Fodder radish seed cake biochar for soil amendment

Environmental Science and Pollution Research (2018) 25:25143–25154 https://doi.org/10.1007/s11356-018-2571-4 RESEARCH ARTICLE Fodder radish seed cake biochar for soil amendment Wendel Paulo Silvestre 1,2 & Paula Lúcia Galafassi 2 & Suelem Daiane Ferreira 1 & Marcelo Godinho 1 Gabriel Fernandes Pauletti 2 & Camila Baldasso 1 & Received: 1 June 2017 / Accepted: 18 June 2018 / Published online: 25 June 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract In this work, fodder radish seed cake (FRSC) was pyrolyzed in a rotary kiln reactor at 0, 3, and 6 rpm, at final temperature of 500 °C. Maximum biochar yield was observed at 0 rpm (≈ 26 wt.%). Increase of the rotary speed decreased the volatile matter content and increased the ash content of the biochars. Biochars exhibited alkaline pH (≈ 9.0), low electrical conductivity (< 105.6 dS m−1), and high cation exchange capacity (69 to 78 cmolc kg−1), as well as high nitrogen contents (≈ 80 g kg−1). FTIR analysis presented biochars with similar spectra, with carboxyl and carbonyl groups within the structure, along with aromatic rings and nitrogen containing functions (amides). Biochar incubation experiments in an acrisol at different biochar doses (5 g L−1 soil to 40 g L−1 soil) were performed in order to evaluate changes in soil fertility parameters caused by FRSC biochar application. Results indicated that most of macro (N, P, K, Ca, Mg) and micronutrients (S, Cu, Zn, Mn, B, Na) increased with increase of the dosage, along with the decrease in Al and H+ Al contents. An increase in pH (from 4.25 to 5.33) was also observed, in electric conductivity (from 30.0 to 45.7 dS m−1), and a decrease in soil real density (from 3.67 to 2.99 kg L−1) at the dosage of 40 g char L−1 soil. Keywords Fodder radish . Organic fertilization . Biochar . Pyrolysis . Rotary kiln Introduction Biochar is a carbon-rich solid product generated through the pyrolysis of biomass. Biochar represents an emerging technology that is being recognized for its potential role in carbon Highlights (1) First study about fodder radish seed cake pyrolysis in a rotary kiln. (2) Rotary speed modified some chemical characteristics of the biochars. (3) Biochars presented high nitrogen content, high CEC, low EC, and alkaline pH. (4) Most of macro and micronutrients contents increased with the increase in biochar dosage. (5) Al content decreased, along with improvement of soil physical characteristics. Responsible editor: Hailong Wang * Marcelo Godinho mgodinho@ucs.br 1 Post-graduate Program in Process Engineering and Technologies, University of Caxias do Sul, Caxias do Sul, Brazil 2 Graduate Program in Agronomy, University of Caxias do Sul, Caxias do Sul, Brazil sequestration, thus presenting potential to reduce greenhouse gas emissions, in waste management and renewable energy production, to promote soil improvement and crop productivity enhancement, and to be used in environmental remediation (Kuppusamy et al. 2016; Lehmann et al. 2011). Biochar application in soil influences soil physicochemical properties, such as pH, porosity, bulk density, cation exchange capacity (CEC), electric conductivity (EC), and water holding capacity (Singh et al. 2010; Zeng et al. 2015). Biochar production (pyrolysis) shows advantages in waste management because it is a safer method to treat organic waste and reduce the quantity of output material generated, in addition to generating energy and forming other products (non-condensable gases and bio-oil) with the production process (Omar and Robinson 2014). Nevertheless, the usefulness of biochar for any application depends on its properties. Biochars with higher contents of available nutrients or higher cation exchange capacity than soil could be used to improve soil fertility (Song and Guo 2012; Singh et al. 2010; Yuan et al. 2011). Biochar, as well as biochar associated with other materials (limestone, compost, diverse manures), is used to improve soil characteristics and to promote remediation of degraded and/or contaminated soils, mainly by heavy metal sequestration 25144 (Liang et al. 2017; Singh et al. 2010). Liang et al. (2017) and Zeng et al. (2015) reported the effect of biochar and compost application on soil. Another important effect is the reduction of heavy metal (Cu, Zn, Cd, Pb) availability to plant absorption, which has an important environmental impact in heavy metal-contaminated areas (Singh et al. 2010). The impact of biochar application on soil microbiota is not well defined. Zhang et al. (2016) cite the negative effects of heavy metals on soil microorganisms, especially Cd and Cr. The authors also state that the effect of soil physicochemical properties on soil microbiota is more important than the heavy metal effect, although both soil properties and heavy metal presence were responsible for microorganism population reduction. Wu et al. (2016) cite the positive effects on soil fertility and microorganisms of biochar application associated with compost or different manures (maturated or not). Fodder radish (Raphanus sativus L.) is an angiosperm from the Brassicaceae family. Its most important use is in biodiesel production, by the seed oil, while the extraction residue (seed cake) is applicated in soil as fertilizer. Fodder radish’ other uses are green fertilizer, by planting or discarding of the biomass, and coverage plant, to avoid the erosion of soil (Ávila and Sodré 2012; Chammoun et al. 2013). The remaining cake from the pressing of the seeds ends being a ‘loose end’ of biodiesel production process. The use of this residue may render it a feedstock with applications in bioenergy (Ávila and Sodré 2012; Chammoun et al. 2013). Although fodder radish seed is widely used as feedstock to obtain oil for biodiesel production, literature lacks works on seed cake characterization, as well as on its pyrolysis. There are studies on rapeseed seed cake (Brassica napus), which belongs to the same botanical family of the fodder radish and which seed has economic importance in biodiesel production. Smets et al. (2011, 2013) performed rapeseed seed cake pyrolysis and characterized both the biomass and the obtained biochar. Char yield was 17 wt.% at final temperature of 550 °C in the flash pyrolysis and 27.9 wt.% in the slow pyrolysis. Ucar and Ozkan (2008) also conducted rapeseed seed cake pyrolysis at final temperature of 500 °C and obtained biochar yield of 33.23 wt.%. Souza et al. (2009) and Santos et al. (2010) presented works on fodder radish seed bagasse as food source (bran) to be used in cattle and fish feeding because of its high protein content. Carvalho et al. (2009) used fodder radish cake as an organic fertilizer. However, the high content of residual oil may cause superficial soil sealing, making it difficult for water percolation along the profile. This problem would not occur with the biochar, because it is oil-free. Literature presents several studies on the agronomic properties of biochar and the potential benefits of its application in soil. Narzari et al. (2017) investigated the yields and the physicochemical properties of biochar from Environ Sci Pollut Res (2018) 25:25143–25154 three different feedstocks (cakes of Jatropha carcus and Pongamia glabra/Jatropha carcus seed cover/noxious weed—Parthenium hysterophorus) through slow pyrolysis at 350 to 650 °C. The biochar produced at higher temperature presented higher water holding capacity (WHC) and pH, suggesting its suitability as an amendment in soil with low water retention capacity. De Conto et al. (2016) investigated the influence of process conditions (rotary speed/temperature) on the performance of a rotary kiln reactor for non-catalytic pyrolysis of a perennial grass (elephant grass). The biochars presented alkaline pH (above 10), 14.9 g N (kg biochar)−1, 799.6 g C (kg biochar)−1, 64.0 g K (kg biochar)−1, 266.1 g ash (kg biochar)−1, as well as low electrical conductivity (2.9 to 3.4 dS m−1), making these biochars potential auxiliary soil amendments. Aziz et al. (2015) investigated biochar production from empty fruit bunch (EFB) pyrolysis. Experiments were carried at temperatures between 400 and 600 °C. Authors reported a char with high pH (10.88), and a cation exchange capacity (CEC) of 350 meq kg−1 at 400 °C. Mohanty et al. (2013) evaluated effectiveness of biochar produced from three different biomasses (wheat straw/timothy grass/pinewood). Biochar was produced in a fixed-bed reactor at 450 °C and two heating rates (450 and 2 °C min−1). Experiments at lower heating rate presented a biochar yield between 41 and 44 wt.%, as well as higher C and H contents. Timothy grass biochar presented considerable potassium content in its composition (46 to 48 g kg−1). No study was found on fodder radish pyrolysis, neither its cake (FRSC). Given the potential of fodder radish to produce biofuels (biodiesel) and consequently a waste (FRSC), the contribution of this study is to evaluate performance of a rotary kiln reactor for biochar production through the FRSC pyrolysis, to characterize the biochars obtained, and to perform biochar incubation experiments in soil to investigate the performance of biochar as a soil amendment/auxiliary liming agent. Materials and methods Biomass preparation Biomass (Raphanus sativus L.) was harvested in the city of Caxias do Sul, state of Rio Grande do Sul, Brazil (geographical coordinates: 28°81′44.13–51°42′55.94). Seeds were gathered and the oil was extracted by pressing. The cake was dried in a kiln for 24 h at 50 °C, being subsequently milled in a hammer mill until the granulometry of the material was smaller than 2.0 mm (mesh/Tyler 10), in accordance with Malavolta et al. (1997). Environ Sci Pollut Res (2018) 25:25143–25154 Biomass characterization Proximate analysis was performed in accordance with the ASTM D3172-13 standard. Extractives content was determined using Tappi T 204 cm-97 standard. A mixture of ethanol and benzene (1:2 in volume) was used as extracting solution in a soxhlet system, with extracting time of 5 h. Cellulose and hemicellulose contents were determined by the Van Soest method (Lictra et al., 1996). Lignin content was determined in accordance with Tappi T-222 om-02 standard. FTIR analysis was carried out in a Nicolet instrument, Thermo Scientific model IS10, following the procedures described by Bottomé et al. (2017). Ultimate analysis was carried out following the ASTM D5373-02 standard for the determination of carbon, hydrogen, and nitrogen determination, and following the ASTM D423914e2 standard for the determination of sulfur. Oxygen content was calculated by mass balance. Other elements were determined according to the methods proposed by Malavolta et al. (1997). Nitrogen was determined by sulfuric digestion, followed by Kjehldahl determination using a Brand digital burette. Boron was determined by burning in a Quimis muffle furnace, and by colorimetry with azometine-H using a ProAnálise 1800 Spectrophotometer. The other elements were mineralized by nitric-perchloric digestion. Sodium and potassium were determined by flame spectroscopy using a Micronal B-542 photometer. Calcium, magnesium, copper, Fig. 1 Scheme of pyrolysis system 25145 zinc, iron, and manganese were determined by atomic absorption spectroscopy (AAS) in an Agilent equipment, model 55AA. Phosphorous was determined by colorimetry using a metavanadate/molibdate and sulfur was determined by turbidimetry with barium chloride, both determinations carried out using a Pro-Análise 1800 Spectrophotometer. CEC determination was performed following the methodology proposed by Singh et al. (2010), after removing soluble salts and carbonates from the sample. Pyrolysis process Pyrolysis assays were performed following an experimental planning in three levels (0, 3, and 6 rpm), being each level carried out with triplicate. A total of 150 g of dried seed cake was used in each pyrolysis assay. The rotary kiln reactor (Sanchis, Brazil) was inerted for 30 min to remove the air present in the system (Fig. 1). A heating rate of 50 °C min−1 was used, starting from room temperature until 500 °C. Once final temperature was reached, the isotherm was started and kept for more 30 min to assure the completion of the reaction. Then, the resistances were turned off to start the cooling. The carrier gas (nitrogen) was kept connected and in constant flux of 1.0 L min−1 during all the pyrolysis process, from inertization to cooling of the system to room temperature. More details 25146 about the reactor can be obtained elsewhere (Perondi et al. 2017; De Conto et al. 2016). In the assays that the reactor tube was rotated, the rotary speeds were 3 and 6 rpm, to avoid biomass accumulation in one of the sides of the tube. The rotation started in the beginning of the intertization, being turned off when the system started to cool. After cooling of the reactor to temperature lower than 40 °C (to avoid spontaneous combustion of the char), the flux of inert gas was stopped, the biochar was collected and weighted to calculate its yield. Bio-oil yield was determined by weighting of the impinger system (with isopropanol) before and after the pyrolysis. The mass gain of the system was considered condensed bio-oil. The non-condensable gas yield was obtained by mass balance. Biochar analysis The obtained biochars were milled until granulometry was smaller than 2.0 mm (mesh/Tyler 10). Chemical characterization was performed following the same methods of biomass characterization. Electrical conductivity (EC) and pH were determined in accordance with the ASTM D4980-89 standard. Test of the impact of different biochar dosages in the soil Five biochar treatments, plus positive and negative blanks, were kept in a greenhouse for 60 days to verify the change in the soil fertility parameters caused by biochar application. The A horizon (top layer) of a never-used acrisol with moisture content of 35% of maximum water holding capacity was collected in Fazenda Sousa, a rural district of Caxias do Sul (geographical coordinates: 28°81′44.13–51°42′55.94). The test was carried out using 500-mL vases, in four replicates for each treatment. The negative blank (T0-) was just soil. In the positive blank (T0+), 15 g of limestone was added to each liter of soil. Since the three biochars had very similar physical chemical properties, they were mixed to form a single sample. In the treatments using biochar, mixed and milled biochar (particle size smaller than 2 mm) were mixed to soil samples to obtain concentrations of 5, 10, 15, 20, and 40 g of biochar per liter of soil, totalizing 7 treatments with 4 replicates each. Samples of all treatments were irrigated up to half of maximum water holding capacity every 2 days with distilled water, avoiding loss of percolate. Greenhouse temperature ranged from 17 to 29 °C during incubation. The treatments are resumed in Table 1. After the incubation period, the mixture of soil and incubated biochars had their nutritional parameters evaluated by the methodology described by Tedesco et al. (1995). Clay was determined by the densimeter method, Environ Sci Pollut Res (2018) 25:25143–25154 Table 1 Treatment codification for the agronomic study Treatment Dosage T0+ T0− T1 T2 T3 15 g limestone L−1 of soil – 5 g biochar L−1 of soil 10 g biochar L−1 of soil 15 g biochar L−1 of soil T4 T5 20 g biochar L−1 of soil 40 g biochar L−1 of soil pH was determined in a mixture of dry milled soil with water in a 1:1 proportion, pH-SMP was determined by buffering the mixture of pH determination with SMP (Shoemaker, McLean, Pratt) buffer (to simulate plant own buffering effect in relation to the soil and, thus, lime requirement). Both pH measurements were performed in a Digimed, DM-22 pHmeter with an Ag/AgCl electrode. Ca, Mg, Mn, and Al were extracted with potassium chloride 1 mol L−1. Ca, Mg, and Mn were determined by AAS using an Agilent AA-55 spectrophotometer, and Al was determined by titration using a Brand digital burette. Cu, Zn P, K, and Na were extracted with the Mehlich-1 solution. P was determined by colorimetry with a Pro-Análise 1800 Spectrophotometer, Na and K by flame photometry using a Micronal B-542 photometer, and Cu and Zn by AAS using an Agilent AA-55 spectrophotometer. Sulfur was extracted with Ca(H2PO4)2, 0.01 mol L−1 and determined by turbidimetry with barium chloride using a ProAnálise 1800 Spectrophotometer. Boron was extracted with hot water and determined by colorimetry with azometine-H using a Pro-Análise 1800 Spectrophotometer. Nitrogen was determined by sulfuric digestion and Kjeldahl determination using a Brand digital burette. O rga n i c m a t t er ( O M ) w a s d et er m i n e d us i n g a sulfochromic solution following UV-Visible spectrometry using a Pro-Análise 1800 Spectrophotometer. H + Al and CEC at pH 7 were calculated. Bulk and real densities were determined by the volumetric flask method. Porosity was calculated and water retention capacity was determined after saturating the soil and determining the mass difference after a 24-h period of drainage, in accordance with Gubiani et al. (2006). Statistical analysis All parameters were analyzed with four replicates. Results underwent analysis of variance (ANOVA), and the parameters that presented statistical significance were analyzed by Duncan’s multiple range test at 95% confidence interval (α = 0.05). The data was processed by Statistica software version 10. Environ Sci Pollut Res (2018) 25:25143–25154 25147 Results and discussion Biochar characterization Effect of rotary speed on biochar yield When compared to the unpyrolyzed biomass, C and N contents of the biochars were higher (Table 3). Although nitrogen can be lost because of volatilization at high temperatures (Smets et al. 2011), an increase in nitrogen content was observed. The high nitrogen content in the unpyrolyzed biomass (6.4 wt.%) was partially retained in the biochar despite its partial volatilization. The relatively low pyrolysis temperature (500 °C) can be also responsible for nitrogen retention (De Conto et al. 2016; Jendoubi et al. 2011). Ultimate (elemental) analysis reported by Smets et al. (2011, 2013) for rapeseed cake was as follows (g kg−1): C–550, H–78, N–47, S–14, O– 311. Both rapeseed and fodder radish presented high sulfur and nitrogen contents, which differed considerably from lignocellulosic biomasses (Stedile et al. 2015). Sulfur had high volatilization during the pyrolysis process, being released possibly as reduced sulfur compounds (hydrogen sulfide in particular) (Jendoubi et al. 2011). The H/C ratio of the biochars (0.56 ± 0.01) was lower than the observed for the unpyrolyzed FRSC biomass (2.1); this may be an increase of the aromaticity, while the O/C ratio of the biochars was 0.13 ± 0.01 and for the unpyrolyzed FRSC biomass, it was 0.5, suggesting that the biochar became more hydrophobic. Rotary speeds of 0, 3, and 6 rpm were used in the experiments. Low rotary speeds were used to simulate the process conditions that occur in industrial kilns, such as in cement factories. Product yields of the pyrolysis process at each rotary speed are presented in Table 2 According to the experimental planning, with a confidence interval of 95%, the experiments at 0 rpm (reactor stationary) presented higher biochar yield, while for the other rotation speeds (3–6 rpm), there was no significant change in biochar yield. The lowest bio-oil yield occurred without rotation, and there was also no significant change between 3 and 6 rpm. This phenomenon may be linked to the releasing of the primary pyrolytic vapors being facilitated with the movement of the biomass, which decreased the residence time of the primary vapors in the bed and, consequently, avoided the secondary cracking reactions (Brigdwater 2012; Smets et al. 2013; Ucar and Ozkan 2008). Ucar and Ozkan (2008), working with rapeseed seed cake, observed that at 500 °C biochar yield was 33.23 wt.%, and at 900 °C the yield lowered to 30.04 wt.%. Smets et al. (2011), working with flash pyrolysis of rapeseed seed cake, obtained biochar yield of 12.5 wt.% at 550 °C. Smets et al. (2013), carrying out slow pyrolysis of the same biomass, obtained 23.0 wt%. of biochar yield at the same temperature (550 °C). Literature shows the importance of the final pyrolysis temperature for biochar yield, once the thermolysis of the macromolecules of the biomass is fundamentally product of the process temperature. Rotation facilitates release of the primary pyrolytic vapors but does not change the temperature gradient within the biomass particles. Since temperature is the main factor for the splitting of the molecules that form the biomass, rotation ends up changing significantly the biochar yield. However, the increase of rotation (3 to 6 rpm) should not have increased the release rate of the primary vapors, and consequently, there was no significant change in the biochar yield between 3 and 6 rpm (Aziz et al. 2015; Kuppusamy et al. 2016; Song and Guo 2012). Table 2 level Average yield (wt.%) for pyrolysis products in each rotation Rotation Biochar Non-condensable gases Bio-oil 0 rpm 3 rpm 6 rpm 26.00 ± 0.16 a 24.89 ± 0.30 b 24.90 ± 0.09 b 16.20 ± 2.09 a 14.13 ± 1.09 a 13.45 ± 0.43 a 57.80 ± 1.09 b 60.99 ± 0.79 a 61.74 ± 0.65 a The means in each column followed by the same letter do not show significant statistical difference when compared by Duncan’s multiple range test Table 3 Fodder radish seed cake biomass and biochars’ proximate and ultimate (elemental) analyses Parameter (g kg−1) Biomass 0 rpm 3 rpm 6 rpm Moisture (M) Volatile matter (VM) Fixed carbon (FC)1 59.5 756.6 133.9 43.0a 228.5a 554.2a 43.6a 224.2ab 554.0a 42.7a 220.4b 558.9a Ash (A) C H N S O2 H/C3 N/C3 O/C3 Cellulose Hemicellulose Lignin Extractives HHV (MJ kg−1) 50.1 479.7 85.0 64.3 20.9 300.0 2.11 0.11 0.50 35.5 41.5 59.6 239.0 – 174.3b 606.8a 29.4a 81.3a 7.3a 100.9a 0.58a 0.11a 0.12a – – – – 23.08a 178.2a 598.6a 27.8a 79.7a 6.7a 109.0a 0.55a 0.11a 0.14a – – – – 22.98a 178.1a 605.1a 28.3a 80.2a 6.8a 101.5a 0.56a 0.11a 0.13a – – – – 22.97a Means in each row followed by the same letter do not show significant statistical difference when compared by Duncan’s multiple range test 1 Calculated as follows: FC = 1000 − (M + VM + A) 2 Calculated as follows: O = 1000 − (C + H + N + S + A) 3 Molar ratio 25148 The FRSC biomass compositional analysis presented high content of extractives (239 g kg−1), being mostly residual oil from the extraction process by pressing of the seed. Smets et al. (2011, 2013) reported for rapeseed seed cake (g kg−1) 15 of moisture, 755 of volatile matter, 181 of fixed carbon, 49 of ash, 90 of cellulose, 80 of hemicellulose, and 40 of lignin, results similar to values obtained for fodder radish. Cellulose and hemicellulose contents in rapeseed were higher than the ones in fodder radish; this may be because of plant growth, age of crop, or environmental and nutritional factors (Chammoun et al. 2013). There was an increase in carbon and nitrogen contents and a decrease in oxygen, hydrogen, and sulfur contents of the biochar in relation to the unpyrolyzed FRSC biomass. Most of the evaluated parameters did not present statistical difference among different rotation speeds, being exceptions volatile matter, which decreased, and ash content, which increased with increase in rotation. Rotary speed had effect neither over the distribution of the elements nor over the high heating values (HHV) of the biochars (Table 3). FTIR analysis of the FRSC and its biochars are presented in Fig. 2. Unpyrolyzed FRSC FTIR spectrum presented a broad band between 2800 and 3400 cm−1, which is related to the presence of free and bonded hydroxyl groups and structural hydroxyl groups (–COOH and –COH) (Al-Wabel et al. 2013). This band was probably from the fatty carboxylic acids present in the biomass. It was not present in the biochars, suggesting the loss of molecules containing –COOH and –COH groups (Al-Wabel et al. 2013; Mohammed et al. 2015). The volatilization of the fatty acids in the biomass during the process explained the absence of the signal. Peaks which appeared at approximately 2920 cm−1 are attributed to the C–H stretching vibration from CH and CH2 in cellulose and hemicellulose components, derived mainly from the fatty acids. Such peaks were not observed in any of the biochars, which is possibly the result of cellulose and hemicellulose degradation at these temperatures, as well as the volatilization of the carboxylic acids, also present in the feedstock. The peaks at 1534–1620 cm−1 provided the evidence of double bonds of alkene and aromatics, strongly present in the unpyrolyzed biomass FTIR spectrum, but very weak in all of the biochars. Thermal destruction of cellulose and lignin in the feedstock may result in the exposure of alkyl, methyl, hydroxyl, carbonyl, and aromatic functional groups in biochars (Chen et al. 2014). The presence of oxygen containing functions in the feedstock was confirmed by the C–O stretching around 1044 cm−1. All the biochars presented a broad and weak band near 1100 cm−1, which can be attributed to C–O functional groups (ketones, aldehydes) (Saikia et al. 2015). Such band is also associated to the C–N stretching vibration in primary aliphatic amines (C–NH2). The char produced at 6 rpm presented a band at 2323 cm−1. This band is attributed to triple C– Environ Sci Pollut Res (2018) 25:25143–25154 N bond (nitriles), as well as to triple C–C bond (alkynes) (Chen et al. 2014; Saikia et al. 2015). A strong band was found peaking at 1447 cm−1, which was attributed to C6 ring modes, thus, providing good evidence for the presence of aromatic structures (Ghani et al. 2013). The biochars presented a very weak band in this region, meaning most of the aromatic structures had been lost as aerosols or as a result of the affinity with lipophilic substances (fatty acids from the biomass that volatilized and condensed in the liquid fraction) (Fig. 2). The presence of carboxyl, carbonyl, and amine groups in the biochar is interesting, since they act as chelating agents, and can enhance the ion-exchange capacity. The presence of these groups may increase the CEC and EC of the biochar, helping to chelate and to slowly release cations, thus preventing losses from leaching (Cely et al. 2015; Ghani et al. 2013). Although FTIR spectra did not show a strong presence of functional groups in the biochar (such as carboxyl, amine, and carbonyl groups), the incorporation and subsequent degradation of the material by the microorganisms of the soil transform the biochars in humic and fulvic acids. Such substances act as organic ion exchange resins (Li et al. 2015; Jindo et al. 2016). This way, application of biochar in soil can enhance its fertility. Agronomical applications of the biochars As seen in Table 4, all elements had higher concentrations in the biochar than in the feedstock, since the non-volatile inorganics remained in the solid fraction (Jendoubi et al. 2011). For element distribution, most of the parameters did not present statistical difference in relation to rotation speed, except for P, Cl, Mn, Na, electrical conductivity (EC), pH, and cation exchange capacity (CEC). The high content (70 to 80 g kg−1) of nitrogen in the biochars is remarkable. The loss of nitrogen in biochars was a result of nitrogenous organic compound reduction to ammonia and its further volatilization; lower pyrolysis temperature may have reduced nitrogen releasing (Cely et al. 2015; Jendoubi et al. 2011) (Table 4). The pH of the feedstock (FSRC) was 5.85 and increased to 9.10 in the biochars. Generally, pH increases with increase in pyrolysis temperature, but the magnitude of this increment depends on the feedstock characteristics, such as inorganics and moisture content (Singh et al. 2010). The pH increase resulted from the formation of Ca, Mg, Na, and K oxides and hydroxides, as it occurs in combustion ashes (Jendoubi et al. 2011). The rotary speed had influenced the pH of the biochars; however, a clear trend could not be identified. In accordance with Bordoloi et al. (2015) and Singh et al. (2010), biochars with high pH may correct acidity problems in soil. Therefore, it may be used as a liming agent or an auxiliary liming agent when combined with other compost (manure, humus). The higher the pH content, the greater the Environ Sci Pollut Res (2018) 25:25143–25154 25149 Fig. 2 FTIR spectra of the biomass and the biochars liming effect on soil. In general, manures, and especially poultry litter, have high pH values (> 9.0) and present great liming potential when compared to other feedstocks (woods and leaves), which have smaller ammoniacal nitrogen content, and are less alkaline. A joint use of biochar and manure may Table 4 Fodder radish seed cake biomass and biochars elemental compositions and physicochemical characteristics Parameter Biomass Biochars* 0 rpm 3 rpm 6 rpm N Kjehldahl (g kg−1) P (g kg−1) K (g kg−1) Ca (g kg−1) Mg (g kg−1) Cl (g kg−1) Zn (mg kg−1) Cu (mg kg−1) Mn (mg kg−1) Fe (mg kg−1) B (mg kg−1) 60.04 10.01 10.94 3.44 4.61 2.31 4.76 56.55 39.52 431.80 7.28 76.59a 28.11b 35.73a 9.72a 14.02a 6.28b 15.25a 182.17a 142.49b 1677.67a 33.93a 76.54a 30.18a 34.67a 10.13a 14.42a 6.54a 16.69a 192.11a 149.73ab 1785.86a 30.49a 76.14a 29.54ab 35.73a 10.30a 14.67a 6.62a 14.84a 187.09a 166.16a 2146.71a 27.85a Na (mg kg−1) EC (dS m−1) pH CEC (cmolc kg−1) 68.22 310.8 5.85 18.27 222.55a 105.6a 8.78b 53.48b 199.00ab 93.6b 9.12a 57.12ab 175.46b 88.4b 9.02a 61.83a *Means in each row followed by the same letter do not show significant statistical difference when compared by Duncan’s multiple range test be useful to achieve special fertility requirements for demanding cultivations (Cely et al. 2015). EC roughly estimates total dissolved salts, which are immediately available for plant absorption. EC decreased with increase in rotary speed, from 310.8 dS m −1 in the unpyrolyzed biomass to 88.4 dS m−1 in the biochar obtained at 6 rpm. This phenomenon may be related to the liberation of aerosols, which can be dragged with the pyrolysis vapors to the liquid fraction. Most of the ‘free’ inorganic fraction is in the aerosols (Jendoubi et al. 2011; Mortensen et al. 2011). Other possibility is biochar sintering because of the rotation, which accommodated the bed and packed more the particles (De Conto et al. 2016; Narzari et al. 2017). Biochars with high EC may increase soil salinity when applied. This may provide undesirable impacts on plant growth. Negative impacts of high salinity on plant growth could stem from the low osmotic potential of the soil solution, which leads to water stress; specific ion effects, which result in salt stress (especially Na); to nutrient imbalances because of competition between ions for absorption (Al-Wabel et al. 2013). On the other hand, low EC indicates that the char matrix must be incorporated/ decomposed by soil microbiota or by chemical ways to release the inorganic content. These chars have a longer-term effect associated to recalcitrance and can be associated with other organic and inorganic fertilizers and liming agents for application (Aziz et al. 2015; Kuppusamy et al. 2016). Cation exchange capacity (CEC) evaluates the quantity of cations that can be adsorbed by a matrix. In general, mineral soils have CEC values lower than 15 cmolc kg−1, while humic substances may have CEC greater than 100 cmolc kg−1 25150 (Rakhsh et al. 2017). Soils with high CEC values are able to retain cationic nutrients near the root, thus preventing nutrient leaching and presenting potential to aid nutrient uptake by plants (Irfan et al. 2016; Srinivasan et al. 2015). CEC values observed in the biochars ranged from 53.48 cmolc kg−1 at 0 rpm to 61.83 cmolc kg−1 at 6 rpm. Rotary speed influenced the CEC of the biochars (which were higher than of the biomass, which presented 18.27 cmolc kg−1) and presented a trend of increase with increase of rotation. Biochars with CEC higher than the soil may also reduce groundwater contamination, minimizing the leaching of nutrients by chelation and enhancing their cycles and recycling in soil (Cely et al. 2015). Macronutrient (N, Ca, Mg, P, K, and S) concentrations in the char will be one of the parameters to calculate the quantity of biochar to be applied in soil for fertilizing purposes, considering that most or all biochar’s nutrients will be incorporated into the soil and, subsequently, become available to plant absorption. The quantity to be used depends on the cultivation demand and soil own fertility. In general, plants have a high demand for the N-P-K triad. Ca and Mg must also be available in high contents, especially during fructification, while S can be obtained occasionally from air. N in ammonia form is easily leached and volatilized, thus being lost. Nitrate is the major nitrogen specie absorbed by plants, being, in general, associated with the organic matter (OM) (Cely et al. 2015; Malavolta et al. 1997; Singh et al. 2010). Micronutrient (Cu, Zn, Mn, Fe, B, and Na) importance will depend on the cultivation. It is an important issue because of heavy metal presence, especially in sewage sludge, and some agricultural residues. Although literatures talk about some chelating effect of the biochar from the surface groups, soil pH, and redox potential also play an important role in heavy metal availability and fixation (Liang et al. 2017; Singh et al. 2010; Song and Guo 2012). However, biochars with high heavy metal content (both microelements and toxic metals) may not be used as soil amendment at risk of increasing the contamination of an already contaminated area (Reverchon et al. 2014; Yang et al. 2016). Regarding micronutrients, high contents can cause toxicity. Nonetheless, the range between production and toxic levels can be quite large, depending on soil chemistry, plant susceptibility, and char incorporation by the soil. Near acidic to neutral pH soils (6.0 to 7.0) make Al unavailable (which is toxic to most plants) and allow high macronutrient availability without micronutrient deficiency. At higher soil pH, micronutrients start to become insoluble because of hydroxide formation (in case of metals, especially heavy metals), and of chelation, which may provoke micronutrient deficiency. In general, grain crops have a low demand for micronutrients, in contrast to vegetable crops, which have higher demands for micronutrients, especially boron (Enders et al. 2012). Environ Sci Pollut Res (2018) 25:25143–25154 This effect of heavy metal unavailability by pH increase and chelation may be interesting for remediating contaminated areas, in particular industrial ones. The formation of insoluble species prevents the heavy metal ions from reaching the water tables by leaching and avoiding long-term contamination of agricultural areas (Kuppusamy et al. 2016; Meier et al. 2017; Randolph et al. 2017). The increase of the rotary speed did not change macro and micronutrient contents, except for P, Cl, Mn, and Na. P did not present a clear trend. Cl and Mn increased with increase of rotation speed and Na content decreased with increase of rotation speed. This increase of inorganics in the char in comparison to the unpyrolyzed FRSC biomass resulted from loss of volatile matter, as the inorganics remained in the solid fraction. For agronomic purposes, higher pyrolysis temperature is interesting, as the produced char will have higher contents of nutrients, and the rotary speed does not interfere with inorganics content. Problems with biochar produced at higher temperature are smaller nitrogen content, which may be complemented with chemical nitrogenous fertilizers or composts, and higher heavy metal content in cases where the biomass already presents these elements (biomagnification). In this case, in order to avoid high heavy metal contamination, different biochars (with different constitution and/or feedstocks) or composts may be mixed to provide liming effect, thus enhancing soil fertility and preventing heavy metal accumulation in soil (Singh et al. 2010; Zeng et al. 2015; Zheng et al. 2016). Biochar application and its effect in the soil After the incubation period (60 days), the five treatments, in addition to the positive (T0+) and the negative (T0−) blanks, were analyzed to evaluate the fertility parameters of each sample (Table 5). Nitrogen content presented a trend of increase with the increase of the quantity of biochar applied in the soil. However, only the biochar dose of 40 g L−1, in which N content was 20 g L−1, presented statistical difference when compared to both positive and negative controls. N content remained steady in both dosages of 15 and 20 g L −1 . Prapagdee and Tawinteung (2017), working with biochar of cassava stem, and evaluating soil parameters observed a similar trend. Randolph et al. (2017), evaluating agronomical capacities of biochars form diverse wastes, also reported increase in N content in soil that is correlated to biochar dosage. Taking the EC under consideration, the applied dosages that presented statistical difference were the positive control (limestone addition—68.11 dS m−1) and the biochar dosage of 40 g L−1 (from 31.95 dS m−1 in the non-treated soil to 45.70 dS m−1 at the biochar dosage of 40 g L−1). It is important to notice that the biochar addition did not increase considerably the EC, to avoid salinity problems, but as the Fertility parameters of the soil treated with biochar after 60 days of incubation period Parameter Unit1 Treatment* T0+ (15 g limestone L−1 soil) T0− (only soil) T1 (5 g char L−1 soil) T2 (10 g char L−1 soil) T3 (15 g char L−1 soil) T4 (20 g char L−1 soil) T5 (40 g char L−1 soil) Clay pH in H2O pH-SMP2 OM3 N P K Na S B Cu Zn Mn Ca Mg Al H + Al CEC pH 7 Electrical conductivity Water retention Bulk density Real density Porosity g L−1 410.0a – 6.23a – 6.25a g L−1 44.5d g L−1 5.77b mg L−1 1.28e mg L−1 82.00g mg L−1 49.00b mg L−1 32.43b mg L−1 0.70b −1 mg L 1.70b mg L−1 6.80d mg L−1 1.50d −1 cmolc L 7.48a cmolc L−1 7.43a cmolc L−1 0.03e cmolc L−1 3.48e cmolc L−1 18.58d dS m−1 68.11a g kg−1 506.8bcd kg L−1 0.92ab −1 kg L 3.29a % 71.52ab 387.5abc 4.25e 4.33c 45.0cd 6.21b 1.05e 91.00f 53.00ab 15.53d 0.70b 3.13a 13.65ab 19.00a 1.28c 0.60b 5.13a 29.88a 31.95a 29.97d 500.6cde 0.93a 3.67a 74.79a 367.5bc 4.20e 4.33c 47.0bc 6.05b 2.48d 167.00e 54.25ab 19.98cd 0.70b 2.85a 13.65ab 16.75b 1.20c 0.58b 5.00a 29.88a 32.05a 34.35c 480.7e 0.92ab 2.70c 65.67c 372.5bc 4.38de 4.40c 49.5a 6.80b 3.53c 233.00d 58.50a 20.23cd 0.70b 3.03a 14.98a 16.25b 1.28c 0.70b 4.50b 27.40b 29.98b 36.64c 491.3de 0.91ab 2.78bc 67.33bc 375.0abc 4.48cd 4.40c 47.3b 8.49b 5.45b 283.50c 55.75ab 26.08bc 0.73b 3.13a 14.65a 15.75b 1.30c 0.70b 4.38b 27.40b 30.13b 36.59c 526.1ab 0.90bc 2.75bc 67.37bc 392.5ab 4.60c 4.50c 44.5d 8.98b 6.08b 325.75b 55.50ab 26.08bc 0.68b 2.95a 11.05bc 16.75b 1.23c 0.83b 3.95c 24.40c 27.25c 37.45c 521.9abc 0.90b 2.79bc 67.75bc 352.5c 5.33b 5.20b 43.9d 20.61a 40.53a 736.25a 53.00ab 52.93a 1.33a 2.73a 10.28c 7.50c 1.88b 1.40b 0.98d 10.90d 17.00e 45.70b 542.7a 0.88c 2.99b 70.54b Environ Sci Pollut Res (2018) 25:25143–25154 Table 5 *Means in each row followed by the same letter do not show significant statistical difference when compared by Duncan’s multiple range test 1 Volume expressed in soil + incubated biochar (unless cited otherwise) 2 pH of the soil in presence of SMP buffer 3 Organic matter 25151 25152 biochars also presented low EC (105.6 dS m−1), there would be no important increase in soil salinity. Randolph et al. (2017) reported no significant increase in EC, even at higher biochar dosages, probably as a consequence of the recalcitrant effect of some biochars. Clay presented a slight trend of decrease with increase of biochar dosage. The highest clay content was in the positive control, probably as a result of limestone density. Na, Cu, and Zn contents did not show any major change in relation to the biochar content in the soil, only the positive control presented smaller contents, result of pH increase (Singh et al. 2010). Mn content presented a trend of decrease with increase of biochar application, probably from pH soil increase, presenting the same pattern of other heavy metals (Cu, Zn) (Liang et al. 2017). pH in H2O presented a trend of increase with increase of biochar dosage, with statistical differences among most of the dosages. pH-SMP also presented a weak trend, only the 40 g L−1 dosage presented statistical difference in relation to other treatments. Organic matter did not present a trend, with oscillations among treatments. Ca and Mg contents presented a small increase at char dosage of 40 g L−1 (however, considering only Mg, the treatments using biochar were not statistically different). P, K, and S contents increased with increase of biochar dosage, and all P and K treatments differed statistically from each other. The high P and K contents in biochar, in addition to its fast availability (probably because K and P are widely used in microorganism metabolism), may cause the increase in each dosage (Singh et al. 2010; Zhang et al. 2016). S also presented a trend of increase, but it was less sensitive, which may be because of sulfur metabolism in soil and biotransformation from sulfate to other S forms, thus delaying S availability (Fitzpatrick et al. 2017). Al and H + Al contents and CEC at pH 7 diminished with the increase of the amount of char applied, presenting a trend similar to the heavy metals. This is also a result of increasing in soil pH because of biochar liming effect. Meier et al. (2017) and Reverchon et al. (2014) also reported this trend and said that biochar application increased soil pH, tending to make available some macronutrients (Ca, Mg, K, P) as well. The functional groups in the biochar may chelate some heavy metals (Cu, Zn, Mn), and, along with pH increase, reduce their availability to plant absorption and leaching. Since FRSC biochars presented Cu contents ranging from 182 to 192 mg kg−1, chelating and liming effects of the biochar become important to prevent heavy metal contamination. Randolph et al. (2017) also observed similar increase trends with P, K, and pH, thus stating the potential effect of biochar application to improve soil fertility. Shepherd et al. (2017) talked about possible toxicity of high nutrient contents and heavy metals, as well as observed a trend of increase of pH, and P and K contents with increased biochar dosage. Yang et al. (2016) and Zeng et al. (2015) commented about the influence of biochar in the bioavailability of heavy metals (Cu, Zn, Mn, Fe) and their Environ Sci Pollut Res (2018) 25:25143–25154 stabilization because of the biochar alkaline properties (unavailability of heavy metals as a result of hydroxide formation when solution pH is higher than 5.8). While using the 40 g L−1 treatment of char, there was an important increase in pH and pH-SMP levels, along with the reduction of Al and H + Al levels. The increase in P, K, S, and B contents at 40 g L−1 was also remarkable. In addition to P and K, at 40 g L−1 of biochar, there was a leap in the concentration of S and Mg. There was also an increase in Ca concentration. The expressive increase in macronutrient concentration at 40 g L−1, along with a decrease of Al and H + Al made the biochar applied in soil at the biochar dosage of 40 g L−1 an interesting alternative fertilizer, capable of modifying the nutritional state of a soil with poor availability of nutrients for plant growth. In relation to the physical properties, the application of biochar caused a decrease in the real density, from 3.67 kg L−1 in the soil without char to 2.70 kg L−1 in the soil with char. Char application showed influence among the treatments and presented statistical difference between both positive and negative control to the biochar-treated soils. T1 to T5 presented similar results. Bulk density presented a trend of decrease with increase in biochar content applied. Soil porosity did not present a clear trend among treatments; nevertheless, biochar application reduced the porosity when compared to the soil without char addition (both controls). Water retention capacity presented statistical difference among treatments, and a trend of increase with higher biochar dosages, thus presenting its maximum value at 40 g L−1 (542.7 g water (kg soil)−1). This may be the result of the biochar intrinsic porosity, which is capable to retain water, reducing soil drying. Randolph et al. (2017) talked about the reduction of bulk density with the increase in biochar dosage, although real density was not evaluated. The authors observed the same trend of increasing with water retention capacity and commented on the role of biochar porosity in water holding. The increase in nutrient contents, along with the improvement of some physical characteristics at higher dosages, without important increase in the EC and heavy metal content, make the fodder radish seed cake biochar suitable for application in soil and an interesting potential auxiliary fertilizer, which is possible to combine with other composts to enhance its potential on soil. Conclusions The modification of the rotary speed influenced significantly biochar yield (from 26 wt.% at 0 rpm to 24.89 wt.% at 3 rpm), and some physicochemical properties of the biochars (volatile matter, ash, P, Cl, Mn and Na contents; EC, pH and CEC values). From an agronomic view, biochar addition helped to correct soil acidity, lowering Al and H + Al contents. The Environ Sci Pollut Res (2018) 25:25143–25154 obtained biochars concentrated the inorganic fraction present in the biomass. Nitrogen and sulfur contents, as a result from volatilization, were smaller than of the unpyrolyzed biomass. FRSC biochar application in soil increased the contents of N, P, K, S, B, Ca, and Mg and decreased Cu, Zn, and Mn contents. Therefore, FRSC biochar may be used as an auxiliary fertilizer. 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