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African Journal of Environmental Economics and Management ISSN 2375-0707 Vol. 7 (4), pp. 001-007, April, 
2019. Available online at www.internationalscholarsjournals.org © International Scholars Journals 

 

Author(s) retain the copyright of this article. 
 
 
 

Full Length Research Paper 

 

Effect of soil pH on sorption of salinomycin in clay 
and sandy soils 

 
R. Jayashree*, Shiv O. Prasher, Rajvinder Kaur and Ramanbhai M. Patel 

 
Department of Bioresource Engineering, MacDonald campus of McGill University, 21111, Lakeshore 

road, Sainte Anne de Bellevue, Quebec, Canada- H9X 3V9, Canada. 
 

Accepted 20 September, 2018 
 
Salinomycin is polyether ionophore, commonly used in poultry industry for the prevention of coccidial 
infections and promotion of growth. A large amount of the administered antibiotic is excreted as parent 
compound, eventually reaching agricultural lands. This makes it imperative for researchers to understand the 
behavior of the compound in soil environment by conducting sorption-desorption studies. In this study, 
sorption of salinomycin was measured in four agricultural soils, a clay soil with low organic matter content 
(LOM), a clay soil with high organic matter content (HOM), a sandy soil with HOM, and a loamy sandy (LOM) 
soils, at three pH levels, namely 4, 7 and 9. Desorption studies was carried out using the batch equilibration 
technique. It was observed that more than 98% salinomycin was strongly sorbed by all soils, irrespective of 
the soil organic matter content or soil pH. The sorption of salinomycin to the sandy soil marginally increased 
as the pH decreased, while the sorption to the two clay soils marginally increased as the pH increased. 
Desorption of salinomycin with methanol over a 72 h period was < 0.2% of the amount added; however, it was 
> 70% with a phosphate buffer (pH 7). Since the phosphate buffer would mimic, to some extent, the quality of 
water flowing through field soils containing various salts, it was concluded that salinomycin could pose 
significant threats to both shallow ground water and surface water bodies. 

 
Key words: Salinomycin, sorption, pH, desorption, environmental pollution, phosphate buffer. 

 
INTRODUCTION 

 
Antibiotics have been detected worldwide in soil, surface 
water, ground water, and sediment (Kolpin et al., 2002; 
Christian et al., 2003; Kim and Carlson, 2006). The 
persistence of antibiotics in terrestrial environments ranges 
from less than one day to weeks or even months, depending 
primarily on temperature and the chemical structure of the 
antibiotic (Rabolle and Splid, 2000). According to the rate of 
degradation and the sorptive properties, the parent 
substance or its metabolites may reach aquatic 
environments through surface runoff or leaching through the 

soil profile. Key chemical properties, such as water 
solubility, soil pH, volatility, and sorption, influence 
antibiotic transport in soils. Typical manure may also 
contain high levels of ammonia that would tend to 
increase the pH of soil solution, thus affecting the  
 
 
 
*Corresponding author. E- mail: jayashree.ramaswamy@gmail. 

com. 

 
 
 

 
sorption of certain compounds.  

Salinomycin is a naturally occurring, monocarboxilic, 
polyether, and antibiotic, elaborated by a strain of 
Streptomyces abbes (ATCC – 21838). Salinomycin is used 
for the prevention of coccidiosis in broiler chickens. The 
highest usage in USA is estimated to be 454 tons of 
salinomycin (active ingredient). Salinomycin is also 
approved for use as a cattle feed additive (BIO GRO) in the 
range of 5 - 10 g ton

-1
 of complete feed. Once administered, 

it can persist in organisms. A study conducted using radio 

labelled (C
14

) reported that the salinomycin persists in 
food chain organisms, with moderately low 
biodegradability (EIA, 1987). Salinomycin is resistant to 
aerobic degradation. Its relatively low aqueous solubility 

(3.4 mg ml
-1

) limits its availability for biological 
degradation, causing it to persist for longer periods in the 
environment (European Food Safety Authority (EFSA), 
2004).  

The potential antibacterial effect of salinomycin in the 
environment would be against gram positive bacteria, at 



   
 
 
 
 
 
 
 
 
 
 
 

Figure 1. Structure of salinomycin. 
 

 

Table 1. Characteristics of salinomycin.  
 

Salinomycin Free Acid 

Molecular formula C42H70O11 

Molecular weight 750 

Melting point (C°) 112.5 – 113.5 

Pka 6.4 

Water solubility 3.4 mg mL
-1

 readily soluble in methanol 

Stability Unstable in acidic condition stable in alkali condition 
 

a
 Pka = acid dissociation constant.

 

 

 

levels of 0.39 µg ml
-1

 and above. It inhibits bacterial 

protein synthesis through the attachment to 9 transfer 
RNA binding sites on the 30 s ribosomal unit (Cater et al., 
2000). It could change microbial populations in the 
environment. This, in turn, could undermine the ability of 
microbes to degrade other pollutants such as pesticides 
(Boxall et al., 2003). This could also have deleterious 
effects on important chemical cycles, such as nitrification 
and denitrification (Frestegard et al., 1997). Salinomycin 
is unstable under acidic conditions and stable under 
alkaline conditions (EFSA, 2004); therefore, its soil 
sorption is expected to be pH-dependent. Currently, 
information about sorption of salinomycin to soil is not 
readily available. Therefore, the objective of this study 
was to investigate the sorption behavior of salinomycin in 
four agricultural soils at three different pH levels (4, 7 and 
9). 
 

 
MATERIALS AND METHODS 
 
Chemicals 
 
Only analytical reagent grade or high purity chemicals were used in 
this study. Sodium phosphate (Mono and di basic), methanol, 
ammonium hydroxide and acetic acid were purchased from Sigma. 
The salinomycin standard was obtained from Sigma Aldrich 
Company. The structure of salinomycin is shown in Figure 1 and 
selected properties are given in Table 1. The stock solution of 
salinomycin was prepared by dissolving 10 mg salinomycin in 10 ml 
of methanol (MeOH), and stored at 4°C. Standard solutions were 
freshly prepared by diluting the stock solution with methanol in vials. 
 

 
Soil characterization 
 
Four agricultural soils  with  no  previous  history  of exposure  to 

 
 
 

 

salinomycin were collected from the Macdonald Campus Farm of 
McGill University in Ste-Anne de Bellevue, Quebec, Canada. 
Bearbrook soil is a dark-brown to grayish-brown clay having 
granular structure. Generally free carbonate is absent in upper part 
of this soil. Soils having high organic matter (HOM-clay) and low 
organic matter (LOM-clay) were collected from this soil series. The 
third soil was a Dalhousie sandy soil with high organic matter 
(HOM-sand). Dalhousie soil was developed from lacustrine 
material, deposited as a thick covering. It is slightly alkaline. The 
fourth soil was Chicot loamy sand having low organic matter (LOM-
loamy sand). This soil is formed from thin alluvial material. All the 
soils were air-dried, passed through 2 mm sieve, and stored in 
plastic containers at room temperature for further analysis. Physical 
and chemical properties of the soils were determined using 
standard methods: pH in a 2:5 soil:water slurry (Trivedy and Goel, 

1986), organic carbon by wet digestion with K2Cr2O7 and H2SO4 
(Walkey and Black, 1934), texture (by hydrometer method), and 
cation exchange capacity (CEC) by atomic adsorption 
spectrometer. The various soil characteristics are listed in Tables 2 
and 3. 

 

pH adjustment 
 
The initial pH of the two clay (LOM and HOM), loamy sand and 
sandy soils were 6.7, 6.4, 6.9 and 6.7, respectively, (Table 2). All 
four soils were equilibrated with 1 M concentrations of HCL and 
NaOH. The pH was adjusted eight times over 20 days to stabilize at 
desired levels of 4 and 9. Excess liquid from each soil sample was 
drained after pH adjustment, and soils were allowed to air dry for 48 
h. Soils were pulverized, rechecked for pH, and used for sorption 
studies. 
 
 
Sorption 

 
The sorption experiments were performed with four different 

concentrations of salinomycin, 0.5, 5, 10 and 25 mg L
-1

, in 
methanol. The UV-radiated sterile soils were used for sorption 
studies in order to minimize the effect of microbial growth and 
activity on the results. 20 mL of salinomycin solution was 



  
 
 

 
Table 2. Physio-chemical characteristics of soils used in this study.  

 

Soil type OC (%) TOC (%) OM (%) pH 
 Soil Texture  

 

Sand (%) Silt (%) Clay (%) 
 

     
 

Clay 4.31 5.7 9.88 6.7 30 20 50 
 

Clay 5.35 7.13 12.3 6.4 10 20 70 
 

Loamy sand 2.25 3 5.17 6.9 87.5 5.1 7.5 
 

Sand 3.9 5.2 8.96 6.7 92.5 2.5 5 
  

a
 OC:Organic carbon, TOC:Total organic carbon, OM:Organic matter.

 

 
 
 

 

A
m

o
u

n
t 

a
d

s
o

rb
e
d

 (
µ

g
/g

) 

 
 

 

10.1 
 
10.0 
 

9.9 
 

9.8 
 

9.7 
 

9.6 
 

9.5 
 

pH4 

 
 

 
 

 101 

(%
)  

 100 
 

 99 

ad
so

rb
ed
 

 

  
 

 98  
 

 97 

A
m

o
u

n

t 

 

 96  

  
 

 95  
 

pH7 pH9  
  

 
 

 Clay (HOM)  Clay (LOM)  Sand (HOM)  Loamy sand(LOM) 
 

Figure 2. Effect of pH on adsorption of salinomycin at 10 µg/g soil. 
 
 
 
added to the 1 g of sterile soil in a 100 ml conical flask to obtain 

final concentrations of 10, 100, 250, and 500 µg g
-1

 of soil. 
Moreover, 20 ml of methanol was added to 1 g of soil for the blank, 
and 20 ml of each concentration of salinomycin solutions were kept 
in flasks, without soil, as controls. All samples were agitated for 24 
h on a rotary shaker. After centrifugation at 3000 g for 30 min, 
supernatants were transferred to glass vials, filtered through 0.45 
µm filters, and subsequently analyzed by HPLC-CAD (Charged 
Aerosol Detector), to determine the amount of salinomycin 
remaining in solution. The sorbed salinomycin concentration was 
calculated using the following equation:  

VAQ  CI -C  
 

S     
 

MS (1)  
 

  
 
Where S is salinomycin sorbed to soil (µg g

-1
), Vaq is the initial 

volume (L), Ms is the soil mass (g), Ci is the initial concentration (µg 

L
-1

) and C is the concentration remaining in the solution phase after 

equilibrium (µg L
-1

).  
As will be evidenced by the results presented later on, the four 

soils used in this study fully adsorbed the antibiotic. Therefore, it 
became necessary to carry out further sorption tests by increasing 
antibiotic concentrations in order to make sure that the sorption 
capacity of soil is reached and there is antibiotic left in the solution 
phase, and, more importantly, to rule out any experimental errors or 
extremely fast chemical degradation (no microbial degradation was 
possible because all soils were sterilized before use). The high-
concentration sorption tests were done on the sandy soil only. 
Since this soil had HOM, it was decided to purchase a LOM-sand 
from a commercial outfit (Reno Department, Pointe Claire, Quebec, 

 
 

 
Canada) and to include it in the sorption/desorption study. The 

highest concentration used was 5,000 µg g
-1

. An equal amount (20 
ml) of methanol was added to the soil, and the flasks were agitated 
for 24 h, centrifuged, as previously described, and this procedure 
was repeated up to 72 h. 
 

 
Analysis of salinomycin 

 
The filtered supernatants were analyzed for salinomycin using 
HPLC CAD (Charged Aerosol Detector), equipped with C18 
column. An elution gradient with methanol (80%), water (13%), 
ammonium hydroxide and acetic acid buffer (7%) (pH 5), and flow 
rate of 1 ml/min was followed. Salinomycin concentration was 
determined using calibration curves. 
 

 
Statistical analysis 
 
Results obtained in this study were analyzed using two-way  
ANOVA and multiple comparisons least significant difference (LSD).  
These tests were done using MATLAB, version 7.8. 
 

 

RESULTS AND DISCUSSION 

 
The sorption of salinomycin in two clays, loamy sand and 
sandy soils at three different pH levels is presented in 

Figures. 2, 3, 4 and 5 for 10, 100, 200, and 500 µg g
-1

 
initial concentrations, respectively. Figure 2 shows that, 



 
 
 
 
 

A
m

o
u

n
t 

a
d

s
o

rb
e
d

 (
µ

g
/g

) 

 
 
 

 

100.2 
 
100.0 
 

99.8 
 

99.6 
 

99.4 
 

99.2 
 

99.0 
 

pH4 

 
 
 

 
 

 100.2 

(%
)  

 100.0  

 

ad
so

rb

ed
 

 

 99.8  

  
 

 99.6  
 

 99.4 

A
m

o
u

n

t 

 

 99.2  

  
 

 99.0  
 

pH7 pH9  
  

 
 

 Clay (HOM)  Clay (LOM)  Sand (HOM)  Loamy sand(LOM) 

 
Figure 3. Effect of pH on adsorption of salinomycin at 100 µg/g soil. 

 
 

 

A
m

o
u

n
t 

a
d

s
o

rb
e

d
 (

µ
g

/g
) 

 
 

 
 

200.2  

200.0  

199.8  

199.6  

199.4  

199.2  

199.0  

pH4 pH7  

 
 

 
 

100.1 

(%
) 

 

100.0 
 

99.9 ad
so

rb

ed
 

 

 
 

99.8  
 

99.7 

A
m

o
u

n
t 

 

99.6 
 

 
 

99.5  
 

pH9  
  

 
 

 Clay (HOM)  Clay (LOM)  Sand (HOM)  Loamy sand(LOM) 
 

Figure 4. Effect of pH on adsorption of salinomycin at 200 µg/g soil. 
 
 

 

A
m

o
u

n
t 

a
d

s
o

rb
e
d

 (
µ

g
/g

) 

 
 

 

500.5 
 

500.0 
 

499.5 
 

499.0 
 

498.5 
 

498.0 
 

497.5 
 

pH4 

 
 

 
 

 100.1 
(%

) 
 

 100.0  

 

ad
so

rb
e

d 

 

 99.9 
 

  
 

 99.8 

A
m

o
u

n
t 

 

 99.7 
 

  
 

 99.6  
 

 99.5  
 

pH7 pH9  
  

 
 

 Clay (HOM)  Clay (LOM)  Sand (HOM)  Loamy sand(LOM) 
 

Figure 5. Effect of pH on adsorption of salinomycin at 500 µg/g soil. 



  
 
 

 

Table 3. Chemical characteristics of soils used in this study (in cmol(+) kg
-1

).  
 

 Soil Type Ca Mg K Na Mn Fe CEC 

 Clay (HOM) 12.77 3.03 0.31 0.213 0.05 0.0166 16.4 

 Clay (LOM) 16.87 5.95 0.453 0.233 0.03 0.02 23.553 

 Sand (HOM) 3.323 1.02 0.12 0.06 0.01 0.02 4.656 

 Loamy sand (LOM) 2.426 0.07 0.053 0.133 0.01 0.02 2.686 
 

a
 LOM-Low organic matter content; HOM-High organic matter content, CEC-Cation Exchange Capacity.

 

 
 

 

at 10 µg/g concentration, the sorption of salinomycin was 
higher (99.3~99.8%) in the sandy and loamy sandy soils, 
irrespective of organic matter content, at pH 4 and 7. It 
was slightly lower (95.5 - 97.0%) for the same at pH 9. 
Multiple comparison LSD analysis showed salinomycin 
sorption in HOM-clay to be significantly different (P < 
0.05) from that in LOM-clay, LOM-loamy sand, and HOM-

sandy soils. In the sandy soil, at 100 µg g
-1

 concentration, 
the sorption of salinomycin was 100% at pH 4, followed 
by 99.7-99.8% at pH 7 and 99.4-99.5% at pH 9 (Figure 

3). At 200 and 500 µg g
-1

 levels also, the trend was 
similar, although, sorption was slightly higher (Figures 4 
and 5). This shows that salinomycin is strongly sorbed to 
the sandy soil; more than 95% of its initial amount is 
sorbed, irrespective of the organic matter content or pH 
level. One noticeable observation is that the amount 
sorbed to both sandy and loamy sandy soils was 

relatively small at pH 9 with 10 µg g
-1

, as compared to the 
other pH levels. It was also observed that sorption was 
relatively higher in the soil with the higher organic matter 
content, as compared to the soil with the lower organic 
matter content. A significant difference (P < 0.05) was 
observed between HOM-clay and LOM-clay soils at 200 

µg g
-1

, and no significant difference was observed in the 
sandy and loamy sandy soils with low and high organic 
matter content.  

In the two clay soils, at 10 µg g
-1

 level, the sorption was 
slightly lower (97.8 - 98.3%) at pH 4, as compared to the 
other pH levels (99.9%) (Figure 2). A similar trend was 
also observed for the other three concentrations (100, 

200 and 500µg g
-1

), although 100% sorption was 
observed at pH 7 and 9 (Figures 3, 4 and 5). The results 
show that salinomycin was also strongly sorbed to the 
clay soils, irrespective of the organic matter content. 
However, at pH 4, the sorption of salinomycin was higher 
in the HOM-clay soil; the sorption was quite high at pH 7 
and 9 for both levels of organic matter content. A clear 
contrast was observed between the sandy and clay soils 

at 10 µg g
-1

 concentrations. The sorption was lowest in 
the case of clay soils at pH 4, whereas it occurred at pH 9 
in sandy soils. A significant difference (P < 0.05) was 
observed between pH 4, 7 and 9 in HOM- and LOM-clays 
and HOM-sandy and LOM-loamy sandy soils.  

Sorption decreased with increasing pH in sandy and 
loamy sandy soils but not in the two clay soils. As the clay 
soils have high OM content (9.88 - 12.3%), when pH 

 
 
 

 

increases, the humic molecules could have deprotonated, 
thereby increasing their negative charge. Also, aluminum 
ions that are initially complexed to the organic matter 
could have started to hydrolyze and eventually 
precipitated as aluminium hydroxide. These processes 
would also contribute to the increase in negative charge. 
Thus, clay soils have a higher pH-dependent CEC. To 
some extent, the increasing exposure of negative charge 
on humic molecules with increasing pH may over-
compensate the declining positive charge of salinomycin.  

Theoretically, the maximum sorption of salinomycin on 
the variable charge of organic matter surfaces would 

occur at a pH value close to the Pka value (acid 
dissociation constant). However, there may still be a 
substantial sorption, even at pH values which are several 

units higher than Pka, due to selective sorption of charged 
molecules by the organic matter (Berglof et al., 2002). 
Higher sorption of antibiotics on clay-rich soils has also 
been reported previously (Rabolle and Splid, 2000; Loke 
et al., 2002; Kumar et al., 2004). Thus, higher antibiotic 
adsorption on clay soils is mainly due to their greater 
exchange capacity because of the higher clay content 
and organic matter. Loke et al. (2002) also reported that 
binding of salinomycin was influenced by ionic binding to 

divalent metal ions such as Mg
2+

 and Ca
2+

. In clay loam 
soils, these ions were higher than in the sandy soils, thus 
this may have contributed to the higher sorption (Table 
3). Organic matter was found to have a small effect on 
the sorption of salinomycin, and it was evident from all 
four soils (Figures 2, 3, 4, and 5). All four soils used in 
this study were field soils and they had relatively high 
organic matter content (Table 2). Therefore, it was 
decided to investigate sorption behavior of salinomycin 
under low organic matter content conditions. To this end, 
construction sand was purchased from a local store with 
0.3% organic matter content. Again, the antibiotic 
sorption was found to be very high. It was decided to use 
even higher initial concentrations, that is, 5,000 and 

10,000 µg g
-1

), but still most of the antibiotic was sorbed. 
This may be due to salinomycin’s greater ability to 
coordinate with inorganic cations in solution, resulting in 
increased hydrophobicity of salinomycin. This effectively 
neutralizes the carboxylic acid’s negative charge. Also, 
salinomycin might interact with cations on soil exchange 
sites through cation bridging. To estimate the saturation 
point of salinomycin sorption in LOM-sandy soil, a very 



 
 
 

 

A
m

o
u

n
t 

d
e

s
o

rb
e

d
 (

u
g

/g
) 

 
 
 
 

 

4000  
 
3000 
 

2000 
 

1000 
 

0 
 

0 20 40 60 80 
 

Time (hours)  
 

 

Methanol Phosphate buffer 
 

Figure 6. Desorption of salinomycin in sandy soil. 
 

 

high initial concentration of 50,000 µg g
-1

 was used on 
the sandy soil since it was the least adsorbent amongst 

the four soils tested. It was found that nearly 35,000 µg g
-

1
 was sorbed in 24 h. This indicates the sandy soil has 

very high sorption capacity for salinomycin.  
Tolls (2001) reported that veterinary antibiotic sorption 

to soil cannot be predicted from compound Kow (Otanol-

water partition coefficient) and soil organic carbon (OC) 
content. Sithole et al. (2003) suggested that antibiotic 
interaction with soil organic matter occurs as a result of 
binding to divalent cations, ion exchange interactions, 
and hydrogen bonding between acidic groups in humic 
acids and polar groups on salinomycin. Binding to 
divalent metal cations has also been suggested to explain 
salinomycin sorption in both marine and manure 
sediments. In addition, the carbonyl and either oxygen 
atoms can coordinate with metal ions, resulting in some 
degree of encapsulation (Volmer and Lock, 1998; Paulus 
et al., 1998)). This coordination gives ionophores the 
unique ability to transport metal ions across cell 
membranes as either undissociated acids or neutral 
complexes and gives the molecule a largely hydrophobic 
exterior (Shen and Bradbelt, 2000; Paulus et al., 1998; 
Miao et al., 2003). Salinomycin has a higher affinity for 

Na
+
, K

+
, Ca

2+
 and Mg

2+
 with a strong preference for 

sodium (EFSA, 2004).  
Desorption of salinomycin was also performed in HOM-

sandy soil and the construction sand (LOM) at higher 

concentration (5000 µg g
-1

) using methanol. The 

desorption results are shown in Figure 6. While 
desorption was performed over a 72 h period, it was 
observed that only < 0.2% of the added amount desorbed 
from the soil (Figure 6). This suggests that once 
salinomycin is sorbed on to these soils, its leaching to 
lower depths may not occur. However, since the 
desorption was being done with deionized water which 
may not be a good representative of rainwater making its 
way through the soil profile, it was decided to use a 
phosphate buffer for desorption. The percolating water 

 
 

 

will tend to dissolve several salts, commonly present in 
field soils, (especially phosphates) this condition is well 
mimicked by the phosphate buffer solution.  

Shen and Bradbelt (2000) have explained the 
mechanism of desorption using a phosphate buffer. The 
polar functionality of the antibiotic molecule, such as 
salinomycin, is oriented towards the cation at the centre, 
while the outer solvent-accessible portion of the molecule 
is largely hydrophobic (Dobler, 1981; Sobott et al., 1997). 
Also, the carboxylic acid or ester groups, in addition to 
the tetrahydropyran or tetrahydrofuran groups in the 
backbone of ionophores like salinomycin can undergo 
anion exchange with soil to have a considerable anion 
exchange capacity, especially the high organic matter 
content soils. Therefore, highly selective phosphate 
anions are capable of desorbing salinomycin. The results 

from our study showed that, out of 5,000 µg g
-1

 

salinomycin, 3,450 µg g
-1

 had desorbed from the 
construction sand with the phosphate buffer after 24 h. 
When the desorption period was extended to 72 h, there 
was a minimal further increase in desorption - a total of 

3,525 µg g
-1

 was desorbed (Figure 6). This showed that 
although, salinomycin is strongly sorbed to soil, it could 
also readily move into underground aquifers and water 
bodies under field conditions. 
 

 

Conclusions 

 

Sorption of salinomycin in clay and two sandy soils was 
very high (> 98%). It also tended to slightly increase with 
increasing soil organic matter and clay contents. The 
influence of pH on the sorption of salinomycin in various 
soils was quite different. Sorption on the clay soil 
marginally increased as the pH increased, while sorption 
on the sandy and loamy sandy soil decreased as pH 
increased. If the rainwater were without any salts, 
salinomycin will strongly sorbed on to soil, with very little 
chances of it leaching into ground water. However, it was 



 
 
 

 

shown in this study that a phosphate buffer (pH 7) easily 
desorbed very tightly sorbed salinomycin. Since actual 
rainwater would contain dissolved salts, there is a high 
likelihood that it may desorb salinomycin, thus posing a 
greater threat to groundwater and, to some degree, 
surface water contamination. 
 

 
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