Prospect of managing factory tea waste as biochar

Introduction

Biochar is a fine-grained, carbon-rich, porous material left after thermochemical conversion process (pyrolysis) of plant biomass at temperatures about 350-600 °C in a condition with little or no oxygen (Amonette and Joseph, 2009) [2] . Biochar is not a pure carbon (C), but it is a mixture of ash, Hydrogen (H), Nitrogen (N), Sulphur (S), Phosphorus (P), Potassium (K), etc. (Masek, 2009) [26] . Now-a-days, biochar has gained popularity due to its potential to sequester carbon and enhance soil quality, as well as for land reclamation, water remediation, and soil amendment (Barrow, 2012; Meyer et al., 2011) [5,28] . There are different methods to produce biochar, but all of them involve heating the biomass with little or no oxygen to run off volatile gasses and leave carbon behind. This simple method is called thermal decomposition, typically achieved from pyrolysis or gasification. Biochar's characteristics are variable depending upon the composition of biomass used for its preparation and on the conditions of pyrolysis (Chowdhury et al., 2016) [8] . Some workers have reported about many key properties for the evaluation of biochar i.e. pH, content of volatile compounds, ash content, water-holding capacity, bulk density, pore volume, and specific surface area. All these properties mainly depend upon the quality of the feed stock that is used for the biochar production. In addition, pyrolysis temperature and duration of pyrolysis are the other two most considerable factors that affect the physico-chemical quality of biochar (Okimori et al., 2003; Sohi et al., 2010) [30,35] . The wastes obtained during the processing of tea are regarded as tea waste which includes unwanted tea leaves, buds, stems and the factory waste (Chowdhury et al., 2016) [8] . The minimum volume of tea waste and made tea should be at the ratio of 2:100 (Tea Waste [8] . Though, Assam is the largest producer of tea in India, there is no report of proper management of factory tea waste. Currently, tea waste is disposed off through inefficient incineration, which leads to environment pollution. So, these wastes are suitable for assessment as biochar (Akgul et al., 2018) [1] . At present, tea waste is sometimes used for the extraction of caffeine. In Assam, tea wastes are sometimes used as poultry and piggery feed (Chowdhury et al., 2016) [8] . Thus, there is a scope to study the potential of tea waste for production of biochar which has a significant importance for opening a new area of research for alternative use of tea waste. Therefore, the present study was conducted to explore the prospect of managing factory tea waste from a selected Tea Garden of Assam as Biochar.

Materials and Methods

The factory tea waste was collected from Cinamara Tea Estate, Jorhat, Assam. The collected factory tea waste was sun dried till the moisture content came down to less than 10%, fresh basis. In this present study, Biochar was produced by two methods-(a) method 1-using charring device and (b) method 2-using muffle furnace. Under the Method-1, Biochar production unit (charring device) available at the Department of Agricultural Engineering, Assam Agricultural University, Jorhat was used for production of biochar. The kiln was filled with sun dried tea waste, covered with a lid and pyrolyzed at 300-350 °C under limited oxygen condition. After pyrolysis, the biochar was allowed to cool in the kiln overnight at room temperature. The next day, the biochar was collected, sundried and weighed. Under the Method-2, Muffle furnace, available at the Department of Biochemistry and Agricultural Chemistry, Assam Agricultural University, Jorhat was used to make biochar. The sundried biomass was taken in a preweighed silica crucible and pyrolyzed at 350 °C for 45 minutes. The crucible containing the biochar was cooled to room temperature by placing these in a desiccator and then weighed. The average of three replications was calculated. Moisture, ash and nitrogen content were determined by using method of AOAC (1980) [1] . Total potassium in the mineral solution was determined by using a double beam atomic absorption spectrophotometer (Koeing and Johnson, 1942) [22] . Total P was determined by vanadomolybdate method (ISO, 2017) [17] . Bulk density of the sample was determined by core method or volumetric cylinder method (Baruah and Borthakur, 1997) [6] . The maximum water holding capacity of the sample (biochar and factory tea waste) was determined using Keen Rackzowski box following the method described by Baruah and Borthakur (1997) [6] and expressed in percentage. The pH of the dry biochar and factory tea waste were determined with a glass electrode pH meter, in a 1:2 sample water suspension as described by Jackson (1973) [18] . Cation exchange capacity of biochar was measured by leaching it with neutral normal ammonium acetate solution. The titrimetric determination, also known as the "wet-digestion method" involves a rapid titration procedure for the estimation of organic carbon content of dried sample (biochar and tea waste) (Walkley and Black 1934). Crude fibre was determined by the method given by Maynard (1970) [27] . Each estimation was replicated thrice. The data obtained from laboratory analysis were analyzed statistically by Completely Randomized Design (CRD). Average of 3 replications was taken for oneway analysis of variance (ANOVA).

Results and Discussion

Yield of biochar

The yield of biochar was recorded to be 71.07% (charring device) and 77.21%, (muffle furnace), respectively (Table 1). Loss of more carbon in charring device than muffle furnace might attribute to the observed lower yield of biochar. The yield of biochar depends on various factors like technique chosen for production, type of biomass and also the process conditions such as heating rate, temperature, residence time, etc. In the slow pyrolysis, rate of heating is very low with a longer residence time. Hence, the slow pyrolysis has a better yield of char relative to different pyrolysis and carbonization strategies (Yaashika et al., 2020, Daful and Chandraratne 2018) [41,9] . According to Akgul et al. (2018) [1] yield of biochar decreased as the pyrolysis temperature increased. Demirbas (2004) [11] stated that the high lignin composition of biomass resulted in higher char formation. The present findings were found to be comparable with Akgul et al. (2017) [43] . They prepared biochar from industrial tea waste using muffle furnace at temperatures 300 °C and 400 °C and yield was reported to be 60-64%.

Physical properties

The moisture content of tea waste biochar and factory tea waste were 6.10% (method 1-charring device), 3.34% (method 2-muffle furnace) and 6.98%, respectively (Table 2). The moisture content of tea waste biomass were found to be comparable with those of Nagaraja et al. (2013) [29] , Rajapaksha et al. (2014) [33] , Akgul et al. (2018) [1] , who reported that moisture content (FW basis) of tea waste were 7.26%, 10.86% and 7.20%, respectively. However, in the present study, the moisture content of biochar from tea waste biomass was found to be higher than those reported by Rajapaksha et al. (2014) [33] , Akgul et al. (2018) [1] , Peiris et al. (2019) [31] , who reported that moisture content of biochar was 2.80% (at temperature 300 °C), 1.5% (at 500 °C) and 6.33% (at 300 °C), respectively. The differences in moisture content of biochar might be due to differences in bulk density of the biochar. Biochar with low bulk density, an indication of highly porous structure, has the potential to hold moisture (Hernandez-Mena et al., 2014) [16] . The bulk density of biochar and factory tea waste (Table 2) were 0.19 g/cm 3 (method 1-charring device), 0.21 g/cm 3 (method 2-muffle furnace) and 0.09 g/cm 3 , respectively (Table 2). Deka et al. (2018) [10] reported that bulk density of biochar produced from rice husk, rice straw, toria stover and bamboo were 0.629 g/cm 3 , 0.729 g/cm 3 , 0.641 g/cm 3 and 0.178 g/cm 3 , respectively. Ghorbani et al. (2019) [14] reported about bulk density of biochar from rice husk at 500 °C to be 0.84 g/cm 3 . However, there is no report of bulk density of biochar produced from factory tea waste. Studies have shown that the properties of biochar were greatly dependent on the production procedure and type of raw material used (Lehmann and Joseph, 2009; Liu and Balasubramanian, 2013) [29,25] .

In the present study, the water holding capacity for biochar and factory tea waste (Table 2) were 80.17% (method 1charring device,), 84.67% (method 2-muffle furnace) and 63.33%, respectively (Table 2). Though, there are reports of changes in water holding capacity of soil after application of biochar, there were hardly any report on water holding capacity of biochar. Liao and Thomas, 2019, reported that water retention capacity of biochar was influenced by processing method of biochar. They observed water retention capacity of biochar to be increased with reduction in particle size; from 47% to 360% in comparison to sand (37%). Sieved biochar and conventional biochar had higher water retention capacity than ground biochar and pelletized biochar of similar particle sizes, respectively.

Chemical properties pH

In the present study, the pH observed for biochar and the factory tea waste were 7.88 (method 1-charring device), 7.90 (method 2-muffle furnace) and 5.2 (tea waste), respectively (Table 3). Rajapaksha et al. (2014) [33] and Yasar et al. (2017) [43] , stated that the pH of tea waste biomass was 5.14 and 5.09, respectively. Rajapaksha et al.

2014 [33] , stated that pH of tea waste biochar produced at 300 °C and 700 °C were 7.93 and 11.05, respectively. Yasar et al. (2017) [43] reported that the pH of biochar produced from tea waste biomass at temperatures 300 °C and 400 °C were 9.15 and 9.28, respectively. Pyrolysis temperature had a strong influence on pH of biochar, higher temperature resulted in an increase of pH of biochar (Tomczyk et al., 2020) [38] . Higher pH with increasing temperature has been associated with the increase in ash content and oxygen functional group that occur during pyrolysis (Ghorbani et al., 2019) [14] .

The ash content of biochar was 6.20% (method 1-charring device), 6.37 % (method 2-muffle furnace) and 4.23 % (tea waste biomass), respectively. The present findings for ash content (%) of tea waste was found to be comparable with Rajapaksha et al. (2014) [33] and Akgul et al. (2018) [1] which were 3.22% and 3.74%, respectively (Table 3). However, Rajapaksha et al. (2014) [33] stated that the ash content (%) of tea waste biochar produced at 300 °C and 700 °C were 5.69% and 10.87%, respectively. Peiris et al. (2019) [31] , reported that ash content (%) of biochar produced from tea waste at temperatures 300 °C, 500 °C, 700 °C were 6.15%, 11.40% and 9.26%, respectively. Biochar pyrolyzed at low temperature contains low ash content (Rafiq et al., 2016) [32] . Types of feedstocks also affect the ash content of biochar (Tomczyk et al., 2020) [38] . Studies have also shown that there was an increase in the ash content increase in pyrolysis temperature from 250 to 500 °C for 3 h (Borgohain et al., 2020) [7] . However, the significant differences in ash content of biochar prepared by two methods, as observed in the present study might involve the differences in device used for pyrolysis leading to differences in pyrolysis temperature, retention time and availability of oxygen during pyrolysis.

In the present study, the carbon content (%) of biochar and tea waste biomass were 50.9% (method 1-charring device), 60.24% (method 2-muffle furnace) and 65.11% (tea waste biomass), respectively (Table 3). Yasar et al. (2017) [43] reported that carbon content of biochar produced from tea waste at 300 °C and 400 °C were 60.5% and 66.4%, respectively. Peiris et al. (2019) [31] , stated that carbon content of tea waste biochar produced at 300 °C, 500 °C and 700 °C were 57.80%, 69.66% and 71.03%, respectively. Increased carbon content with an increase temperature occurs due to high degree of polymerization (Domingues et al., 2017) [12] leading to a more condensed carbon structure in the biochar (Lehmann and Joseph, 2009) [29] . The observed lower values of carbon content of biochar than the biomass (tea waste) might indicate loss of carbon during pyrolysis in both the methods, being higher in charring device than that in muffle furnace.

In the present study, the nitrogen content of biochar and tea waste were 3.94% (method 1-charring device), 4.58% (method 2-muffle furnace) and 2.57% (tea waste), respectively (Table 3). The present findings for nitrogen content of tea waste found to be comparable with Rajapaksha et al. (2014) [33] , Peiris et al. (2019) [31] and Yasar et al. (2017) [43] . Akgul et al. (2017) [43] reported that nitrogen content of biochar produced from tea waste biomass at 300 °C and 400 °C were 3.4% and 3.6%, respectively. And Peiris et al. (2019) [31] which also stated that nitrogen content of tea waste biochar produced at 300 °C, 500 °C and 700 °C were 3.66%, 2.55% and 3.12%, respectively. The higher nitrogen content in biochar produced by using muffle furnace might indicate higher retention of nitrogen

In the present study, the data of the phosphorus content of both the biochar and tea waste were 0.0377% (method 1charring device), 0.0441% (method 2-muffle furnace) and 0.021% (tea waste), respectively (Table 3). The present study of phosphorus content of biochar found to be comparable with Deka et al. (2018) [10] , who reported that phosphorus content of biochar produced from rice husk, rice straw, toria stover and bamboo were 0.0237%, 0.0185%, 0.0324% and 0.0173%, respectively. However, Gondim et al. (2018) [15] and Severo et al. (2020) [34] also reported that phosphorus content of biochar prepared from cashew and rice husk were 1.10g/kg (0.11%) and 1.2 g/kg (0.12%), respectively. The nature of biomass and pyrolysis temperature influenced the variation in phosphorus content in biochar (Kamara et al., 2015) [20] .

In the present study, the potassium content of biochar and tea waste were 0.071% (method 1-charring kiln), 0.073% (method 2-muffle furnace) and 0.067% (tea waste), respectively (Table 3) which were lower than those reported by Deka et al. (2018) [10] , Gondim et al. (2018) [15] and Severo et al. (2020) [34] . The observed variation in potassium content of biochar might be due to differences in potassium content of raw biomass. The total mineral content in biochar were found to be higher than the biomass (Judd, 2016) [19] , which might be due to the process of making biochar. Biochar production reduced the product weight and thus the mineral element proportion became higher (Evans et al., 2017) [13] . Kloss et al. (2012) [21] , reported that the level of K increase with greater carbonization. As reported by Borgohain et al. (2020) [7] total P and K, increased consistently with increasing pyrolysis temperature

The cation exchange capacity of biochar and factory tea waste were 11.87 cmole/kg (method 1-charring kiln), 15.77 cmole/kg (method 2-muffle furnace) and 5.58 cmole/kg (tea waste), respectively (Table 3). In the present study, the cation exchange capacity of biochar was found to be comparable with Ghorbani et al. (2019) [14] , Venegas et al. (2015) [39] and Zama et al. (2017) [44] who analysed tree bark biochar, buckwheat husk biochar and rice husk biochar, respectively. Deka et al. (2018) [10] also reported that cation exchange capacity of biochar produced from rice husk, rice straw, toria stover and bamboo to be 12.74 cmole (p+)/kg,15.67 cmole (p+)/kg, 14.72 cmole (p+)/kg and 16.68 cmole (p+)/kg, respectively. Biomass with a high content of ash can produce biochar with a higher cation exchange capacity. It might be due to alkali and alkali metals in biomass which promote the formation of oxygencontaining surface functional groups (Yang et al., 2015; Tag et al., 2016) [42,36] .

The crude fibre content of biochar and factory tea waste were 1.13% (method 1-charring kiln), 1.63% (method 2muffle furnace) and 22% (tea waste), respectively (Table 3).

In the present study, crude fibre content was found to be comparable with Angga et al. (2018) [3] , who reported the same in tea waste to be 17.405% (dry basis). However, presence of crude fibre in biochar might indicate need for increasing pyrolysis time to complete the combustion.

Conclusion

The yield of biochar, for method 1(using charring device) and method 2 (using muffle furnace) were recorded to be 71.07% and 77.21%, respectively. Considering the physical and chemical properties of biochar, the biochar produced by muffle furnace was found to be better as higher bulk density and water holding capacity were observed in addition to higher values for ash content, carbon content, mineral content (nitrogen, potassium, phosphorus) and cation exchange capacity. Thus, factory tea waste can be profitably converted to biochar using furnace and thereby contributing to carbon sequestration. Furthermore, the biochar produced from tea waste can be effectively used to improve soil health.

Add-on information

Authorship contribution: N. Goswami conducted the field as well as laboratory experiment and based on the final results had drafted the research manuscript. Dr. P. Das and Dr. G. K. Saikia outlined the overall research theme and standardized the methodology for successfully performing the experiment. Dr. T. Nath and Dr. K. Sarmah was actively involved in designing the final version of the said manuscript.

Conflict of interest:

The authors declare that there is no conflict of interest.

Funding:

No funding has been received to conduct the study Ethical Approval: Not applicable

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