- Open Access
Impact of particle size, temperature and humic acid on sorption of uranium in agricultural soils of Punjab
© Kumar et al. 2015
- Received: 7 January 2015
- Accepted: 20 May 2015
- Published: 17 June 2015
Batch experiments were conducted to study the sorption of uranium (U) onto soil in deionised water as a function of its dosage, temperature and humic acid (HA). Furthermore, soils were characterized for particle sizes in the form of sand (>63 µm), silt (>2–<63 µm) and clay (<2 µm). The textural analysis revealed that soils were admixture of mainly sand and silt along with a small abundance of clay. X-ray diffraction analysis indicates that clay factions ranging from 2.8 to 5% dominated by quartz and montmorillonite. Experimental results indicated that soil with high abundance of clays and low sand content has relatively high U sorption which could be due to availability of high exchange surfaces for metal ions. However, at low concentration of HA, sorption of U was maximum and thereby decreased as the HA concentration increased. The maximum sorption may be due to increase in the negative active surface sites on HA and further decrease could be attributed to saturation of sorption site and surface precipitation. Conversely, the thermodynamic data suggested that the sorption is spontaneous and enhanced at higher temperature.
- Particle size
During the past decades, agricultural activities in Punjab widely expanded causing an escalation in the application of inorganic fertilizers, pesticides and other agricultural chemicals to increase crop production and to enhance soil properties. The contaminants accumulation in soil due to long-continued agricultural activities will depend on its concentrations in fertilizers, annual application rate of fertilizers, physical and chemical properties of soil and geochemical properties of the contaminant itself. The distribution of U in soil are generally influenced by sorption, complexation processes on inorganic soil constituents such as clay minerals, oxides and hydroxides (silica, aluminium, iron and manganese), biological fixation and transformation of organic matter (Belivermis et al. 2009; Bolivar et al. 1995). The abundances of radionuclides and their occurrences in the environment are a result of anthropogenic activities as well as natural processes (Bolivar et al. 1995). The migration of U through soil is enhanced by rainwater (precipitation) and greatest in areas with heavy rainfall. Since the textural and mineralogical information of soils is also essential for understanding soil genesis and for developing appropriate management practices in the maintenance of soil fertility (Marsonia et al. 2008). Therefore, attempts have been made to study the textural and mineralogical characteristics of agricultural soils in the uranium sorption studies. Uranium (VI) forms very stable carbonato complexes in solution and as a consequence uranium sorption in the presence of dissolved CO2 is strongly suppressed in comparison to the carbonate free system (Kowal-Fouchard et al. 2004; Katsoyiannis 2007; Dong et al. 2005; Alliot et al. 2005; Hartmann et al. 2008). Due to very low solubility of the tetravalent uranium [U(IV)], it has a strong tendency toward hydrolysis under relevant natural aquatic system conditions (Choppin 2006). This leads to a strong interaction (sorption) with any kind of surfaces, even at low pH (Clark et al. 2011; Landa et al. 1995; Murphy et al. 1999). Precipitation or polymer/colloid formation due to oversaturation (Dähn et al. 2002; Carroll et al. 1992; Neck et al. 2003) has to be expected as side reactions in sorption studies of tetravalent uranium.
Furthermore, natural organic matters (NOMs) present in soil also play an important role in the fate and transport behaviour of uranium in which they form strong complexes, which are affected by the extent of organic interaction with mineral surfaces and thereby depends on pH. The sorption of uranium onto mineral surfaces in the presence of humic substances had been reported by many researchers (Schmeide et al. 1999; Pompe et al. 1999). There are a large number of possible reactions and interactions of uranium with OM which depends on the pH of the soil, the cation concentration in the soil, the functional group and the degree of saturation of the potential sorption sites.
Studies on the effect of temperature and uranium concentration on the sorption of uranium to a number of pure minerals were conducted where idealized distribution coefficients (kd) are calculated from Freundlich isotherms (Langmuir 1978; Syed 1999; Choppin 2007; His and Langmuir 1985). In the present study, besides the textural and mineralogical characteristics of soils, the sorption of uranium was examined as a function of its concentrations, temperature and HA using batch experiment techniques.
A total of 8 representative agricultural surface soil samples (a depth range of 5–30 cm) were collected from Bathinda district in Punjab in the month of March, 2014. The sampling was done using an auger soil sampler, stored in polyethylene bags and transported to the laboratory. The geographical location of the sampling area is south-west of Punjab between latitude 29°07′N–30°57′N and longitude 74°05E−76°55′E at an average elevation of 200 m from the sea level. Average annual rainfall is 500 mm of which 80% is received during the period of June–October. The soil of the study area is loose, sandy, calcareous and alluvial, which is an admixture of gravel, sand, silt and clay in various proportions.
Soil sampling and pre-treatment
The collected soil samples were dried at 110°C for 24 h, powdered, homogenized and sieved through 110 mesh sizes. The powdered samples were thoroughly mixed with each other and prepared for two sets (Set-1 and Set-2). Each set was washed thrice with deionised water. The solid phase was allowed to settle by centrifugation and the washing solution was discarded. After washing, samples were further dried at 110°C, placed in conical flasks and stored as stock samples for experimental work.
In the similar fashion, a batch experiment was also conducted to determine the kd values of U in both sets of soil as a function of HA concentrations (2.5, 5, 10, 25, 50, 100 and 125 mgL−1) spiked with 1 mgL−1 of U standard. The pH of the equilibrated solution was maintained within the range of 5.5–6 throughout all experiments after addition of desired amount of 0.1 M of NaOH or 0.1 M HNO3 (Merck, Mumbai, India) using an automated titrator (Metrohm-798 MPT Titrino, Switzerland) in “pH–stat” mode. Blank samples were also run in absence of soils at different HA concentrations. Duplicate samples of each soil and one experimental blank were also analyzed and served as an internal check on the precision of the analytical results.
The concentration of U in aliquots of equilibrium solution was measured by uranium analyser UA-2 (Quantalase, Indore, India) in which LED (Light Emission Diode) is used to excite uranyl species present in the sample, which on de-excitation gives out fluorescence peak. Finally standard addition technique was followed for the estimation of U in the samples. The instrument was calibrated in the range of 1–100 μgL−1 using a stock solution of (1 gL−1) UO2 (NO3)2∙6H2O standard (USA). 5% sodium pyrophosphate in ultra pure water was used as fluorescence reagent (Kumar et al. 2014a). All the experimental data were the averages of duplicate or triplicate experiments. The relative standard deviation (RSD) was calculated to be 5–8%. Quality assurance was carried out by spike recovery, replicate analysis and cross method checking.
The mineralogical study of soil samples were also carried out using X-ray diffractometer (XRD, Model: GNR, Italy). The XRD data were collected on an APD-2000 diffractometer equipped with a 6-position sample holder, theta–theta goniometer and a NaI (Tl) scintillation detector. A Ni-filtered CuKɑ radiation (λ = 0.154 nm) at applied voltage of 40 kV and current of 30 mA was used. For phase identification, Search and Match procedure was performed by using GNR’s SAX software with ICDD Reference Database.
Particle size distribution
The particle size distribution of soil samples was determined using a laser diffraction particle size analyzer (CILAS, France, Model 1190). For soil texture analysis, three different laser diffraction methods identified as LDM 1, LDM 2 and LDM 3 were considered. In LDM 1, the soil sample is thoroughly mixed before analysis. In LDM 2, the sand fraction is sieved out and analyzed separately from the silt–clay fraction. LDM 3 is similar to LDM 2 except that the silt–clay fraction is diluted so that a large sample volume can be used while maintaining an acceptable level of obscuration. LDM 2 and LDM 3 improve the particle size distribution (PSD) in comparison to LDM 1, without the need of altering the Mie theory parameters. Finally, the PSD of the silt–clay and sand were quantified in terms of percent (%) based on the relative weight of each fraction. PSD was performed with a small angle light scattering apparatus equipped with a low-power (2 mW) Helium–Neon laser with a wavelength of 633 nm as the light source. The apparatus has active beam length of 2.4 mm, and it operates in the range 0.04–2,500 lm. The obscuration levels of samples in the laser diffractometry analysis were kept between 15 and 25%. Maintaining this obscuration levels in sediments with high clay contents (20%) compelled to use small volumes because of the high optical density of clay. A 2 g aliquot of the soil sample was introduced into the ultrasonic bath. Finally, the PSD was obtained using two optical models, the Fraunhofer diffraction model and the Mie theory. Because the Fraunhofer model is not accurate enough for the determination of the clay–size fraction. The Mie theory applies rigorously to spherical, homogeneous particles and fits less satisfactorily nonspherical or non homogenous particles as commonly found in sediments. The details of particle size distribution methods are also described in Kumar et al. (2014b).
The total carbon, nitrogen and hydrogen in soil and HA were estimated using C H N S–O elemental analyser (Flash EA 1112 Series, Thermo Finnigan, Italy). The elemental analyzer was calibrated and standardized using BBOT Standard [2, 5-bis (5-tert-butyl-benzoxazol-2-yl)-thiopen, C26H26N2O2S, Thermo Finnigan, Italy)]. The minimum detection limit for C, N and H was calculated to be 0.08%. The other elements (K, Ca, Fe, Cu, Ni, Co, Mn and S) in soil were also quantified using Bench top Energy Dispersive X-ray fluorescence technique (EDXRF, Oxford Instrument, X- 5000, Germany). The sample targets were excited using the incident beam from the X-ray tube (10 W long-fine-focus Rh-anode) operated at anode voltage of 50 kV.
Effect of U concentrations
Effect of particle size
In general, the soils were mainly composed of sand and silt. Particle sizes of soils of both sets were characterized as sand (>63 µm), silt (>2–<63 µm) and clay (<2 µm). Soils of set-1 were sandy- silt loam in the form of 54.2% sand, 42% silt and 3.8% clay whereas set-2 were silty-sand with the distribution of 36% sand, 59% silt and 5% clay. The mean diameter of particle size of soils ranged from 53 to 86 µm (Mean: 69.4 ± 11.5 µm) along the studied area. The comparatively higher sorption of U onto soil of set-2 might be due to presence of high amount of finer particles in the form of clays and low sand content. This can be confirmed by obtaining a strong positive correlation between U and clays content in the past study. However, sand and silt did not show any particular significant correlation (Kumar et al. 2015). In literatures, the huge variation observed in kd values of uranium in various soil types (Kaplan et al. 1998; Gamerdinger et al. 1998; USEPA 1999).
Effect of temperature
Thermodynamic parameters for the sorption of U onto soil (set-2)
C0 (mg L−1)
∆Hº (kJ mol−1)
∆Gº (kJ mol−1)
Effect of HA
Literatures have reported that at acidic pH range (3.5–6), the sorption of U generally increased in the presence of HA which is due to increase in the negative sorption active sites on HA. However, sorption decreases at higher pH (>6), probably due to formation of soluble uranyl humate complexes species (Pompe et al. 1999). As pH increases, the increased deprotonation makes the HA more negatively charged. This negative charge creates an electrical field which influences the complexation reaction. HA concentration must be high enough to influence the uranium adsorption onto minerals due to the competition from other anionic ligands especially in the slightly acidic to alkaline pH range.
In the previous study, FTIR spectra of soils of the studied area were recorded and confirmed a silicate group only (Kumar et al. 2013). The obtained absorption bands for soils were in poor agreement with the HA and thus poorly enriched with respect to organic matters. This is also confirmed by the presence of C content in soils which showed the mean value of 0.89%. Due to low abundances of carbon, soils of set 1 might have shown poor sorption with U leading to relatively lower kd values.
XRD spectra of soil
The presence of Ca-rich and Na-rich plagioclase and Ca-montmorillonite mineral in the soil might be responsible for the high sorption of uranium due to exchange with Ca2+ ions in the mineral lattice. Generally, sorption of the uranium is known to take place primarily as an exchange reaction with metal ions, particularly Ca2+, Na+ and K+ present in the clay minerals such as Ca -montmorillonite, Na-montmorillonite, illite respectively. Zeolites are also known for their high adsorption capacity for many cations, however, is unable to adsorb relatively high concentration of uranium.
Chemical composition of soil
Cu (mg kg−1)
Ni (mg kg−1)
Co (mg kg−1)
Mn (mg kg−1)
S (mg kg−1)
The sorption of U onto soils increases at its low concentration range thereafter decreases at sufficiently high concentration range. Furthermore, the relatively higher sorption of U onto soil might be also affected by high abundances of finer particles in the form of clays. Results also indicated that sorption is strongly dependent on kind of clay minerals, temperature and presence of HA. The thermodynamic data suggested that the sorption reaction is spontaneous and endothermic. The HA appears to be a key-component when the objective of the study is to assess the potential mobility of U in natural systems. It is also suggested that the migration of uranium in soils in the presence of HA can be either accelerated by formation of the humic compounds or partly retarded by sorption of humic compounds. This study also reveals the susceptibility of U toxicity depending on sorption capacity of soil.
AK has conducted the experiment and drafted the manuscript properly. SR and RK participated in analytical work, MKM was involved in sampling program, PMR and RMT read the draft critically and advised. All authors read and approved the final manuscript.
The authors sincerely acknowledge the guidance and help provided by Dr. D. N. Sharma, Director, H, S and E Group, for constant encouragement.
Compliance with ethical guidelines
Competing interests The authors declare that they have no competing interests.
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