Use of Carbonized Fallen Leaves of Jatropha Curcas L. as a Soil Conditioner for Acidic and Undernourished Soil

Round 1

Reviewer 1 Report

The paper is well organized and results are clearly exposed and discussed. Conclusions are supported by results.

Authors must clarify if soils have been analyzed 44 days after plant transplantation (line 131) or 45 days after transplantation((line 253)

Author Response

The correct duration was “44 days”. We apologize for this simple mistake for writing. The value was revised to “44 days” (line 254 in the revised manuscript).

Author Response File: Author Response.pdf

Reviewer 2 Report

Authors present an interesting paper about the production of fertilizer/soil conditioner from Jatropha curcas fallen leaves. I am not pretty sure whether the use of the term "biochar" throughout this paper is correct since biochar refers to the carbonaceous residue obtained after pyrolysis, and the process described in this paper is more likely a combustion. In spite of the well presented data, the good English style and the pertinent analytical methods used, this paper has a weak point, which is the homemade "reactor" used for the combustion. It is true that it is very cheap and the purpose of this paper is that it may be used in poor areas of developing countries, where there is not budget to buy a real reactor, but from the oil drum used as reactor can arise some problems, such as the leaching of metals from the drum to leaves during the process.

 

On the other hand, the introduction is a little short (also the conclusions), and in this section as well as in the conclusions section, some important applications for Jatropha curcas biomass are missing. For example, the pyrolysis of Jatropha curcas leads to the production of a biochar that has catalytic properties:

García Martín J.F., Alés-Álvarez F.J., Torres García M., Feng C., Álvarez Mateos P. 2019. Production of oxygenated fuel additives from residual glycerine using biocatalysts obtained from heavy-metal-contaminated Jatropha curcas L. roots. Energies, 12(4):740.

Álvarez Mateos P., Alés Álvarez F.J., García Martín J.F. 2019. Phytoremediation of highly contaminated mining soils by Jatropha curcas L. and production of catalytic carbons from the generated biomass. Journal of Environmental Management, 231, 886-895.

Fig. 9 of this latter paper could be useful to discuss the results obtained in the present paper.

 

Other minor points:

 

Lines 48-49: “jatropha trees produce 48 550 kg of fallen leaves per plant at 1 year old”. This must be a mistake.

Lines 84-85: “The fallen jatropha leaves were tightly packed into the drum up to the height of the upper rim by stamping them down with the body weight of an adult”. How many kg (average weight) were introduced?

Lines 93-95: “After 48 h, the carbonization was stopped by opening the lid and immediately applying approximately 10 L of water to the top of the pile”. If this is performed manually, it seems to be very dangerous.

Line 127. Change “second true leaves had emerged” by “second true leaves emerged”.

Line 151. “The soils were then poured back into the original pots and 500 mL of water was added”. Indicate the mass of soil introduced (1 kg based on the information given in line 280).

Tables 1 and 2. Why the concentration of Al in fallen leaf "biochar" and in the experimental oil is so high? In the case of fallen leaves "biochar", is Al coming from the "reactor"?

Table 1 and Fig. 6 caption. Delete (Jatropa curcas) after the word jatropha.

Lines 205-207. Authors are comparing biochars obtained from pyrolysis processes with the residue obtained after the proposed carbonization scheme, which authors consider as biochar.

Lines 247-248. Change “application of jatropha fallen leaf biochar to the acidic experimental soil improved the growth 247 performance of Swiss chard, particularly in the aerial parts of the plants.” by “application of jatropha fallen leaf biochar to the acidic experimental soil improved the growth performance of stalk and leaves of Swiss chard.”. As aforementioned, I have doubts about the pertinence of using the term "biochar".

Captions for figures 2, 5 and 7 are too long. They provide information already given into the text, which is not needed to be repeated.

Author Response

Comments on the manuscript:

“I am not pretty sure whether the use of the term “biochar” throughout this paper is correct since biochar refers to the carbonaceous residue obtained after pyrolysis, and the process described in this paper is more likely a combustion.”

 

Response to the Comments:

As the Reviewer pointed out, this paper included a process of “combustion” at the beginning of heating process (1.5 hr), which was followed by a 46.5-hr oxygen-starved heating condition. This scheme has been known as the auto-thermal process, which utilizes “burning part of the raw material with a controlled air inlet provides the energy necessary for the (pyrolysis) process (Boateng et al., 2015). As Boateng et al. (2015) pointed out in their chapter “Biochar production technology” in the book “Biochar for Environmental Management” (Lehmann and Joseph (eds), 2015), this method “is most common for small-scale operation”. Historically, numerous examples for the carbonizers applying the partial combustion method for biochar production have been described, such as traditional pit kiln, mound kiln, and brick kiln (FAO, 1987; Brown et al, 2015).

In this study, all air inlets of the oil drum carbonizer were closed after the first 1.5 hr combustion phase, and the oxygen-starved condition (pyrolysis) was maintained for the next 46.5 hr. This condition fulfils a definition by Lehmann and Joseph (2015), that “Biochar is the product of heating biomass in the absence of or with limited air to above 250°C”. The internal thermal kinetics of the drum, which were maintained in the range of 100~450°C during the 46.5 hr phase, suggested that the biomass was treated with a temperature typical for pyrolysis (Brown et al., 2015; Álvarez-Mateos et al., 2019).

In the revised manuscript, these background and discussion were concisely added to the text, together with new citations described above (lines 162-164, 311-312). Moreover, more descriptions were added concerning the procedure for air cutoff after the 1.5 hr combustion (lines 89, 160-162), to emphasize that oxygen supply was restricted during pyrolysis.

 

 

Comments on the manuscript

“this paper has a weak point, which is the homemade “reactor” used for the combustion. It is true that it is very cheap and the purpose of this paper is that it may be used in poor areas of developing countries, where there is not budget to buy a real reactor, but from the oil drum used as a reactor can arise some problems, such as the leaching of metals from the drum to leaves during the process.”

 

Response to the Comments:

As described in the three references [39-41] in the previous version of the manuscript, utilizing the used oil drum as carbonizer was not rare. However, as pointed out by the Reviewer, the oil drum may have some potential drawbacks, such as metal leaching, which should be evaluated in future research. Therefore, discussion regarding this point was included in the revised manuscript (lines 225-227), in relation to the observed high Al content in the jatropha fallen leaves biochar.

 

 

Comments on the manuscript

“the introduction is a little short (also the conclusions), and in this section as well as in the conclusions section, some important applications for Jatropha curcas biomass are missing. For example, the pyrolysis of Jatropha curcas leads to the production of a biochar that has catalytic properties:”

“Fig. 9 of this latter paper could be useful to discuss the results obtained in the present paper.”

 

Response to the Comments:

We appreciated the Reviewer recommending these two nice references. In the revised manuscript, these two studies were mentioned in the Introduction section (lines 45-46), and the latter paper was cited in the Results and Discussion section again (lines 190-193) to discuss pyrolysis characteristics at different temperatures. Moreover, additional sentences were provided in the Conclusions section (lines 310-318).

 

 

Other minor points:

Comment on the manuscript:

Lines 48-49: “jatropha trees produce 550 kg of fallen leaves per plant at 1 year old”. This must be a mistake.

 

Response to the Comment:

We thank to the Reviewer for pointing out our mistake. The value in the original report was 550 “g” per plant, not 550 “kg”. These mistakes were corrected in the revised manuscript (lines 50-51).

 

 

Comment on the manuscript:

Lines 84-85: “The fallen jatropha leaves were tightly packed into the drum up to the height of the upper rim by stamping them down with the body weight of an adult”. How many kg (average weight) were introduced.

 

Response to the Comment:

The weight was approximately 44 kg, thus this information was added to the beginning of the sentence as “Approximately 44 kg of the jatropha fallen leaves were…” (line 84).

 

 

Comment on the manuscript:

Lines 93-95: “After 48 h, the carbonization was stopped by opening the lid and immediately applying 10L of water to the top of the pile”. If this is performed manually, it seems to be very dangerous.

 

Response to the Comment:

In our experiences, manual handlings for opening the lid and applying water did not resulted in the accidents such as vapor/flame explosion, burn injuries, or fume inhalation. We actually performed these processes very carefully by waring refractory gloves. To describe this point, we added two sentences in the revised manuscript (line 95, 194-198).

 

 

Comment on the manuscript:

Line 127. Change “second true leaves had emerged” by “second true leaves emerged”.

 

Response to the Comment:

The sentence was changed as suggested (line 129).

 

 

Comment on the manuscript:

Line 151. “The soils were then poured back into the original pots and 500 mL of water was added”. Indicate the mass of soil introduced (1 kg based on the information given in line 280).

 

Response to the Comment:

As suggested, information on the mass of soil (1 kg) was added to the sentence (line 153).

 

 

Comment on the manuscript:

Tables 1 and 2. Why the concentration of Al in fallen leaf “biochar” and in the experimental oil is so high? In the case of fallen leaves “Biochar”, is Al coming from the “reactor”?

 

Response to the Comment:

It is not surprising that the concentration of aluminum (Al) in the experimental soil (69,400 mg/kg) was highest among the measured metal elements, because Al is known to be the third most abundant element of the earth’s crust, after oxygen and silicon, and the value was similar to the reported median value of Al content in the soils on earth (71,000 mg/kg soil; Bowen, 1979). However, the observed high concentration of Al in fallen leaf biochar (52,900 ± 700 mg kg⁻¹) was unexpected. Reasons for the occurrence of high concentration of Al in fallen leaf biochar is currently unknown. One possibility is that jatropha fallen leaves feedstock might be contaminated with Al-rich soil particles during harvest in the field. Alternatively, Al in fallen leaf biochar might be derived from intrinsic Al accumulated within jatropha leaves, although absorption and accumulation of Al in plant leaves are normally inefficient due to its phytotoxicity. Third possibility is that Al might be derived from leaching from the surface of used oil drum. Although we extensively washed the inside of drum before usage, this possibility is not totally excluded. These possibilities were described in the revised manuscript (lines 220-227).

 

 

Comment on the manuscript:

Table 1 and Fig. 6 caption. Delete (Jatropha curcas) after the word jatropha.

 

Response to the Comment:

The phrase “(Jatropha curcas)” were deleted as suggested (lines 212, 286).

 

 

Comment on the manuscript:

Lines 205-207. Authors are comparing biochars obtained from pyrolysis processes with the residue obtained after the proposed carbonization scheme, which authors consider as biochar.

Lines 247-248. Change “application of jatropha fallen leaf biochar to the acidic experimental soil improved the growth performance of Swiss chard, particularly in the aerial parts of the plants.” by “application of jatropha fallen leaf biochar to the acidic experimental soil improved the growth performance of stalk and leaves of Swiss chard.” As aforementioned, I have doubts about the pertinence of using the term “biochar”.

 

Response to the Comment:

As recommended, the phrase “particularly in the aerial parts of the plants” was replaced by “stalk and leaves of Swiss chard” (line 266). Concerning the term “biochar”, please see the point above in the first and second pages of this letter, that the term ”biochar” has been historically applied to the products made by traditional kilns, in which thermal energy necessary for pyrolysis is provided by partial combustion of the biomass, followed by the thermal conversion of biomass to carbonaceous residues in oxygen-limited condition. From this standpoint, the term “biochar” should be applied to the product in the present study as well, and the comparisons with the previous studies should be justified.

 

 

Comment on the manuscript:

Captions for figures 2, 5 and 7 are too long. They provide information already given into the text, which is not needed to be repeated.

 

Response to the Comment:

As suggested by the Reviewer, we shorten the captions for Figures 2 (lines 176-178), 5 (lines 268-271), and 7 (lines 305-308) in the revised manuscript.

Author Response File: Author Response.pdf