Chemosphere 364 (2024) 143300

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Humic acid enhances the co-transport of colloids and phosphorus insaturated porous media
Changxi Wang a, Simin Yao a , Renkuan Liao a,* , Jirka Šimůnek babCollege of Land Science and Technology, China Agricultural University, Beijing, 100083, PR China
Department of Environmental Sciences, University of California Riverside, Riverside, CA, 92521, United States

H I G H L I G H T S

G R A P H I C A L A B S T R A C T• Humic acid significantly increased
montmorillonite colloids’ phosphateadsorption and desorption capacity.
• Humic acid enhanced the co-transportabilityofthemontmorillonitephosphate mixture through the quartz
sand column.
• The transport parameters fitted using
HYDRUS-1D indicated that humic acids
decreased the deposition coefficients ofcolloids.
• The potential mechanism of ground­water phosphate pollution in humusrich areas was discussed.

A R T I C L E I N F O

A B S T R A C T

Handling editor: X. Cao

Phosphorus (P) has been widely recognized as a substance that is difficult to transport due to its tendency to
become easily fixed in the soil. However, many reports demonstrate that groundwater P pollution is rising inhumus-rich areas. Research is urgently needed to confirm (or reject) the hypothesis that increased P pollution isrelated to humus, as there is currently limited quantitative research on this topic. In this study, we conducted aseries of batch equilibrium adsorption-desorption experiments and column experiments to quantify the effects ofmontmorillonite colloids (MCs) and humic acids (HCs, the main components of humus) on the P transport
behavior. The results indicate that P’s adsorption and desorption behavior on MCs can be well simulated using
the Langmuir and Temkin models (R2 > 0.91). Compared to the non-HC treatments, HCs significantly increased
MCs’ P adsorption and desorption capacity 5.18 and 7.21 times, respectively. Moreover, HCs facilitated thetransport ability of the MC-P mixture through the saturated quartz sand column. In a 0.1 M NaCl solution, the
MC-P mixture is nearly completely adsorbed on the surface of quartz sand, with a penetration rate of only 0.5%.
In contrast, the HC-MC-P mixture can evidently penetrate further at a rate of 26.1%. The transport parameters
fitted using HYDRUS-1D further indicated that the presence of humic acids significantly decreased the deposition

Keywords:
Phosphorus
Humic acids
Montmorillonite colloids
Breakthrough curve
Co-transport* Corresponding author.
E-mail addresses: renkuanliao@cau.edu.cn, renkuanliao@163.com (R. Liao).
https://doi.org/10.1016/j.chemosphere.2024.143300
Received 15 May 2024; Received in revised form 28 August 2024; Accepted 6 September 2024
Available online 7 September 20240045-6535/© 2024 Elsevier Ltd. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

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coefficients of colloids, thereby enhancing the co-transport of colloids and P through the quartz sand porous
medium. The potential mechanism of P pollution in humus-rich areas is likely enhanced by the formation of an
HC-colloid-P mixture, which greatly increases the adsorption amount of P on colloids and enhances the elec­
trostatic and spatial repulsion between colloids as well as between colloids and quartz sand. It reduces the ag­
gregation and adsorption of colloids, ultimately transferring P into groundwater through colloid-facilitated cotransport. The findings of this study clarified the relationship between the transport of P, colloids, and HCs,
which provides a theoretical basis for explaining the P pollution mechanism in humus-rich areas.

1. Introduction

molecules and the surface of magnetite. However, Perassi and Borgnino(2014) discovered that HA in the solution decreased phosphateadsorption by montmorillonite. This is because phosphate and HA
compete for adsorption sites on montmorillonite particles, reducing theefficiency of phosphate adsorption. Similarly, the competition between
humic acid and phosphate for adsorption sites may lead to a decrease in
the adsorption capacity of phosphate on surfaces of montmorillonite,goethite, and other adsorbents (Antelo et al., 2007; Fu et al., 2013; Chenet al., 2022).
Many efforts have been made to study the co-transport behavior of
colloids and adsorbed P. Colloids have long been recognized as a key
pathway for P transport in agricultural systems (Adediran et al., 2021;
Eltohamy et al., 2023). Missong et al. (2018) conducted column irriga­
tion migration experiments on forest topsoils and found that the leachednatural colloids contained high concentrations of phosphorus. Thissuggests that phosphorus leaching in forest topsoils primarily depends
on the action of colloids, emphasizing their role in promoting phos­
phorus leaching. In their packed box experiments, Sharma et al. (2017)
reached similar conclusions, indicating that particulate phosphorus
accounted for 52%–93% of the total phosphorus loss in runoff. Addi­
tionally, Uusitalo et al. (2001) observed that most leached phosphorus
existed in particulate form, further confirming the importance of theparticulate matter for phosphorus transport processes.
Despite extensive research on the co-transport of colloids and P,
there is currently limited quantitative research on how colloids transport
P vertically into groundwater in areas enriched with humus and how
humus influences the co-transport behavior of colloids and P. Therefore,
the objectives of this study are to examine the impact of humic acid, amain component of humus, on the adsorption and desorption charac­
teristics of colloids and the co-transport behavior of colloids and P usingbatch equilibrium adsorption-desorption experiments and column ex­
periments, thereby elucidate the potential mechanisms of groundwaterP pollution in areas where humus is enriched. This study’s findings will
provide a theoretical basis for preventing and treating groundwater P
pollution in these areas.

Phosphorus (P) fertilizer, a crucial nutrient essential for crop growth,has been widely used in agricultural practices for many years, playing asignificant role in boosting crop yields (Zhu et al., 2018; Han et al.,2022). However, P is also acknowledged as a major contributor to
freshwater eutrophication, promoting algae growth in surface waters(Scavia et al., 2014; Wurtsbaugh et al., 2019). In recent years, there has
been an increasing number of reports indicating that the impacts ofhuman activities and climate change have extended these eutrophica­tion issues to groundwater ecosystems, resulting in unpredictableecological impacts (Penuelas et al., 2013; Maavara et al., 2015; Wanget al., 2021). Specifically, groundwater P pollution is notably severe in
regions enriched with humus, such as black soil areas (Yang and Yan,
2010; Liu et al., 2023).
Typically, soil can mitigate excess P by adsorbing and binding it,
thereby reducing the likelihood of P entering water bodies (Bindraban
et al., 2020). The enhanced influx of P entering water bodies may be
attributed to the abundance of colloids in the soil. Colloids, with anenormous specific surface area and strong adsorption capacity, aresuspended particles ranging from 1 nm to 10 μm in size (McCarthy andZachara, 1989; Ouyang et al., 1996). They can be classified as abiotic
colloids (including clay, metal oxides, and humic substances) andbio-colloids (such as viruses) (McCarthy and Zachara, 1989; Ouyang
et al., 1996; Foppen and Schijven, 2006). P can combine with thesecolloids to form a colloidal-phosphorus mixture. These mobile colloidal
particles can then act as effective P “carriers” in soils and groundwateraquifers, enhancing the transport of absorbed P and reducing retarda­
tion effects (Pang et al., 2016; Ling et al., 2021; Deb and Chakma, 2023;
Liu et al., 2024).
Under the influence of human activities and precipitation, the soil
colloidal-phosphorus mixture can undergo horizontal transport withrunoff drainage (Barbosa et al., 2009; He et al., 2019) and verticaltransport with soil dominant flow (Jacobsen et al., 1997; Djodjic et al.,
2004; Buesseler et al., 2009; VandeVoort et al., 2013; Liang et al., 2016).
This may be one of the main reasons for the loss of P into groundwaterand causing environmental problems. Studies have shown that colloidal
phosphorus accounts for a significant fraction of P loss (Heathwaite
et al., 2005; VandeVoort et al., 2013). The adsorption and desorption
capacity of colloids for P, as well as the co-transport behavior of colloids
and adsorbed P, may determine the ability of P to enter water bodies.
Environmental factors, such as solution pH, temperature, and colloid
type, can influence the colloids’ adsorption and desorption properties
towards P, with humus being recognized as a significant factor affecting
this process. Wang and Wang (2012) studied the effects of different
organic components on P adsorption in riverbed sediments. They
showed that the tightly-combined humus in organic matter was the main
factor affecting P adsorption in sediments. After removing organic
matter, the adsorption capacity of sediments for P significantly
decreased. (Rashid et al., 2017) have achieved similar results. They
employed humic acid (HA) coated magnetite materials to remove
phosphate from eutrophic water. Additionally, Gai et al. (2024) foundthat the addition of HA increased phosphate adsorption capacity by
15%. This suggests that the presence of HA enhances the affinity ofadsorbents for phosphate, possibly due to the formation of morehydrogen bonds or electrostatic interactions between humic acid

2. Materials and methods
2.1. Materials
Montmorillonite powder (Al2O9Si3) used in the experiments waspurchased from the Shanghai Macklin Biochemical Technology Co.
Briefly, the specific surface area (SSA) is 240 m2 g− 1, and the molecularweight is 282.2. To obtain montmorillonite colloids, 2.5 g montmoril­
lonite powder was initially suspended in 0.4 L of deionized (DI) water.
The suspension was then subjected to ultrasound for 1 h using an ul­
trasonic bath (KQ-300DA, 300 W, 40 kHz, Kunshan, China) to preparemontmorillonite colloids (MCs). Subsequently, the suspension wastransferred to a 1 L graduated cylinder and diluted with DI water up to
the 1 L mark. After thorough mixing, it was left undisturbed for 16 h.
Finally, after 16 h, the liquid located 0–10 cm below the liquid surface
was collected to obtain MCs. The concentration of collected MCs was
gravimetrically determined, and the colloid suspension was diluted to 2g L− 1 for further use.
Humic acid was purchased from the Tianjin Guangfu Fine Chemical
Research Institute. According to the information provided by the2

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Chemosphere 364 (2024) 143300

company, humic acid contains 10% ash content, 8% moisture, and 0.3%
iron content. To prepare humic acid colloids (HCs), 2.5 g of humic acidpowder was mixed with 0.4 L of DI water and subjected to the same
sonication process as the montmorillonite colloids. The suspension wasthen transferred to a 1 L graduated cylinder, diluted to 1 L with DI water,
thoroughly mixed, and left to settle for 16 h. The liquid layer 0–10 cm
below the surface was collected to obtain the HCs. The concentration ofcollected HCs was gravimetrically determined, and the colloid suspen­sion was diluted to 2 g L− 1 for further use.
Montmorillonite-humic acid hybrid colloids (MHCs) were obtainedby mixing equal volumes of the prepared HCs and MCs. The two colloid
suspensions were combined in a 1:1 volume ratio, ensuring a homoge­
neous mixture, and then used for further experimentation.

oven at 105 ◦ C and stored in covered Pyrex beakers until utilized in thecolumn experiments.
The transport experiments were conducted using acrylic columns
with dimensions of 3 cm inner diameter and 28 cm length. The columns
were meticulously packed with clean sand in 2 cm-thick layers, resulting
in an average bulk density and porosity of approximately 1.58 g cm− 3
and 0.40, respectively. A peristaltic pump facilitated a continuous up­
ward water supply, maintaining an average pore water velocity of 5 mLmin− 1 and a flow residence time of 16 min within the column. The
column tracer experiment involved injecting 3 pore volumes (PVs) of a
3 mM NaNO3 solution into the column, followed by elution with 5 PVs of
DI water to elucidate the hydrodynamics of the column experiment.
Effluent NO3− concentrations were measured using a UV–visible spec­
trophotometer (TU-1900, YOKE, China) at 235 nm to obtain the
breakthrough curve (BTC) of NaNO3 (Shang et al., 2010; Yang et al.,2017). The dispersivity of the column was obtained by fitting the NaNO3
breakthrough curves (BTCs) using the convection-dispersion equation(CDE) numerically solved by Hydrus-1D (Šimůnek et al., 2008, 2016).
In all experiments, the columns underwent a pre-equilibration phase
wherein more than 5 PVs of the designated electrolyte solution werepumped through it. Following this pre-equilibration step, 3 PVs of asolution of MCs/MHCs with adsorbed P, at a concentration of 100 mg
L− 1, was injected into the column (Phase 1). This was succeeded byinjecting 5 PVs of the colloid-free background electrolyte solution(Phase 2). To simulate the effect of rain or irrigation events leading to adecrease in soil solution ionic strength, the column was systematicallyflushed with 5 PVs of DI water (Phase 3) to investigate the release
behavior of retained colloids under transient changes in solution ionicstrength (Bradford et al., 2012; Li et al., 2020).
Effluent samples were collected at intervals of 3 min. The collectedeffluent was centrifuged and filtered to measure the concentration of P
in the supernatant. Detailed procedures for determining the P content
can be found in the Supporting Information(SI). The concentrations of
MCs and MHCs were determined using a UV–visible spectrophotometerat 260 nm and 630 nm, respectively. The calibration curves for thecolloids are provided in Fig. S1.

2.2. Characterization of MCs and MHCs
The morphology of MCs and MHCs was determined using trans­mission electron microscopy (TEM, JEOL Jem-2100, JPN). The func­
tional groups on MCs and MHCs surfaces were obtained using a Fouriertransformation infrared (FTIR) Spectrometer (Nicolet iS50, Thermo
Fisher, USA) equipped with a diamond ATR crystal in the 400-4000
cm− 1 range. XPS energy spectra were obtained using an X-ray spec­
trometer (Escalab 250Xi, Thermo-Fisher Scientific, USA), and thespectra were analyzed using the Thermo Avantage software. The col­loids with a concentration of 100 mg L− 1 were measured for their hy­
drodynamic diameter (DH) using a Zetasizer (Nano ZS90, Malvern
Instruments, UK) under conditions where the ionic strength (IS) was
0.01 M and 0.1 M. The scattering angle was fixed at 90◦ , and the tem­
perature was maintained at 25 ◦ C.
2.3. Adsorption and desorption experiments
Batch sorption experiments were conducted to investigate the impactof IS and the presence of humic acid (HCs) on P sorption onto MCs. P
sorption experiments at concentrations of 2, 5, 10, 20, 40, 60, and 80 mg
L− 1 on 1 g L− 1 MCs/MHCs with a total volume of 20 mL were conducted
in 50 mL centrifuge tubes. The mixtures were shaken at 200 rpm for 24 h
in a temperature-controlled orbital shaker under pH 7.0 ± 0.2 condi­
tions, with varying IS achieved using 0.01 M and 0.1 M NaCl solutions(Antelo et al., 2005; Zou et al., 2020). All adsorption tests were per­formed in triplicate. Following the shaking, the tubes were centrifuged
at a speed of 12000 rpm for 10 min. Subsequently, the supernatant was
filtered through a 0.45 μm membrane, and the P content in the filtrate
was determined.
In the desorption experiment, the supernatant obtained after
centrifugation was decanted. The colloids were washed three times with
NaCl solution of the same ionic strength to remove surface-free P.
Subsequently, 20 mL of NaCl solution with equivalent ionic strength wasadded to each centrifuge tube for desorption testing(An et al., 2016).
The colloid solution was shaken at a constant temperature of 25 ± 0.1 ◦ Cwith a constant speed of 200 rpm for 24 h(Li Hu et al., 2015). Aftershaking, the solution was centrifuged at 12000 rpm for 10 min, and thesupernatant was filtered through a 0.45 μm membrane. The P content in
the filtrate was measured. Each experiment was conducted with three
replicates. Detailed steps for P content determination can be found in the
Supporting Information (SI).

2.5. Data analysis
The Langmuir, Freundlich, and Temkin models were applied to fit
the P adsorption isotherms on MCs and MHCs due to their establishedeffectiveness in describing a wide range of adsorption behaviors insimilar studies(Liu et al., 2015; Zhou et al., 2020). These models accountfor different adsorption mechanisms: Langmuir for monolayer adsorp­
tion, Freundlich for heterogeneous surfaces, and Temkin for
adsorbate-adsorbent interactions. The desorption capacity and desorp­
tion hysteresis index (HI) were used to quantify the degrees of desorp­
tion in the desorption processes (Huang et al., 2019; Zhou et al., 2020).
The detailed information is described in the SI.
The transport of colloids in the porous media was modeled using the
one-dimensional convection-dispersion equation (CDE) combined witha two-site kinetic adsorption-desorption model(Bradford et al., 2003;
Wang et al., 2019). It was assumed that there were two kinetic
adsorption sites: one site (S1) representing a reversible attachment sitewith both adsorption and desorption and the other site (S2) representing
an irreversible attachment site with only depth-dependent blocking. The
calculation formulas are provided in the SI.
In the colloidal migration experiments, we assess the behavior of
colloids during different phases by calculating the Fraction of reversible
adsorption (FRA)(Li et al., 2020). The calculation of FRA is based on
formula (1):2.4. Column transport experiments
The porous media for the column experiments consisted of quartz
sands (425–600 μm)(Yan et al., 2019). The sand beads underwent a
pre-treatment process involving immersion in 2 M HCl solutions at 90 ◦ C
for 24 h to eliminate organic and iron impurities(Wang et al., 2019; Zhao
et al., 2022). Additionally, residual fine particles adhering to the sandbeads were meticulously removed through sequential rinsing with DI
water. After these cleansing procedures, the sand beads were dried in an

FRA =

M3
1 − M12(1)

where M12 represents the colloid recovery rate in phases 1 and 2, and M3
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Chemosphere 364 (2024) 143300

represents the colloid recovery rate in phase 3. The rate of irreversibly
adsorbed colloids Mirr can be obtained using formula (2):Mirr = 1 − M12 − M3

et al., 2013). MHCs exhibited a stretching vibration peak of C––O bonds
near 1700 cm− 1 and stretching vibration peaks of aromatic CH bonds
between 910 cm− 1 and 690 cm− 1 (Uchimiya et al., 2011; Zhang et al.,2020).
FTIR was employed to characterize the surface functional groups ofcolloids before and after adsorption. After phosphates adsorption, thepeaks corresponding to –OH bonds at wavelengths of 1640 cm− 1, 3430
cm− 1, and 3620 cm− 1, as well as the Si–O bond peak at 1024 cm− 1 in
MCs weakened and disappeared. New stretching vibration peaks
appeared at 1280 cm− 1, 1305 cm− 1, and 1410 cm− 1, all of which are
attributed to the stretching vibrations of phosphate groups (Elzinga andSparks, 2007; Cheng et al., 2022). This indicates a ligand exchange be­
tween hydroxyl groups on the montmorillonite surface and negatively
charged phosphate ions in the aqueous solution, suggesting an
inner-sphere complexation adsorption mechanism. Given that the initial
pH of the experimental solution was 7, under which the phosphate ionsexist primarily as H2PO−4 and HPO2−
4 (Fig. S3), the ligand exchange
process can be attributed to (Yang et al., 2006):(2)

3. Results and discussion
3.1. Characterization of colloids
The TEM (Transmission Electron Microscopy) image in Fig. 1aillustrated MCs with an extremely thin sheet-like structure, exhibiting
both clear and blurry outlines with numerous edges, with an averagehydrodynamic diameter of 474 ± 18 nm (Fig. 1b). Meanwhile, Fig. 1c
depicted the TEM image of MHCs, where humic acid adhered to themontmorillonite surface, forming varied-sized clusters with clear out­lines, diminishing the sharp edges of the particles. The average hydro­
dynamic diameter of MHCs was 542 ± 29 nm (Fig. 1d), which was larger
than that of MCs due to humic acid attachment to the surface of themontmorillonite particles.
X-ray photoelectron spectroscopy (XPS) full spectrum scan results
are shown in Fig. S2. The main elements of MCs include O, Si, C, and Al,
among others, with a significant abundance of silicon, attributed to thepresence of silicon-oxygen tetrahedra on the montmorillonite surface.
The main elements of MHCs are similar to MCs, but with decreased
oxygen and silicon peaks and increased carbon peak, indicating thepresence of a large amount of carbon in the humic acid within MHCs.
The Fourier Transform InfraRed (FTIR) spectra of MCs and MHCs areshown in Fig. 2. The infrared spectrum of MCs exhibits a prominent
absorption peak near 3620 cm− 1, which is attributed to the stretchingvibration of Al–OH bonds. Peaks at 3430 cm− 1 and 1630 cm− 1 corre­
spond to the stretching and bending vibrations of interlayer watermolecules (H–OH), indicating the presence of physically adsorbed watermolecules in the sample (Tan et al., 2008). The stretching vibration peakof Si–O bonds was observed between 1100 cm− 1 and 800 cm− 1 (Zou

≡S–OH + H2PO−4 → ≡S-OPO3H2 + OH−−−≡S–OH + HPO2−
4 → ≡S-OPO3H + OH

The symbol ≡ S represents the surface of MCs. After adsorption ofphosphates, MHCs also exhibited similar features, with a weakening of
the –OH peak at around 1600 cm− 1 and 3410 cm− 1, as well as the Si–O
peak at 1024 cm− 1. However, due to the presence of humic acid in
MHCs, the peaks corresponding to –CH and aromatic –CH overlap withthe characteristic peaks of phosphate ions, making it difficult to identify
them.
3.2. Phosphorus sorption on colloids
The adsorption efficiency (η) of different colloids at various initial P

Fig. 1. The Transmission Electron Microscopy (TEM) image (a) and particle size distribution (b) of MCs, and the TEM image (c) and particle size distribution (d)of MHCs.
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Chemosphere 364 (2024) 143300

Fig. 2. FTIR spectra of MCs with IS of 0.01 M (a) and 0.1 M (b) and of MHCs with IS of 0.01 M (c) and 0.1 M (d) before and after adsorption.

concentrations (C0) is illustrated in Fig. 3a. Across all types of colloids
and C0 variations, there was an overall decreasing trend in adsorption
efficiency with increasing initial P concentration. At lower C0 (e.g., 2
mg L− 1), the higher adsorption efficiency may be attributed to thelimited availability of P, allowing for more available sites on theadsorbent and resulting in higher efficiency. As C0 increases, the avail­able adsorption sites may become saturated, leading to decreased effi­
ciency. Under the same C0 conditions, the adsorption efficiency of MHCs
was significantly higher than that of MCs, possibly due to theenhancement of colloidal adsorption capacity with the addition of
humic acid. At an ionic strength of 0.1 M, the adsorption efficiency of
MHCs was highest, indicating that humic acid may contribute tomaintaining high adsorption efficiency under higher ionic strength
conditions.
For MCs, the influence of ionic strength was insignificant, while for
MHCs, increasing ionic strength enhances the adsorption efficiency,
especially under low C0. These results suggest that both the addition of
humic acid and changes in ionic strength can affect the adsorption ef­
ficiency of P.
The equilibrium adsorption capacity of colloids for P increased with

the initial P concentration. This is because the mass transfer driving
force becomes stronger when the phosphate concentration increases(Wang et al., 2016b). The Langmuir, Freundlich, and Temkin adsorption
isotherm models were applied to delineate the relationship between
adsorbents and adsorbates under equilibrium conditions. The adsorp­
tion isotherms are illustrated in Fig. S4, while the corresponding model
parameters are listed in Table 1. The Langmuir adsorption isotherms
revealed that when the NaCl concentration increased from 0.01 M to 0.1
M, MCs’ maximum adsorption capacity (qm) decreased from 0.437 mg
g− 1 to 0.418 mg g− 1. Similarly, the qm value for MHCs decreased from2.70 mg g− 1 to 2.69 mg g− 1. This decline could be attributed to theimpact of IS on the adsorption density of phosphate ions on the mineralsurface. More ions in the solution may lead to competition with phos­
phate ions for adsorption sites on the adsorbent surface. In the presenceof humic acid, qm values of MHCs exhibited a substantial increase, with
qm values at IS of 0.01 M and 0.1 M being 6.19 and 6.43 times that ofMCs, respectively. This phenomenon could be attributed to the provisionof additional phosphate adsorption sites by humic acid or alterations inthe surface properties of montmorillonite.
It was observed for the Freundlich model that the KF values increased

Fig. 3. Adsorption efficiency (η) of phosphorus onto colloids (a) and desorption efficiency (R) of phosphorus from colloids (b).
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Chemosphere 364 (2024) 143300

Table 1
Regression parameters of the phosphorus adsorption isotherms onto the mont­
morillonite colloids (MCs) and montmorillonite-humic acid hybrid colloids(MHCs), fitted by the Langmuir, Freundlich, and Temkin models.
Isotherm models
LangmuirFreundlich
TemkinParametersqm(mg g− 1)
KL
R2
KF
1/n
R2
bT
KT
R2

MCs

Table 2
Desorption hysteresis index (HI) of phosphorus from montmorillonite colloids(MCs) and montmorillonite-humic acid hybrid colloids (MHCs).

MHCs

0.01 M

0.1 M

0.01 M

0.1 M

0.437
0.033
0.990
0.030
0.550
0.977
29.865
0.481
0.977

0.418
0.056
0.983
0.048
0.471
0.939
27.852
0.617
0.987

2.704
0.106
0.960
0.589
0.340
0.949
4.909
1.653
0.958

2.689
0.180
0.972
0.809
0.279
0.972
5.208
3.171
0.992

C0(mg L− 1)

0.01 M MCs

0.1 M MCs

0.01 M MHCs

0.1 M MHCs

2
5
10
20
40
60
80

0.906 ± 0.064
0.952 ± 0.013
0.951 ± 0.009
0.922 ± 0.014
0.916 ± 0.031
0.932 ± 0.012
0.938 ± 0.002

0.716 ± 0.027
0.739 ± 0.015
0.821 ± 0.005
0.737 ± 0.009
0.762 ± 0.053
0.750 ± 0.007
0.764 ± 0.013

0.757 ± 0.005
0.725 ± 0.003
0.746 ± 0.001
0.698 ± 0.001
0.758 ± 0.008
0.750 ± 0.002
0.750 ± 0.002

0.748 ± 0.003
0.734 ± 0.003
0.741 ± 0.009
0.725 ± 0.001
0.706 ± 0.007
0.698 ± 0.003
0.697 ± 0.004

values slightly increase or remain relatively stable with increasing initialconcentrations. This suggests that at higher C0, the adsorption process
may involve stronger or deeper adsorption sites, making adsorption atthese sites more difficult to reverse.
The HI values for MHCs are generally lower than for MCs for all IS.
This indicates that MHCs have relatively higher desorption efficiency,
possibly because MHCs have a higher adsorption capacity, allowing
more phosphate ions to desorb from the surface of MHCs. Within thesame type of colloids, the HI values at 0.1 M are generally lower thanthose at 0.01 M. This suggests that at higher IS, the bonding formed
during the adsorption process may not be as strong due to ion compe­
tition effects, making the desorption process easier to occur than atlower IS.

as follows: 0.01 M MCs <0.1 M MCs <0.01 M MHCs <0.1 M MHCs.
Additionally, the 1/n values, representing the adsorption intensity
index, follow a sequence: 0.01 M MCs >0.1 M MCs >0.01 M MHCs >0.1
M MHCs. Smaller 1/n values indicate a more favorable adsorption
process and a stronger heterogeneity on the adsorbent surface. The re­sults suggest that MHCs exhibit weaker surface heterogeneity than MCs,
which is consistent with the TEM findings. The highest KF and lowest 1/
n values for MHCs at an ionic strength of 0.1 M indicate the predomi­nance of high-affinity sites.
The Temkin model describes the experimental data very well for
MHCs at an ionic strength of 0.1 M. The Temkin constant (KT) signifi­
cantly increases in the case of MHCs compared to MCs, indicating asubstantial enhancement in adsorption capacity upon adding humic
acid. The R2 values for different adsorption models were used toinvestigate the mechanisms of phosphate adsorption on colloids undervarious conditions. Our study found that all R2 correlation coefficientsfor the Langmuir, Freundlich, and Temkin models were high, indicatingthat phosphate adsorption on colloids can occur due to multiple mech­anisms. When the IS was low, the Langmuir model exhibited the highest
R2 among the three adsorption models (R2 > 0.95), suggesting that
phosphate adsorption on colloids may involve monolayer physicaladsorption under these conditions. However, when the IS was 0.1 M, theTemkin model showed a higher R2, indicating that phosphate adsorption
on colloids for higher values of IS became more complex, transitioning
to non-uniform adsorption.

3.4. Co-transport of colloids and phosphorus in porous media
Fig. 4 depicts the breakthrough and release curves (BTCs) of MCs (a,b) and MHCs (c, d) after P adsorption for 0.01 M and 0.1 M. Fig. 4 in­dicates that the colloid breaks through at ~0.95 PV, the effluent con­centrations continuously increase with colloid injection and do notexhibit a plateau phase. This behavior has been attributed to blocking
behavior, characterized by the continuous occupation of available
retention sites on the sand surface(Camesano et al., 1999). The model
effectively captures this behavior. In phase 2, the effluent concentrationof MCs rapidly drops to the background solution level, and the BTC
shows no significant tailing, indicating minimal desorption of MCs
under steady-state flow and chemical conditions. In phase 3, the NaClsolution is replaced with DI water, resulting in the rapid release ofsolid-phase adsorbed colloids, demonstrating that the solid-phaseadsorption of some colloids is reversible.
The BTCs of MCs showed significant differences under different IS.
When the solution concentration was increased to 0.1 M (Fig. 4b), there
was no noticeable colloid breakthrough in both phases 1 and 2. The
colloid recovery in the effluent of phases 1 and 2 (M12) decreased by
63.3% (Table S1). This may be attributed to the higher ionic concen­
tration compressing the double layer, weakening the mutual repulsion
between colloids and sands (Table 3), and thus causing more colloids tobe trapped in the pores. Under high IS conditions, the concentration ofdesorbed colloids in phase 3 was significantly higher than that under
low IS conditions, as more colloids were retained in phases 1 and 2 athigher IS, and more colloids could be desorbed when IS was reduced(Pazmino et al., 2014; Wang et al., 2016a). At 0.01 M, the M12 for MHCs
(Fig. 4c) slightly exceeded that of MCs, showing a 16.7% increase. Thisphenomenon arises from the presence of humic acids (HCs), which
render the colloids’ zeta potential more negative. Consequently, thisintensifies the maximum repulsive barrier (φmax) between the colloids
and quartz sand (Fig. S5 and Table 3). At 0.1 M (Fig. 4d), M12 for MHCs
is 25.6%, which is significantly higher than for MCs (0.158%). Thissuggests that the presence of humic acid may inhibit the aggregation of
MHCs due to electrostatic and steric hindrance mechanisms, thereby
enhancing the transport of the colloids in porous media. Similar result
were obtained in previous research, showing that humic acid enhanced
the ferrihydrite colloid transport under neutral conditions (Ma et al.,2018). Meanwhile, we observed that the Fraction of reversible

3.3. Phosphorus desorption from colloids
The desorption efficiencies (R) of P from MCs and MHCs are shown in
Fig. 3b. The desorption efficiency of MCs varies significantly for
different ISs. Under 0.01 M and 0.1 M conditions with MCs, there was a
noticeable difference in R, especially at low initial P concentrations(from 2 mg L− 1 to 20 mg L− 1). This suggests P adsorption to MCs may bestronger at low IS, resulting in lower desorption efficiency. In contrast,
the desorption efficiency of P may increase due to ion competition for
adsorption sites or the charge shielding effect at increasing IS.
On the other hand, the desorption efficiency of MHCs showed slightvariations under different ionic strengths, especially at 0.01 M and 0.1
M, with minimal R differences. This may suggest that the presence of
humic acid reduces the influence of IS on the adsorption-desorption
process of P. The presence of humic acid may enhance the hydrationof the montmorillonite surface, allowing water molecules to interveneand further stabilize the phosphate ions adsorbed on the surface, making
them less prone to displacement by other ions in high ionicenvironments.
This study employed the desorption hysteresis index (HI) to quantify
the ease of P desorption from colloids (Table 2). An HI value of 0 in­
dicates a complete overlap between the adsorption and desorption iso­therms, indicating no desorption hysteresis. Higher HI values indicate
more pronounced desorption hysteresis phenomena. In most cases, HI
6

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Chemosphere 364 (2024) 143300

Fig. 4. Observed and fitted breakthrough and release curves for montmorillonite colloids (MCs) (a,b) and montmorillonite-humic acid hybrid colloids (MHCs) (c,d)
with phosphorus. Phase 1: attachment of the colloids at different ISs; Phase 2: elution with background electrolyte solution; Phase 3: elution with DI water. Note a
different concentration scale in the Phase 3 graph.
Table 3
Hydrodynamic diameter of the colloids, zeta potentials of colloids/sand, energy barrier and secondary minimum calculated by DLVO theory, and fitted parameters of
MCs/MHCs breakthrough curves during Phases 1 and 2.
colloid
type

IS
(M)

DH(nm)a

MCs

0.01

474 ±
18
557 ±
33
542 ±
29
565 ±
28

MCs

0.1

MHCs

0.01

MHCs

0.1

k1d ( × 10− 4min− 1)e

k1det ( × 10− 4min− 1)fk2d ( × 10− 4min− 1)g

R2h

− 0.5

22.50

0.53

3.86

0.994

i

NA

i

i

Zeta potential of
colloids (mV)b

Zeta potential ofsands (mV)

DLVO
φmax
(kT)c

φmin2
(kT)d

− 26 ± 1− 30 ± 3

245.9

NAi

− 21 ± 1

− 9±1

35.1

− 0.9

NA

− 35 ± 2− 30 ± 3

370.7

− 0.5

11.04

1.04

1.12

0.995

− 25 ± 1

− 9±1

49.2

− 0.8

25.12

0.06

6.47

0.960

NA

a

Hydrodynamic diameter of MCs or MHCs.
Zeta potential of colloid with phosphorus.
cEnergy barrier between colloids-sand.
dSecondary minimum of colloids-sand.
eFirst-order retention rate coefficient on site 1.
f
First-order detachment rate coefficient on site 1.
g
First-order retention rate coefficient on site 2.
hSquared Pearson correlation coefficient.
iMeans data not available.
badsorption of colloids increased with the adsorption amount of colloidson the sand surface (100 - M12), as shown in Table S1. This suggests thatcolloids tend to preferentially adsorb onto irreversible adsorption sites
on the solid phase surface. As these irreversible adsorption sites become
progressively occupied, colloids subsequently adsorb onto reversible
adsorption sites. This finding is consistent with previous research results(Li et al., 2017).
Furthermore, by measuring the concentration of P in the effluent, it
was observed that the transport behavior of P closely paralleled that ofcolloids (Fig. 4), indicating that phosphate in the effluent is primarilytransported along with the transport of colloids. The presence of humic
acids not only affects the transport of colloids but also plays a crucialrole in the co-migration of P. The interaction between humic acids andcolloids alters the surface charge of the colloids, making them morenegatively charged (Table 3). This increased negative charge reduces thelikelihood of colloid aggregation and deposition, thereby enhancing
their mobility and facilitating more effective phosphorus transport
through porous media. Additionally, humic acids were found to enhance
the adsorption capacity of colloids for phosphorus, leading to the for­mation of a more stable colloid-phosphorus complex. This stability re­duces the retention of phosphorus in the soil and increases its likelihoodof co-transport with colloids through subsurface environments.
The parameters (k1d, k1det and k2d) fitted using the Hydrus-1D modeland the goodness of fit (R2) for phases 1 and 2 are summarized in7

C. Wang et al.

Chemosphere 364 (2024) 143300

Table 3. The fitting parameters indicated that the two kinetic sites model
can effectively describe the experimental data, with R2 ranging from0.960 to 0.995. The k1d and k2d values for MCs and MHCs both showed
an increase as IS increased from 0.01 M to 0.1 M, indicating that theelectrostatic repulsion between the colloids and sands decreased underhigher IS. This led to a higher rate of deposition. At an ionic strength of
0.01 M, the adsorption rates (k1d and k2d) of MCs are higher than thoseof MHCs, suggesting that humic acids reduce the adsorption of colloidson sands. Due to the extremely small amount of MCs collected in theeffluent for IS of 0.1 M, the breakthrough curve can hardly be analyzed,
resulting in extremely high fitted nonunique adsorption rates (k1d andk2d). Conversely, a certain amount of MHCs can be collected in theeffluent due to the presence of humic acids, allowing for the charac­terization of its breakthrough curve, with the adsorption rates (k1d andk2d) being well fitted by the model.

Data availability

4. Conclusion

References

The montmorillonite-humic acid hybrid colloids (MHCs) exhibit
humic acid attachment on the surface of montmorillonite, forming
various aggregated structures as observed in the TEM images. XPS re­sults indicate a higher carbon content in MHCs compared to MCs, which
was further confirmed by FTIR spectra, which showed the impact ofhumic acid on the surface functional groups of MHCs. The FTIR spec­
troscopy analysis revealed ligand exchange reactions with phosphate (P)ions during adsorption. In 0.01 M and 0.1 M NaCl solutions, themaximum theoretical adsorption capacity of MCs for P were 0.437 and
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in real-world situations. Moreover, the influence of other environmental
factors such as changes in pH levels, composition of organic matter, andtemperature fluctuations should be thoroughly examined. Lastly, thelong-term stability of humic acid-colloid-phosphorus complexes within
groundwater systems requires additional investigation.

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Data will be made available on request.
Acknowledgment
This work was supported by the National Natural Science Foundationof China (42477317), China, Chinese Universities Scientific Fund
(15053343), China and Key R&D Program Projects in Xinjiang(2023B02002-1), China.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.chemosphere.2024.143300.

CRediT authorship contribution statement
Changxi Wang: Writing – review & editing, Writing – original draft,
Visualization, Investigation, Formal analysis. Simin Yao: Writing –original draft, Visualization, Investigation, Formal analysis, Data cura­tion. Renkuan Liao: Writing – review & editing, Writing – originaldraft, Supervision, Resources, Methodology, Funding acquisition, Data
curation, Conceptualization. Jirka Šimůnek: Writing – review & edit­
ing, Supervision, Software.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.

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