the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Effect of colloidal particle size on physicochemical properties and aggregation behaviors of two alkaline soils
Abstract. Soil colloidal particles are the most active components of all, and they also vary in elemental composition and environmental behaviors with the particle size. The purposes of the present study are to clarify how particle size affects the physiochemical properties and aggregation kinetics of soil colloids, and to further reveal the underlying mechanisms. Soil colloidal particles from two alkaline soils—Lou soil and cinnamon soil were subdivided into three ranges: d < 2 μm, d < 1 μm and d < 100 nm. The organic and inorganic carbon contents, clay mineralogy, surface electrochemical properties, including surface functional groups and zeta potentials, were characterized. Through time-resolved light scattering technique, the aggregation kinetics of soil colloidal fractions were investigated, and their critical coagulation concentrations (CCCs) were determined. With decreasing colloidal particle diameter, the total carbon content, organic carbon, organic functional groups content and illite content all increased. The absolute zeta potential values and the charge variability decreased with decreasing particle diameter. The CCC values of Lou soil and cinnamon soil colloids followed the descending order of d < 100 nm, d < 1 μm, d < 2 μm. Compared with the course factions (d < 1 μm and d < 2 μm), soil nanoparticles were more abundant in organic carbon and more stable clay minerals (d < 100 nm), thus they exhibited strongest colloidal suspension stability. The differences in organic matter contents and clay mineralogy are the fundamental reasons for the differences in colloidal suspension stability behind the size effects of Lou soil and cinnamon soil colloids. The present study revealed the size effects of two alkaline soil colloids on carbon content, clay minerals, surface properties and suspension stability, emphasizing that soil nanoparticles are prone to be more stably dispersed instead of being aggregated. These findings can provide references for in-depth understanding of the environmental behaviors of the heterogeneous soil organic-mineral complexes.
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RC1: 'Comment on egusphere-2024-1266', Anonymous Referee #1, 06 Jul 2024
The manuscript by Yan et al. presents findings on the characterization of different colloid particle size fractions extracted from calcareous soils in terms of organic and inorganic C contents and surface functional groups, clay mineralogy, and zeta potential, as well as their implications on aggregation kinetics and critical coagulation concentrations. The manuscript is generally well-written and the objectives of the study are noteworthy particularly considering the lack of information on the colloid characteristics and functioning in calcareous soils. However, the potential novelty of this work was not fully achieved due to a number of critical issues that preclude publication.
- No justification was provided for the selection of the two soils (Anthrosol and Calcisol) used for the extraction of colloid particle size fractions and the findings reported were not used to discuss any differences between the two soils. Although the fact that these soils were calcareous could represent the main novelty of this work, the knowledge gaps were not clearly identified and no hypothesis were defined.
- The experimental description does not mention the presence of replicates and no uncertainties associated with the reported data have been provided making it difficult to appreciate the significance of the differences discussed.
- Although the authors set out to separate (by centrifugation) and characterize three colloid particle size fractions of < 2 µm, < 1 µm and < 100 nm, the actual colloid particle diameters reported in Table 1 were very different from the intended cutoffs suggesting that the separation method used was neither calibrated nor checked beforehand (see comments below for possible causes of this).
- XRD spectroscopy is not quantitative (Table 3) due to the different X-ray absorption of the different minerals leading to different peak intensities. If semi-quantitative analysis was actually performed these methods must be detailed.
- The authors often refer to different soil types using what seems like a national classification systems (e.g. red, yellow, purple, cinnamon; see Pg 6) that however have little significance when publishing in international journals. It would be more appropriate to use international classification systems (e.g. FAO’s WRB) that also give an indication of the main soil forming processes involved.
Specific issues:
L51: Replace “active parts” with “reactive fractions”
L52: Surface charge is also an important characteristic of colloidal material
L53: What about their role in soil C mobilization and stability
L58: Replace “driving attached” with “mobilizing bound”
L93: Please check this citation. I would have expected the opposite.
L104: “The findings can have important implications”
L109ff: These soils and the processes driving their formation must be clearly described. It is not clear why they have been chosen for this specific study. Other characteristics like texture and cation exchange capacity would have been important to report. What do you mean by free Fe/Al oxides? How were these quantified? It would be better to use more consolidated terms like “pedogenetic”, “oxalate-extractable”, “dithionite-citrate-bicarbonate extractable” etc.
L123ff: Use past tense
L129; “sieved”. Please indicate pore size of the sieve rather than mesh number.
L138: When using Stokes’ law the density of the soil particles is normally taken to be 2.65 and not 1.65 g cm-3. This could have been the cause for the lack of separation of the nominal colloid particle sizes fractions the authors initially planned to achieve.
L160-161: What does this sentence mean?
L196ff and elsewhere: Do not report data in the main text that has already been reported in the tables and figures.
L207ff: Please reword this sentence as the real meaning is not clear.
L228: I think the authors should also consider the effect of higher organic matter contents in reducing the N2-BET surface area of finest colloidal fractions due to surface coverage.
L251: Are these differences really significant?
L280: When referring to zeta potential of colloidal fractions I suggest the authors refer to “less negative” and “more negative” when comparing values. For example, here “zeta potential became more negative with increasing particle diameter”.
L386: The conclusions should summarise the main findings considering the application of this work to understand soil functions and not merely repropose the results.
Citation: https://doi.org/10.5194/egusphere-2024-1266-RC1 -
RC2: 'Comment on egusphere-2024-1266', Anonymous Referee #2, 14 Jul 2024
This study examined the impact of particle size on surface properties and aggregation behaviors of soil colloids, aiming to enhance the understanding of these properties across different particle sizes in natural environments. However, there are still some issues in this study that need to be clarified. What is the background and significance of selecting these two soils (Lou soil and cinnamon soil) in this study? Additionally, while the study considered colloids of varying sizes (d < 2 μm, d < 1 μm, and d < 100 μm), the final measured colloid particle size fell outside of this range (Lines 195-202). The primary objective of the research is to explore the influence of colloid particle size on colloid aggregation; however, the presence of organic matter on colloid surfaces raises questions about how the study differentiates between the effects of colloid size and organic matter on colloid aggregation. Essentially, the study is about the two soils, rather than using the two alkaline soils to explore broader and more generalizable phenomena, and lacks the hypothesis.
Specific comments
Line 21 Delete “most” and “of all”
Line 56 change “of” to “in”?
Line 115 change “are” to “were”?
Lines 109-112: Please add detailed information on sampling points, such as longitude and latitude, crop type, climate, etc..
Line 114-119: Specific methods for determining soil properties need to be given.
Line 123 50 g of dry soil was weighed into a beaker containing 500 mL of distilled water.
Line 204 – 207 delete “The particles in the soil solution are in constant Brownian motion, and when light passes through the colloids, the particles will undergo light scattering, resulting in fluctuations in light intensity, and thus the effective diameter (intensity-weighted diameter) of the particles is calculated (Filella et al., 1997)” and re-written it.
Line 213 higher than
Lines 276-277: What is the reason for this phenomenon?
Lines 380-381: Why does the increase of illite content increase the colloidal stability?
Table 2: Why do the colloids with d < 1 μm have the largest specific surface area?Citation: https://doi.org/10.5194/egusphere-2024-1266-RC2
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