected by weight loss measurement, that LWAs containing spent coffee grounds (Fig. 3b) show higher porosity with respect to brewery sludge (Fig. 3a). In addition, LWAs containing brewery sludge show a more heterogeneous microstructure which could favor the nutrients release in soils (Fig. 3a). Pore’s amount decreases when the fertilizer glass is included (Fig. 3c and Fig. 3d), at the same time an agglomeration and general increasing of grain phases is observed, that is coherent with the marked decreasing of measured porosity moving from B-Ash to B-Glass (or C-Ash to

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C-Glass) as reported in Table 3. Table 3 shows that water absorptions strongly

increase if ashes are included directly as LWAs component with respect to STD samples, because ashes are highly organic and hygroscopic materials. On the contrary water absorptions decrease with the addition of fertilizer glass in LWAs composition for the same reason. Furthermore, fertilizer glass acts as pore closure, with respect of STD and ashes containing samples. This result is also confirmed by the apparent density measurements (Table 3) which show an increase of the apparent density with fertilizer

glass addition, therefore indicating a decreasing of the open porosity available to water permeation. On the contrary not significant variation of apparent density has been found moving from B-STD to B-Ashes or from C-STD to C-Ashes.

The increasing of absolute density moving from STD to ash containing samples and to glass containing samples agrees with the higher crystalline degree of the LWAs microstructure investigated through ESEM (Fig. 3) suggesting a more packed microstructure due to the presence of crystalline phases and vitrified areas.

Table 2. Chemical and elemental analysis of the raw materials and fertilizer glass

(XRF sensibility ± 0.05; ICP sensibility ± 0.01%)

wt(%) Brewery sludge Cattle bone flour ash Vegetable biomass ash Fertilizer glass Chemical Analysis

SiO2 11.86 0.43 2.38 31.97 Al2O3 1.10 <0.05 0.24 3.85 Fe2O3 7.40 <0.05 0.21 0.46 CaO 18.52 53.89 6.07 24.83 MgO 1.17 1.11 0.73 1.49 Na2O 1.82 1.4 0.31 6.04 K2O 0.26 <0.05 61.20 15.96 TiO2 0.18 <0.05 -- 0.02 MnO 0.06 -- -- -- P2O5 2.18 41.24 1.07 14.97 PbO -- -- -- 0.01

L.O.I. 55.44 1.74 18.60 0.00 Total 99.14 99.9 90.81 99.60

Elemental analysis N 3.02 8.34 0.00 / C 23.45 31.72 6.28 / H 3.32 4.32 0.62 / S 0.09 2.16 0.00 /

(a) (b)

Fig. 2. DSC and TG analysis (a) clay; (b) coffee grounds

Table 3. Physical properties and growth test results after burial

in soil and seeding of selected samples

CNTR B-STD B-Ash B-Glass C-STD C-Ash C-Glass pH / 6.72 6.40 8.24 7.06 6.52 8.42 Conductivity (dS m-1) / 0.25 1.39 0.17 0.40 0.97 0.19 Water absorption 24h (%) / 11.65 17.69 9.09 15.34 18.87 11.00 Water absorption 6h (%) / 11.93 21.37 9.92 22.16 24.96 16.45 Absolute density (g cm-3) / 2.62 2.72 2.82 2.49 2.68 2.80 Apparent density (g cm-3) / 1.44 1.45 1.73 1.30 1.27 1.49

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Total porosity (%) / 45.28 47.72 38.60 46.82 52.50 46.81 Weight Loss (%) / 15.7 15.4 9.3 21.1 20.2 14.1

Growth test results Number of plants 5±0.1 / / / 4.7±0.6 5.7±0.6 4±1.4 Highness (cm) 10.2±0.2 / / / 9.4±0.1 10.6±1.5 10.7±1.3 Weight (g) 7.234±2.309 / / / 6.035±1.180 6.349±0.722 5.510±1.329 Number of leaves 25.6±1.4 / / / 21.6±0.4 23.6±0.2 22.4±0.1

Fig. 3. ESEM analysis of LWAs (300x): a) B-Ash, b) C-Ash, c) B-Glass, d) C-Glass

According to results obtained through ESEM and weight loss measurement, spent coffee grounds-containing LWAs show in general higher porosity and water absorption with respect to brewery sludge-containing ones. According to these results the highest porosity is obtained through the sample C-Ash with a value above 50%. As expected, the dynamic water absorption enhances the water uptake, leading to generally higher values of water absorption for all the investigated samples. The water absorption is a key issue for CRFs functionality because it is strictly connected with nutrients release. In fact, a promising mechanism to better understand the working principle of CRFs is the one called multiphase diffusion model (Liu et al., 2008). According to this mechanism, after the fertilizer has been released into the soil, the irrigation water penetrates the coating, condenses at the core and causes the partial, but progressive,

dissolution of the nutrients. Finally, as already reported considering the raw materials employed, no toxic elements have been founded through XRD analysis after the sinterization of the lightweight aggregates.

The results obtained about pH and conductibility are shown in Table 3 and demonstrate that all LWAs designed in this study fall into the category of aggregates that can be used for all type of culture, since they have a pH value between 6 and 7 or very close to this range. Even if slight deviations from this range to a basic pH have been observed (B-Glass and C-Glass), they are not significant considering the requirements needed for the greatest number of cultivations. The same happens for conductivity values which are mainly less than 2 dS m-1, that is the threshold to consider a fertilizer suitable for most type of soils. It must be noted that ashes containing LWA

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shows in general higher conductivity and lower pH due to CaO and MgO in their composition (Table 3). 3.3. Growth test

Germination and growth test results are reported in Table 3, where a notable increment of the germination parameter has been found employing C-Ash with respect to others compounds. This result reflects the fact that the overall PK content of the commercial CRFs (~11 wt%) is lower than the PK content of C-Ash (~14 wt%). In fact, the number of plants germinated is higher employing C-Ash with respect to CNTR. On the other hand, similar results have been found employing C-Ash with respect to CNTR about growth parameters. As expected, sample C-STD is the one that shows lower beneficial effect, due to the absence of PK nutrients, whereas the glass employment (C-Glass) lead to a restrained nutrient release, confirmed by its overall PK content (~8 wt%).

These results suggest that the PK release of the fertilizer employed in this study has a beneficial effect on plant’s germination with respect to a commercial fertilizer soil. With respect to other studies on basil cultivation with standard fertilizers (NH4

+ and commercial NPK) , the present study demonstrates the possibility to obtain similar or enhanced plant’s highness and number of leaves during growth (Frerichs et al., 2019; Setti et al., 2019).

Thereafter, the balanced employment of residues in this field of application overcomes theirs possible limitation due to source variability and chemical composition variation, avoiding possible costs due to residues purification, but strongly promoting a circular economy processing. 4. Conclusions

In this work, lightweight aggregates (LWAs) were successfully designed to host very tailored compounds, allowing the controlled release of nutrients and water when buried in soil. Their obtainment was reached by powders sinterization of mixtures containing local raw materials, agro-residues and red clay, in a circular economy perspective. At the same time a new type of fertilizer glass, from different type of recycled materials was engineered to be enriched in potassium and phosphorous, 16 wt% and 15 wt% respectively, also according to the actual EU regulation about critical raw materials recovery and re-employment.

Porosity evaluation has shown that spent coffee grounds have increased the overall porosity up to 52% which lead to an increasingly higher water absorption value. Therefore, they could be particularly suitable for green roofs and hanging gardens due to their particularly lightweight. Chemical tests have shown that aggregate’s pH and conductivity values fall within the range suitable for agronomic use. The basil germination tests have confirmed the beneficial effect of the aggregates containing ashes (PK content ~14 wt%) with respect to a standard fertilizer (PK content

~11 wt%), whereas growth test showed similar results. Consequently, quantitative life cycle analysis and life cost analysis should be done in comparison with products already on the market to estimate the beneficial effects. Acknowledgements Authors would like to thank Project FAR UNIMORE 2017 entitled “The enhancement of agro-industrial waste between law and science: innovative processes from experimentation to industrialization in the legal context” for financial support. Italian company suppliers for the raw materials supply and Eng. Alessandro Borghi, Eng. Galia Agnoletto, Dr. Romina Farias and Dr. Giulia Santunione for the experimental support. References Al-Zaharani S.M., (1999). Controlled-release of fertilizers:

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