Adsorption of Phosphates onto Mg/Al-Oxide/Hydroxide/Sulfate-Impregnated Douglas Fir Biochar

Round 1

Reviewer 1 Report

1. Please detail out the materials with all the chemical listings.

2. Throughout the study 20ml solution was used except for the pH study. Explain the reason behind this.

3. Line 235 states Biochar has a negligible adsorption or extremely low? As most publication reports a significant adsorption.

4. Line 238 -Physisorption interactions can  be governed by charged metal hydroxide surfaces and hydrogen bonding- Please cite

5. Line 245 -This may be more triggered at higher temperatures and high initial phosphate concentrations -rephrase...

Author Response

Thank you for your comments of suggestions to improve the manuscript.

1. Please detail out the materials with all the chemical listings.

Materials were detailed out in the revised version

2. Throughout the study 20ml solution was used except for the pH study. Explain the reason behind this.

We cannot find a place that we used a volume different from 20 mL in our adsorption studies.

2. Line 235 states Biochar has a negligible adsorption or extremely low? As most publication reports a significant adsorption.

This biochar is created at extremely high temperatures (900–1000 °C), which results in a material with more graphitic structure than one with a lot of functional groups. In order to increase the biochar's ability to adsorb phosphate, Al/Mg oxide/hydroxide nanomaterials are added to the raw biochar in this manuscript. Nowhere have we stated that the neat biochar has a high phosphate adsorption capacity.

3. Line 238 -Physisorption interactions can  be governed by charged metal hydroxide surfaces and hydrogen bonding- Please cite – References were added as suggested
4. Line 245 -This may be more triggered at higher temperatures and high initial phosphate concentrations -rephrase...

This was rephrased to the following

“When temperatures are greater (>35 °C), and the initial phosphate concentration is higher (>100 mg/L), this may be more likely to occur.”

 

Reviewer 2 Report

Please check the attachment.

Comments for author File: Comments.pdf

Author Response

 

 processes-2027516-Review:

In this manuscript, batch phosphate uptake of the AMBC at different initial pH, equilibrium time, equilibration temperature, and initial phosphate concentration was optimized. Sorption data best fitted into the sips isotherm model. The manuscript showed that the exhausted AMBC composite has the potential to recycle as a slow release phosphate fertilizer. This manuscript provided some interesting results. Research background is of significance. On the whole, this is a good work. However, necessary modifications shall be made:

1—In introduction: In this manuscript, the adsorption mechanism was discussed emphatically.At present, a lot of work has been done in this area in recent years, including the adsorption-desorption model, variable seepage velocity and variable pollutant concentration, as well as the co-adsorption of various migrating substances. Relevant work deserves attention: “Cotransport of heavy metals and SiO2particles at different temperatures by seepage. Journal of Hydrology, 2021, 597: 125771”; “Enhanced removal of hexavalent chromium from aqueous solution by functional polymer-wrapped gamma-alumina composite adsorbent. Environmental Technology & Innovation, 2021, 24: 101954”; “The remediation efficiency of heavy metal pollutants in water by industrial red mud particlewaste.Environmental Technology & Innovation, 2022. doi: 10.1016/j.eti.2022.102944”; “Adsorption of synthetic and real Kinetic Hydrate Inhibitors (KHI) wastewaters on activatedcarbon: adsorption kinetics, isotherms, and optimized conditions. Separation Science and Technology, 2021, 56(13): 2266-2277”; “The transport of silica powders and lead ions under unsteady flow and variable injection concentrations. Powder Technology, 2021, 387: 22–30”.

 

Thank you for your comments and suggestions

Unfortunately, the above papers are irrelevant to phosphate adsorption; hence, we cannot justify citing them.

We only cited the top 10% of articles pertinent to the manuscript's core topic.

We have attempted to improve the manuscript’s introduction by adding more relevant discussion and references on the uptake mechanism. But, we believe that the rigorous discussion on the uptake mechanism is more appropriate to leave in the mechanism section instead of the introduction. Hence, the mechanism discussion was further improved by adding a new scheme with step-by-step reactions that illustrate the formation of surface complexes.

 

2—Lines 107-130: The source of references of Eq. 1 should be given for the convenience of readers' judgment and selection.

This equation is an introductory general chemistry equation, and we do not think it requires a reference.

 

3—Lines 124-125: The author sets different temperatures (10, 40 degrees). In fact, in the references suggested above, more analysis has been carried out on the influence degree of temperature effect. What is the effect of temperature on your results? Ask the author to make necessary discussion.

The original manuscript discusses the thermodynamics of adsorption in the isotherm section (3.6). Also, we have included the discussion on high temperature effects on Mg/Al leaching and the formation of stoichiometric phosphate compounds in our uptake mechanism discussion.

The suggested reads deal with pollutants such as heavy metals etc., which are not relevant to our work, and we do not understand why the reviewer suggested papers that are not relevant to this work

4—Lines 251-258: The author has carried out the following work: “Both data sets were fitted well onto pseudo-2nd-order kinetic model with high correlation coefficients.” This is a classical adsorption kinetic process, and the fitting combination is also very good. What is his physical mechanism (adsorption and solidification)? Ask the author to discuss it appropriately. Please provide your innovation points.

The kinetic discussion was improved by explaining mono dentate and bidentate surface complex formation mechanisms. Changes are highlighted in the revised manuscript.

 

5—In practical engineering, the concentration of solution is constantly changing (increasing or decreasing). How to describe the desorption process? This is related to the removal effect. Ask the author to give an appropriate explanation. The references suggested above may have some reference significance.

The adsorption isotherm data is generated when the system reaches an equilibrium with adsorption and desorption. Isotherm models are designed in such a way that the equilibrium data is described. For example, the Langmuir model derivation equations are given in this paper in detail.

https://pubs.acs.org/doi/10.1021/la801839b

Sips isotherm differs slightly from Langmuir but follows the same proof of concept.

The purpose of conducting kinetic experiments is to identify the equilibrium point. Once the phosphate is adsorbed on the metal oxide surfaces, desorption is only possible with competitive species that can strip phosphate out. Therefore, it is implausible that it gets desorbed to give a net desorption effect during the experiment time window in the absence of competitive species. Of course, there can be negligible desorption, which will not drastically change the model predicted capacities.

Reviewer 3 Report

Overall

The manuscript reports on development of Mg-doped biochar for enhanced P uptake, and potentially recycling, from waste streams. The work is novel and practically very important, as improving nutrient use efficiency will be necessary due to resource constraints, population growth and increased focus on environmental impact of agricultural practices. The authors have conducted a suite of interesting methods to address the research question and encouraging results are presented. However, the manuscript requires a major revision before publication can be recommended. First, the writing style is very confusing and jumps between different concepts with little effort to link them. This is particularly the case in the Introduction which veers across many topics and doesn’t flow coherently into the research objectives.   The Results and Discission (currently titled ‘Result’s) also needs a rewrite to clarify and focus the key elements of the research. In addition to the loose writing, the methods are inadequately described and there is no mention of replication, sample numbers or statistical analysis. Consequently, the presentation of data in the Results is somewhat random and incoherent – with unbalanced pH and phosphate concentration data popping up throughout.  Overall, a potentially very interesting study but a lot of work needed to get it to publication standard.

 

Specific comments

The Introduction needs a rewrite – the logic links are poorly articulated and some strange terms are presented. For example, what does this sentence mean?  “Eutrophication is one of the leading causes of fishing impairments in coastal areas that naturally takes place over time in lakes and other bodies of water as sediment settles at the bottom”. “Cultural eutrophication”?

Line 32- Phosphate is not an element

Line 70 – “Zeolites, bentonite, diatomite, sand, resin, activated 70 carbon, and tea-waste has been used to stabilize the agglomeration” But you have no references for this statement

Line 80 – You jump from energy production to biochar modification without linking the two processes. “The slow pyrolysis of biomass demonstrates that renewable fuels can be potentially used in industrial applications such as site remediation, soil fertility, and wastewater management [19]. In addition, impregnating Mg/Al oxides/hydroxides/sulfates onto biochar surfaces prevents 83 particle agglomeration and increases mechanical strength.”

 

Line107 – did you monitor the pH of the isotherm solutions to check if the biochar had a buffering effect?

 

Figure 2 – says pH=7 but then have different pH levels in the x axis

 

Fig 5 B is interesting but we don’t have a BC+P graphic

 

Figure 7 is interesting but note it is a conceptual model

 

Figure 8 – not clear why different solution concentrations

Author Response

Overall

The manuscript reports on development of Mg-doped biochar for enhanced P uptake, and potentially recycling, from waste streams. The work is novel and practically very important, as improving nutrient use efficiency will be necessary due to resource constraints, population growth and increased focus on environmental impact of agricultural practices. The authors have conducted a suite of interesting methods to address the research question and encouraging results are presented. However, the manuscript requires a major revision before publication can be recommended. First, the writing style is very confusing and jumps between different concepts with little effort to link them. This is particularly the case in the Introduction which veers across many topics and doesn’t flow coherently into the research objectives.   The Results and Discission (currently titled ‘Result’s) also needs a rewrite to clarify and focus the key elements of the research. In addition to the loose writing, the methods are inadequately described and there is no mention of replication, sample numbers or statistical analysis. Consequently, the presentation of data in the Results is somewhat random and incoherent – with unbalanced pH and phosphate concentration data popping up throughout.  Overall, a potentially very interesting study but a lot of work needed to get it to publication standard.

Thank you for your comments and suggestions to improve our manuscript

First of all, we would like to bring the reviewer’s attention to the fact that this material is not a Mg-doped material. We never mentioned this anywhere in our original manuscript. This is an Al/Mg oxide/hydroxide impregnated biochar material. Doping a metal onto a biochar surface is typically a challenging task.

We have significantly improved the manuscript’s introduction. Also, the number of experiment replicates and statistical analysis details were mentioned where appropriate. However, we believe that the results section can stand alone as it is written logically in the original submission, and no significant changes were made.

We have prepared 28 different materials at various initial solution pH values, and we have made this clearer in the revised version. Our adsorption experiments are primarily performed at pH 11, as it was the prime pH we screened from our pH studies. However, we have compared pH 7 performance in kinetic studies as it is similar to natural water pH. Therefore, we are unsure what the reviewer implied about unbalanced pH experiments.

Specific comments

The Introduction needs a rewrite – the logic links are poorly articulated and some strange terms are presented. For example, what does this sentence mean?  “Eutrophication is one of the leading causes of fishing impairments in coastal areas that naturally takes place over time in lakes and other bodies of water as sediment settles at the bottom”. “Cultural eutrophication”?

The sentence was modified to the following for the clarity

“Eutrophication is one of the primary consequences of phosphate pollution”

Cultural eutrophication is a well-known phenomenon in our field. Please refer to the following link.

https://www.conserve-energy-future.com/causes-effects-solutions-cultural-eutrophication.php

Line 32- Phosphate is not an element

This was changed to phosphorus

Line 70 – “Zeolites, bentonite, diatomite, sand, resin, activated 70 carbon, and tea-waste has been used to stabilize the agglomeration” But you have no references for this statement

A reference was added

Line 80 – You jump from energy production to biochar modification without linking the two processes. “The slow pyrolysis of biomass demonstrates that renewable fuels can be potentially used in industrial applications such as site remediation, soil fertility, and wastewater management [19]. In addition, impregnating Mg/Al oxides/hydroxides/sulfates onto biochar surfaces prevents 83 particle agglomeration and increases mechanical strength.”

The following paragraph was added, and the writings were rearranged to make the flow better

 “Low-cost biochar adsorbents could serve in large scale agricultural roles where the cost of activated carbon makes their use impossible. The operation cost can be reduced significantly if an alternative adsorbent can be found, which is readily available, easily recovered from solution, and is cheap. Wood, agricultural byproducts, and energy crops provide an abundant and renewable supply of lignocellulosic feeds for biorefineries.”

Line107 – did you monitor the pH of the isotherm solutions to check if the biochar had a buffering effect?

The final pH was measured in each case, and the change was 0.3-0.35 in the majority. This was mentioned in the isotherm figure caption.

Figure 2 – says pH=7 but then has different pH levels in the x-axis

The reviewer has missed the point that figure 2 pH values correspond to various pH values that the materials prepared. Then they were screened for phosphate uptake at pH 7.

Fig 5 B is interesting but we don’t have a BC+P graphic

This figure was not included since the BC has negligible P uptake and the P2p XPS spectra are noise signals. We have included these figures in the supporting material, and a short discussion was added to the link the text.

Figure 7 is interesting but note it is a conceptual model

We have included several examples of surface complex formation mechanisms in the revised version.

Figure 8 – not clear why different solution concentrations

Different initial concentrations were used to study the influence of concentration on the initial phosphate uptake rate. This was made more apparent in the revised version’s kinetics discussion.