**4. Kinetic sorption: mechanisms involved during sorption of ionisable and non-ionisable herbicides on VADS**

Sorption processes are known to be important because they are time-dependent processes with considerable ecosystem impact, influencing the availability of organic pollutants for plant uptake, microbial degradation and transport in soil and consequently the leaching potential. Sorption kinetic studies may provide important information related to weed control, crop toxicity, runoff, sorption mechanisms, solute transport mechanisms and its applied use in decontamination, such us remediation [7, 22]. This process occurs within the boundary layer around the sorbent, being conceptualised as a rapid uptake process to readily available sorption sites and proceeds in the liquid-filled pores (external mass transfer steps; (EMT)) or along the walls of the pores of the sorbent (internal mass transfer steps (IMT)) followed by slow diffusion-immobilisation in micropores or capillaries of the sorbent's internal structure (intra-particle diffusion, IPD), except for small sorbed amounts that appear on the external surface, and can occur during the transport of pesticides in soils. The third stage is the sorption of the solute in the inner surface of the sorbent through mass-action-controlled mechanisms where a rapid uptake occurs or surface reaction through interactions between solute and surface functional groups (such as chemisorption) [7, 9, 27].

The herbicide-VADS interaction is a time-dependent process that often progresses rapidly over the short term (minutes or hours), and it may also take a short time to reach equilibrium on VADS (**Figure 1**). Sorption process has influence on the transport of pesticides in the soil environment during the short term [4, 7, 9, 15]. The *pseudo-second-order* model has been the best sorption kinetic model to establish principal kinetic parameters and modelling of sorption process of INIH on VADS (**Figure 1**). On the other hand, the *Weber-Morris* model is one of the most used models to describe solute transport mechanisms of organic compounds in different sorbents intended for remediation purposes. Nevertheless, the *two-site non-equilibrium* (TSNE) model has been the best kinetic model to describe INIH transport mechanisms on VADS (**Figure 1B**; **Table 1**).

**Figure 1A** and **B** shows different transport mechanisms of MSM and diuron (DI) on ultisol (Collipulli soil, COLL) and andisol (Nueva Braunau soil, NBR). The *Weber-Morris* model indicates that the mass transfer across the boundary layer and IPD control DI sorption on all ChVADSs (**Figure 1A**; **Table 1**). The non-ionic or hydrophobic herbicide sorption on VADS has been described as a two-site equilibrium-kinetic process, where intra-OM diffusion has been suggested to be the predominant factor responsible for non-equilibrium sorption on andisols. In contrast, the MSM sorption on ultisols was controlled exclusively by IPD; thus, the

**111**

*Impact of Physical/Chemical Properties of Volcanic Ash-Derived Soils on Mechanisms Involved…*

first line (at shorter time) will depict macropore diffusion in the boundary layer, with the EMT occurring at short contact times during the retention of MSM into macropores and the second line (at longer time) accounts for the gradual sorption

**Pesticide-VADS Kinetics sorption description and/or model Ref.**

AT sorption on VADS was controlled by instantaneous equilibrium followed by a time-dependent phase. Andisols: two rate-limited phases were established; the first one related to intra-sorbent diffusion in OM and second one related to slow IPD in the organo-mineral complex. Ultisols: only one rate-limited phase attributed to kaolinite associated to slow and

GPS sorption kinetics on ChVADS followed a *pseudo-second-order* model with an apparent equilibrium reached in 10–120 min. The faster GPS sorption of andisols was related to higher OC content. The slower GPS

A rapid adsorption was observed during the first few minutes, followed by a slower process that in all cases reached an apparent equilibrium within 2 h. The order of the kinetic reaction was two. The slow process may be attributed to the diffusion of MBT within the porous of the soil matrix. On the sorption kinetic for non-allophanic soils, OM amendment increased the Xmax values and decreased the rate constant, pointing out a higher porosity

MSM sorption kinetics on ChVADS followed a *pseudo-second-order* and the *Weber-Morris* model, indicating that the mass transfer across the boundary layer and IPD are the two mechanisms controlling MSM sorption on andisol, whereas in ultisols, the rate was controlled exclusively by IPD into macropores/micropores. The *two-site nonequilibrium* (TSNE) model was the best kinetic model to be applied to VADS. Andisol presented an initial phase with a fast trend to equilibrium, where ∼50% of sites accounted for instantaneous MSM sorption. Most ultisol sites corresponded to the time-

The *pseudo-second-order* model was able to describe DI sorption at all time intervals for all soils. The *Weber-Morris* model indicated that mass transfer across the boundary layer and IPD were the two processes controlling sorption kinetics of DI in all ChVADSs, being corroborated by the *Boyd* model. An initial phase, with a fast trend to equilibrium, was established for the andisols through the TSNE model, where ∼ 51% of sites account for instantaneous sorption on andisols. For the ultisols, most of the sites

corresponded to the time-dependent stage of sorption

; R2

Pesticide-VADS Sorption description model Ref.

AT sorption on ChVADS was well described by the *Freundlich* model

of L-type (Nf < 1) and a concentration-dependent AT solid-solution distribution. The mechanisms involved are hydrogen bonding and charge transfer. AT in acidic soil, for its weak basic nature, presents a partial protonation of amine groups, which are involved in the hydrogen bonds with carbonyl and carboxyl groups of soil. These contribute to its protonation, forming a partial positive charge in the aromatic ring

Sorption data were well described by the *Freundlich* model (*Kf* between 2.2

andisols being adsorbed mainly through hydrophobic interactions or van der Waals forces at the phenolic groups of OM (humic acid and humin fraction). AT was weakly sorbed on soil with permanent negative charge (ultisols) through a hydrophobic bonding at the silanol surface sites of the tetrahedral sheets present

on clays, such as chlorite, gibbsite, goethite/hematite and kaolinite

= 0.98), exhibiting non-linear isotherms

; R2 ≥ 0.993). The highest sorption was observed on

sorption kinetic on ultisols was controlled by IPD

AT approached equilibrium in about 12 h [14]

[15]

[4]

[33]

[9]

[7]

[14]

[15]

*DOI: http://dx.doi.org/10.5772/intechopen.81155*

progressive sorption

of the soil matrix

dependent stage of sorption

(*K <sup>f</sup>* = 1.19 mg(1−N)LNkg−<sup>1</sup>

and 15.6 μg1−1/nmL1/ng−<sup>1</sup>

AT-Andisol (Southern, Chile)

AT-acidic andisols and ultisols (Southern Chile). Soils presented negative net charge at pHsoil

GPS-acidic ultisol and andisol (Southern Chile)

T-non-allophanic (Central Chile) and allophanic soils (Southern Chile). Soil pH range: 6.3–7.4

MSM-acidic ultisols and andisols (Southern

DI-acidic ultisols, inceptisol and andisol (Southern

Chile)

Chile)

AT-Andisol (Southern Chile)

AT-acidic andisols and ultisols (Southern Chile). Soils presented negative net charge at pHsoil

first line (at shorter time) will depict macropore diffusion in the boundary layer, with the EMT occurring at short contact times during the retention of MSM into macropores and the second line (at longer time) accounts for the gradual sorption


*Advanced Sorption Process Applications*

(i.e. the adsorbate behaves ideally) [26].

**and non-ionisable herbicides on VADS**

tional groups (such as chemisorption) [7, 9, 27].

mechanisms on VADS (**Figure 1B**; **Table 1**).

INIH on VADS (**Figure 1**). *Freundlich* model with (1/n) < 1 (L-type) indicates a heterogeneous sorption site, a strong adsorbent affinity for the adsorbate, diversity of sorption mechanisms and strong concentration dependence of sorption for the sorption sites [4, 14, 15, 19]. Isotherms with (1/n) > 1 (S-type) indicate competition between solvation water and adsorbate for the sorption sites [19]. The *Langmuir* model assumes: (1) sorption occurs on planar surfaces that have a fixed number of sites that are identical and the sites can hold only one molecule; thus, only monolayer coverage is permitted, which represents maximum sorption; (2) sorption is reversible; (3) there is no lateral movement of molecules on the surface and (4) the sorption energy is the same for all sites and independent of surface coverage (i.e. the surface is homogeneous), and there is no interaction between adsorbate molecules

**4. Kinetic sorption: mechanisms involved during sorption of ionisable** 

processes with considerable ecosystem impact, influencing the availability of organic pollutants for plant uptake, microbial degradation and transport in soil and consequently the leaching potential. Sorption kinetic studies may provide important information related to weed control, crop toxicity, runoff, sorption mechanisms, solute transport mechanisms and its applied use in decontamination, such us remediation [7, 22]. This process occurs within the boundary layer around the sorbent, being conceptualised as a rapid uptake process to readily available sorption sites and proceeds in the liquid-filled pores (external mass transfer steps; (EMT)) or along the walls of the pores of the sorbent (internal mass transfer steps (IMT)) followed by slow diffusion-immobilisation in micropores or capillaries of the sorbent's internal structure (intra-particle diffusion, IPD), except for small sorbed amounts that appear on the external surface, and can occur during the transport of pesticides in soils. The third stage is the sorption of the solute in the inner surface of the sorbent through mass-action-controlled mechanisms where a rapid uptake occurs or surface reaction through interactions between solute and surface func-

The herbicide-VADS interaction is a time-dependent process that often progresses rapidly over the short term (minutes or hours), and it may also take a short time to reach equilibrium on VADS (**Figure 1**). Sorption process has influence on the transport of pesticides in the soil environment during the short term [4, 7, 9, 15]. The *pseudo-second-order* model has been the best sorption kinetic model to establish principal kinetic parameters and modelling of sorption process of INIH on VADS (**Figure 1**). On the other hand, the *Weber-Morris* model is one of the most used models to describe solute transport mechanisms of organic compounds in different sorbents intended for remediation purposes. Nevertheless, the *two-site non-equilibrium* (TSNE) model has been the best kinetic model to describe INIH transport

**Figure 1A** and **B** shows different transport mechanisms of MSM and diuron (DI) on ultisol (Collipulli soil, COLL) and andisol (Nueva Braunau soil, NBR). The *Weber-Morris* model indicates that the mass transfer across the boundary layer and IPD control DI sorption on all ChVADSs (**Figure 1A**; **Table 1**). The non-ionic or hydrophobic herbicide sorption on VADS has been described as a two-site equilibrium-kinetic process, where intra-OM diffusion has been suggested to be the predominant factor responsible for non-equilibrium sorption on andisols. In contrast, the MSM sorption on ultisols was controlled exclusively by IPD; thus, the

Sorption processes are known to be important because they are time-dependent

**110**


**113**

(**Figure 1A**; **Table 1**).

*ultisol (•) and andisol (Ο) [7, 9].*

*Impact of Physical/Chemical Properties of Volcanic Ash-Derived Soils on Mechanisms Involved…*

attracted to the positively charged exchangeable Al ion

*Atrazine (AT), bensulfuon-methyl (BSM), chlorpyrifos (CPF), chlorothalonil (CTL), deethylatrazine (DEA), diazinon (DZN), diuron (DI); glyphosate (GPS) and methabenzthiazuron (MBT), metsulfuron-methyl (MSM), aminomethylphosphonic acid (AMPA), 3,5,6-trichloro-2-pyridinol (TCP); isopropyl-4-methyl-6 hydroxypyrimidine* 

**Pesticide-VADS Kinetics sorption description and/or model Ref.**

Surface area, Al/Fe oxide content, OC content, pH, soil phosphate and exchangeable Al content, and active surface hydroxyls derived from the active and free metal (hydr)oxides, such as allophane, imogolite, ferrihydrite, goethite and metal-SOM complexes might have an important role in the carboxylic acid herbicide sorption. 2,4-D sorption on andosol was regulated by ion exchange reaction and/or a ligand exchange reaction in which the active surface hydroxyls on Al and Fe were replaced by the carboxylic group of 2,4-D. 2,4-D may form surface complexes with exchangeable Al ions (via a cation-bridging mechanism involving an exchangeable Al and the carboxylate group of 2,4-D) or be electrostatically [31]

stage, where the molecules of MSM diffuse through the smaller pores of the soil (IPD) (**Figure 1A**; **Table 1**). On the other hand, the mass transfer across the boundary layer and IPD were the two mechanisms to control MSM sorption on andisol

*(A) IPD plots for MSM kinetic sorption on ultisol (▲) and andisol (Δ); and DI kinetic sorption on ultisol (•) and ndisol (Ο); (B) TSNE model plot for MSM sorption on ultisol (▲) and andisol (Δ) and DI sorption on* 

Time-dependent sorption (or non-ideal sorption) can be a result of physical and chemical non-equilibrium [9]. Non-equilibrium sorption on soils has been attributed to several factors, such as: diffusive mass transfer resistances, non-linearity in sorption isotherms, sorption-desorption non-singularity and rate-limited sorption reactions [8]. The rate-limited diffusion of the sorbate from bulk solution to the external surface of the sorbent, and rate-limited diffusion within mesopores and micropores of the soil matrix, will occur before the equilibrium is reached. The TSNE model (**Figure 1B**; **Table 1**) indicated that MSM sorption on andisol presented an initial phase with a fast trend to equilibrium, where about 50% of sites accounted for the instantaneous stage and the great part of sites on ultisols corresponded to the time-dependent stage of sorption (90%). In contrast, the sorption of non-ionisable herbicide (DI) on andisols presented an initial phase, with a fast trend to equilibrium,

*DOI: http://dx.doi.org/10.5772/intechopen.81155*

*(IMHP) and 2,4-dichlorophenoxyacetic acid (2,4-D).*

*Kinetics sorption and sorption-desorption of INIH on VADS.*

2,4-D-SOM-rich acidic andisol (Japan)

**Table 1.**

**Figure 1.**

*Impact of Physical/Chemical Properties of Volcanic Ash-Derived Soils on Mechanisms Involved… DOI: http://dx.doi.org/10.5772/intechopen.81155*


*Atrazine (AT), bensulfuon-methyl (BSM), chlorpyrifos (CPF), chlorothalonil (CTL), deethylatrazine (DEA), diazinon (DZN), diuron (DI); glyphosate (GPS) and methabenzthiazuron (MBT), metsulfuron-methyl (MSM), aminomethylphosphonic acid (AMPA), 3,5,6-trichloro-2-pyridinol (TCP); isopropyl-4-methyl-6 hydroxypyrimidine (IMHP) and 2,4-dichlorophenoxyacetic acid (2,4-D).*

#### **Table 1.**

*Advanced Sorption Process Applications*

(R2

distribution of SOH, SOH2

AlSiFe-HA

mL1/ng−<sup>1</sup>

ultisol

runoff process

between 3.1 and 14.4 μg1−1/nmL1/ng−<sup>1</sup>

edges (Si-OH) of kaolinite or silicate edges

hydrophobic interactions

R2

BSM-acidic andisols and ultisols (Southern Chile) which presented negative net charge at pHsoil

GPS-acidic ultisol and andisol (Southern Chile)

AT, CPF, CTL, DZN, GPS and MSM, DEA, AMPA, TCP and IMHP-acidic ultisol and andisol (Southern Chile). All soils present a negative net charge at pHsoil

MBT-nonallophanic (Central Chile) and allophanic (Southern Chile) soils. The pH values of natural soils (6.3–7.4)

MSM-ultisol and andisol (Southern

2,4-D-oxisols (Brazil), andisols (South Korea), ultisols (Costa Rica), and alfisols (Toronto)

Chile)

**Pesticide-VADS Kinetics sorption description and/or model Ref.**

+

Data fitted to the *Freundlich* model with R2

andisols; *Kf*>1500 μg1−1/nmL1/ng−<sup>1</sup>

BSM sorption on ChVADS was well described by the *Freundlich* model

 = 0.995), exhibiting non-linear isotherms of L-type. BSM was weakly sorbed on soil with permanent negative charge (ultisols). A multifactorial influence of several properties of soils was demonstrated, through a partial least squares regression model (P value = 0.0042) by use of % OC, % Fe and surface area. The BSM sorption on mineral (AlSi-Fe) and mineralorganic complexes (AlSi-Fe-HA) confirmed the participation of variablecharge materials present in VADS, both with a high sorption capacity, demonstrating the effect of BSM sorption on charge of both sorbents. The

GPS sorption on ChVADS over the extended range of concentrations was well described by *Freundlich* model (*Kf* between 1480 and 3764 μg1−1/n

adsorbed to Fe/Al oxides and allophane by ligand exchange through its phosphonic acid moiety or by hydrogen bonding reacting with polyvalent cations adsorbed on SOM between GPS and humic substances (metal-GPS OM complex). GPS was adsorbed strongly and specifically to kaolinite on

metabolites (except GPS) showed a markedly higher sorption on allophanic soil exhibiting a non-linear isotherm of L-type (Nf < 1) with a trend to the saturation of sorption sites at higher concentrations. The exceptionally high sorption of GPS on ultisols (with lower OM, AlOx and FeOx contents than

acidic pH, posing the need to establish its possible contamination

The isotherms fit the *Freundlich* model (*K <sup>f</sup>* between 5.3 and 82.1 cm3

R2 ≥ 0.998) exhibiting a non-linear isotherm of L-type (Nf < 1). The MBT sorption was markedly higher on allophanic soils with OM being the principal component responsible for MBT sorption. The clay and smectite contents on non-allophanic soils were considered the most important factors governing the MBT sorption. The interaction was predominantly by physical bonding (van der Walls forces or hydrogen bonding) between the amino hydrogen of MBT and hydroxyl groups of humic acids of SOM, or through the carbonyl group, which could act as a strong donor to hydrogens of the alcoholic and phenolic groups on the humic acid of SOM. The presence of MBT in surface waters in Chile would be produced only by losses from the sediment-adsorbed MBT through

The *Freundlich* model described MSM sorption in all ChVADSs (Kf values

capacity on low variable-charge soils (Chilean ultisols) was attributed to the low OM content. The kaolinite mineral group as major constituent of the inorganic fraction of ultisols and minerals, such as allophane, gibbsite, hematite and goethite, contributed to MSM sorption mainly through hydrophilic interactions. The OM and active/free Fe/Al oxides controlled the MSM sorption in andisols mainly through hydrophilic rather than

Linear sorption model fitted to isotherms measured from CaCl2, CaSO4, Ca(H2PO4)2 and KCl systems (Kd values between 1.28 and 18.5 L Kg<sup>−</sup><sup>1</sup>

 > 0.96). The higher 2,4-D sorption was obtained on CaCl2, reflecting both effects of inorganic anion in terms of competition and cation in terms of bridging interactions. 2,4-D sorption via Ca-bridging took place on silanol

; R2

; R2 ≥ 0.99). The highest sorption was observed on andisols being

and SO<sup>−</sup> sites was better characterised for

> 0.97. All herbicides and

> 0.992). The lower MSM sorption

) was related to kaolinite contents and

[16]

[4]

[3]

[33]

[5]

[17]

 g<sup>−</sup><sup>1</sup> ;

;

**112**

*Kinetics sorption and sorption-desorption of INIH on VADS.*

#### **Figure 1.**

*(A) IPD plots for MSM kinetic sorption on ultisol (▲) and andisol (Δ); and DI kinetic sorption on ultisol (•) and ndisol (Ο); (B) TSNE model plot for MSM sorption on ultisol (▲) and andisol (Δ) and DI sorption on ultisol (•) and andisol (Ο) [7, 9].*

stage, where the molecules of MSM diffuse through the smaller pores of the soil (IPD) (**Figure 1A**; **Table 1**). On the other hand, the mass transfer across the boundary layer and IPD were the two mechanisms to control MSM sorption on andisol (**Figure 1A**; **Table 1**).

Time-dependent sorption (or non-ideal sorption) can be a result of physical and chemical non-equilibrium [9]. Non-equilibrium sorption on soils has been attributed to several factors, such as: diffusive mass transfer resistances, non-linearity in sorption isotherms, sorption-desorption non-singularity and rate-limited sorption reactions [8]. The rate-limited diffusion of the sorbate from bulk solution to the external surface of the sorbent, and rate-limited diffusion within mesopores and micropores of the soil matrix, will occur before the equilibrium is reached. The TSNE model (**Figure 1B**; **Table 1**) indicated that MSM sorption on andisol presented an initial phase with a fast trend to equilibrium, where about 50% of sites accounted for the instantaneous stage and the great part of sites on ultisols corresponded to the time-dependent stage of sorption (90%). In contrast, the sorption of non-ionisable herbicide (DI) on andisols presented an initial phase, with a fast trend to equilibrium, where∼20% of sites accounted for instantaneous sorption on andisol. For the ultisols, most of the sites corresponded to the time-dependent stage of sorption (90%).

The higher value in the overall rate constant, k2, of MSM on andisols with respect to DI in the same soil indicates that this value reflects contributions from the favoured electrostatic interactions considering both a retarded IPD as well as intra-OM diffusion. The way minerals present on VADS are interrelated or chemically spatially distributed, either being freely distributed throughout the soil mass or coating silt and clay grains, is determinant of their chemical role in the whole ion sorption-desorption mechanisms [28]. In this sense, the OM content is the principal component to control the pesticide sorption on andisols, as much by instantaneous equilibrium as by IPD, the presence of kaolinite, halloysite and Al/Fe oxides in Ultisols will be significant in the IPD mechanism. According to this analysis, ultisols present a potential risk of ionisable herbicide transport. The different mineral composition of ultisols impacts on their different physical behaviour, influencing the slowest INIH sorption rate, the sorption mechanism involved and the lowest INIH sorption capacity. All of the above must be taken into account to evaluate the potential leaching of INIH in these kinds of soils.
