Destination of urban pruning organic waste by means of biochar applied to the soil : Analysis of the compactation curve

Urban pruning waste can have multiple uses from direct burning wood as energy to its use as organic fertilizers in the composting process. However, the lack of suitable places for the disposal of this type Brazilian Journal of Development Braz. J. of Develop., Curitiba, v. 6, n. 11, p. 92665-92679, nov. 2020. ISSN 2525-8761 92666 of waste is a problem for municipalities with large urbanization. Thus, “biochar” presents itself as an alternative proposal for the disposal of urban pruning organic waste. In this context, the present work aimed to verify the behavior of soil compaction curves through the application of biochar as a form of conditioning and waste disposal. Soil collection was carried out at the Institute of Science and Technology of Sorocaba (ICTS –UNESP Sorocaba) followed by drying and sieving for soil characterization through particle size analysis. The tests were performed with application of biochar in the soil using the Proctor Normal Compaction Test (NBR 7182/1986) at 35 t/ha and 70 t/ha in different textures. The data of the compaction curves and comparative statistical analysis presented satisfactory results. The results expose a maximum specific weight reduction and optimum humidity increase in all samples with application of biochar. Thus, the proposal for the redestination of urban pruning organic waste by means of biochar applied to the soil can be considered an alternative for urban management of pruning organic waste and may help in integrated solid waste management.

of waste is a problem for municipalities with large urbanization. Thus, "biochar" presents itself as an alternative proposal for the disposal of urban pruning organic waste. In this context, the present work aimed to verify the behavior of soil compaction curves through the application of biochar as a form of conditioning and waste disposal. Soil collection was carried out at the Institute of Science and Technology of Sorocaba (ICTS -UNESP Sorocaba) followed by drying and sieving for soil characterization through particle size analysis. The tests were performed with application of biochar in the soil using the Proctor Normal Compaction Test (NBR 7182/1986) at 35 t/ha and 70 t/ha in different textures. The data of the compaction curves and comparative statistical analysis presented satisfactory results. The results expose a maximum specific weight reduction and optimum humidity increase in all samples with application of biochar. Thus, the proposal for the redestination of urban pruning organic waste by means of biochar applied to the soil can be considered an alternative for urban management of pruning organic waste and may help in integrated solid waste management.

INTRODUCTION
In the same trend of urban population growth, there is also an increase in the volume of solid waste generated in cities. The problem related to solid waste is part of an economic, political, social and ecological, whose complexity involves a wide network of interrelations that demand comprehensive approaches on various aspects MEDEIROS et al, 2020). In this context, urban pruning waste, even with multiple uses ranging from burning as firewood for power generation to use as organic fertilizers, presents a major management and disposal challenge within the municipalities. In this scenario, it highlights the volume of material and the lack of adequate places for the disposal of this type of waste.
Considering these factors, there is a shortage of efficient models for the management of pruning residues from urban afforestation and also the high cost of some techniques for their treatment, resulting in numerous environmental problems, due to the inadequate disposal of these materials (CHALUPE, 2016). According to NBR 10. 004/2004004/ (ABNT, 2004, pruning waste is classified as solid class II waste, that is, non-hazardous. If this type of waste is disposed of in open places, such as landfills, it can cause a series of problems, because in addition to reducing the capacity of the landfill, it can mix with other pre-existing waste, causing environmental impacts, such as increased risk fire in landfills and vacant lots, landscape degradation and air and water pollution. In this way, the search for alternatives for the use and redestination of organic waste, especially tree pruning, has become a challenge for sustainable urbanization (ANDREOLI et al, 2014).
In this context, the placement of biochar in soil conditioning can have good results in the environmental management of organic waste in cities. Studies carried out in the last decade prove the capacity of "black earth" or "biochar" to accumulate organic matter, allowing for increased fertility and carbon storage in tropical soils. Among the benefits of applying charcoal as a soil conditioner, in addition to high fertility, are high content of stable carbon in its organic fraction and increased biological activity, contributing to the formation and growth promotion characteristics of these soils (SCHOENHOLTZ et al, 2014;KIM et al, 2019).
Currently, the intensification of the urbanization process can be considered one of the major inducers of environmental problems due to the use and occupation of land, leading to the removal of vegetation and soil degradation (KIM et al, 2017, STEFANOSK et al, 2013STANGANINI and LOLLO, 2018). In that context, knowledge of the dynamics of land use and land cover is increasingly important for understanding space, planning and management of natural resources, which contributes to the conservation and environmental sustainability (Maidjelele et al, 2020).Thus, the biochar incorporated into the soil presents itself as an alternative management practice with great potential for soil improvement, contributing to soil recovery and conservation (TIECHER, 2016;GUILHEN, 2018;Kumar et al, 2019).
The term "biochar" results from "biomass + charcoal" (biomass + coal). This idea arose from the observation of the existence of our "black land of the Indians" in soils of the Brazilian Amazon.
The "biochar" is produced basically by burning organic matter in environments with little or no oxygen.
The process is known as pyrolysis or gasification, the same process that is used to create wood charcoal (Kumar et al, 2019;MANGRICH et al, 2011). The main function of Biochar is based on the following benefits: (1) soil improvement, (2) carbon sequestration, (3) carbon neutral energy production, (4) waste management and (5) water quality protection through greater efficiency in the use of nutrients (BRICK and MADISON, 2010).
Although several studies deal with changes in soil properties resulting from the use of biochar, few assess the effects of applying biochar to the soil's response to compaction and its potential for use in sustainable management (VERHEIJEN et al, 2010). In this scope, the present work aimed to verify the behavior of soil compaction curves through the application of biochar, as a way of conditioning and disposal of waste.

MATERIAL AND METHODS
The soil used in the present study was collected at the Campus of UNESP Sorocaba/SP.
Approximately 40 kg of sub-surface soil was removed, disregarding the organic horizon, as organic matter can influence the results of specific weight. The soil remained on drying for 15 days before test.
Then, the soil was sieved through a 4.75 mm sieve (ASTM 4 / TYLER 4) and soil characterization through granulometric analysis.
The biochar used in the tests was eucalyptus charcoal (Eucalyptus grandis, first the charcoal was broken until it reached a finer texture. Then the charcoal grains were separated into four different diameters, using 2 mm sieves (ASTM 10), 1.18 mm (ASTM 16), 600m (ASTM 30), 300m (ASMT 50), 150m (ASTM 100) and 75m (ASTM 200). The biochar plots with a diameter between 75m e 600m "fine texture", and between 600m and 2 mm "thick texture." As a descriptive element, the letter Ø was used as a symbol of the diameter retained in the sieving, for example: 75 µm <Ø <600 µm.
The rates of application of biochar in the soil were 35 t/ha and 70 t/ha. In this way, it was sought through these dosages to identify the contrast between applications, that is, quantities [tons] and particle sizes [size of coal particles]. The quantity and texture were adjusted according to the surface area of the 0.79 cm 2 cylinder and its depth 12.75 cm with the Proctor Normal Compaction Test cylinder following NBR 7182/1986(ABNT, 1986 and the calculation of the proportion of Biochar applied to the soil in each test he followed the equation below: Where: Mbio = Biochar mass to be applied (g); Mbiocil = Biochar mass for the cylinder surface (g); Msoil = Mass of soil (g); M1000cm³ = Mass of dry and sieved soil in the 1000 cm³ (1288g).

STATISTICAL ANALYSIS OF THE DATA
The statistical analysis of the data was performed with the aid of the ASSISTAT Version 7.6 beta software (2012) in a completely randomized design (DIC). Therefore, the analysis of variance (ANOVA) was carried out, ending with the application of an average test (Tukey, 5%). The variables analyzed were maximum specific weight and optimum humidity for each trial, treatments were (S, SBt, SBT, Sbt, SbT), and 4 repetitions were performed.
With the values of humidity and the specific weight of the dry soil, the graphs were constructed referring to the compaction curves for each test. Figure 1 represents the compaction curves for Tests 1 to 4, referring to the soil without Biochar (Fig. 1). The Compaction Curves of the same soil tests were similar to each other as expected due to the control of the test. The greatest variation between maximum specific weight was 0.033 g/cm³ between tests 1 and 2. The optimum humidity content also had little variation in tests 1 to 4, with 0.58% being the difference between tests (2 and 4) with greater divergence of optimum humidity. Figure 2 presents the graph referring to tests 5,6,7 and 8 represent the soil incorporated with thick biochar at the application rate of 35 t/ha, the curves shown show the maximum specific weights and the optimum humidity content for the same samples. The compaction curves for the soil incorporated with coarse biochar at the rate of 35 t/ha also showed few variations from one test to another, except for test 7, which showed a certain specific weight reduction in the first humidity content. However, the biggest difference between the maximum values of specific weight was 0.031 g/cm³ obtained in tests 6 and 7. The biggest difference between the values of optimum humidity was 2.09%. Figure 3 shows the compaction curves for Proctor Normal Tests 9 to 12, performed with coarse biochar in the amount of 70 t/ha. The compaction curves obtained from the tests with soil and coarse Biocarbon in greater quantity converge more for the wet branch (after reaching the maximum specific weight) than for the dry branch, even though the greatest difference in maximum dry specific weight was 0.026 g/cm between tests 10 and 11. The greatest variation in optimum humidity between tests 10 and 11 was 1.44%.  Figure 4 shows the compaction curves for Tests 13 to 16, performed with fine biochar in the amount of 35 t/ha. For the soil with thinner Biochar at the rate of 35 t/ha, the maximum variation in dry specific weight was even smaller than the previous ones, showing a difference of 0.020 g/cm³ between tests 13 and 15. The biggest difference between the variation of optimum humidity was 1.55%.  In addition to the reduction in specific weight and increase in optimum humidity in the soil with the application of biochar, there is also a lower slope in the wet branch (right side) of the curves of the soil with biochar, demonstrating a smaller variation in specific weight with the variation of moisture.
This property may be associated with a lower susceptibility of these soils to compaction according to the increase in humidity. The retention of water by fine clay particles showed similar characteristics described by VERHEIJEN et al. 201).
To understand the variations of the proposed treatments, a univariate statistic was performed with the assistance of the ASSISTAT software version 7.6 betas (2011) in order to identify the differences between the means in the interpretation of the data obtained from optimum humidity and maximum specific weight in the 5 treatments with 4 repetitions each. The comparison of the averages selected in the program was based on the Tukey test.
The results generated by the program for the statistical analysis of the optimum moisture content in different samples are shown in Table 3. The results generated by the program for the statistical analysis of the maximum specific weight in different samples are shown below in table 4. 0.04632 * significant at the level of 5% probability (.01 = <p <.05) ** significant at the level of 1% probability (p <.01) ns not significant (p> = .05) Comparative analysis for the verification between changes in soil compaction in relation to the application of different amounts and textures of biochar can be seen in figures 6 and 7 that elucidate bar graphs for the treatments. Differences can be observed between the average optimum humidity [%] and also in the average maximum specific weight [g/cm3] for the different treatments. Thus, according to table 6, a Tukey test was applied at a significance level of 5%. Considering the average moisture content, a significant increase in optimum moisture was observed in almost all applications of biochar to the soil. Sample 5 of Soil + fine biochar 70 t/ha was the one that most increased the optimum humidity. This means greater water retention in the soil made possible by coal fines, which for the same amount of water in a soil without biochar and another with biochar, the former will be more likely to reach greater densities when it undergoes some application of energy. Table 6 shows that the means followed by the same letter do not differ statistically from each other by means of the Tukey test at the 5% significance level.  The averages between the maximum specific weights of the different soil treatments with Biochar were reduced considerably when compared to the soil without Biochar, mainly for the applications of Biochar of 70 t/ha. The "SBT" sample (Soil+Biochar coarse 70 t/ha) was the one that most reduced the maximum specific weight of the soil-Biocarbon system, promoting a mass reduction greater than 6%.
Although the specific weight of the Biochar is much less than that of the soil, it can increase the specific weight when incorporated, since the maximum compaction capacity depends on the particle size of the system. As important as disposal and appropriate treatments, it is necessary to produce less and less waste and reuse more and more waste generated, reducing the high rate of waste, thus contributing to a more balanced and responsible society .The explanation for the thicker Biocarbon grains being better in reducing specific weight may be in the arrangement between the Biocarbon coarse particles with the soil particles and the friction between them. In a study on erodibility and compaction (GUILHEN, 2014), observed similar effects in analysis and highlighted that the addition of biochar to the soil resulted in decreased erosion throughout the dry state, while the opposite was observed in the wet state.

CONCLUSION
The results represented by the compaction curves and the statistical analysis of the data show a reduction in specific weight in all soil samples with incorporation of biochar, with the application of thick biochar (600 µm <Ø <2 mm) in the concentration of 70 t/ha more efficient in reducing soil compaction under the same energy as the rest of the tests.
It is also noted that the optimum moisture content, for the maximum specific weight, increases in all additions of biochar in the soil, and samples containing fine biochar (75 µm <Ø <600 µm) reached optimum humidity levels, more higher than other tests. This represents greater water retention in soils with biochar fines, which can be very interesting in very permeable soils. In addition, the increase in optimum humidity content can minimize the risk of maximum compaction in agricultural soil preparation in very rainy regions.
Thus, biochar has the potential to reduce soil compaction and increase water retention. These properties allow the biochar to be used for the disposal of urban organic waste and soil conditioning.
As an indication for the production of future works, it is suggested that tests be carried out in the field, based on biochar from different plant residues and applications on larger scales, as the proposal for the destination of organic urban residues originating from pruning applied to the soil can assist in urban waste management in an integrated manner and with more sustainability.

ACKNOWLEDGMENT
We thank the Laboratory for Research in Land Management and Recovery at the Institute of Science and Technology -ICTS / UNESP for the help and technical support, and the research group for their support in writing and editing the work.