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A Comparison of the Langmuir, Freundlich and Temkin Equations to Describe Phosphate Sorption Characteristics of Some Representative Soils of Bangladesh

Mohammad Z. Afsar
Department of Soil Science, University of Chittagong, Bangladesh.

Sirajul Hoque
Department of Soil, Water and Environment, University of Dhaka, Bangladesh.

K.T. Osman
Department of Soil Science, University of Chittagong, Bangladesh.

Abstract/ Executive Summary:

To study the phosphate sorption potential of soils, an experiment was conducted with eight different soil series of Bangladesh. The soils were equilibrated with 0.01 M calcium chloride solution containing 0, 1, 2, 4, 8, 16, 25, 50, 100 and 150 μg phosphorus mL^-1 and the amount of phosphate sorbed was determined. Calcareous soils sorbed more phosphorus than non-calcareous soils. The sorption curves showed more or less similar pattern of changes. Langmuir model showed better fit to sorption data at higher P concentrations. Phosphate sorption was highly correlated with Fe oxide and clay content of the soils. Freundlich constant k_f and n, Phosphorus Sorption Maximum (S_max) and EPC₀ values were positively correlated with both the above soil properties. On the contrary, phosphate binding strength (b) was negatively correlated. It is suggested that Langmuir equation might be more suitable for commercial soil-testing laboratories for routine determination of phosphate sorption characteristics of soils.

Keywords: Sorption, Sorption maximum, Phosphate binding strength, Available P

Location: Laboratory of the department of soil, water and environment, University of Dhaka

Start Date:

End Date:

Program Area: Crop-Soil-Water Management

Commodity: Soil fertility

Objectives:

(1) Finding out the phosphate sorption potential of some representative soils of Bangladesh.

(2) Developing model sorption behavior using Linear, Freundlich, Langmuir and Temkin models.

(3) Observing the effects of some soil properties on phosphate sorption and

(4) Evaluating the relationships among P sorption parameters and selected soil properties.

Methodology:

With a view to study phosphate sorption characteristics of soils, this experiment was conducted in the laboratory of the department of soil, water and environment, University of Dhaka with eight different bench-mark soil series. Among these, Baliadangi series (eutric cambisol) of old Himalayan Piedmont plain; Gongachara series (eutric fluvisol) of Tista Meander floodplain; Lockdeo (eutric fluvisol), Silmondi (eutric fluvisol) and Ghatail series (eutric fluvisol) of old Brahmaputra floodplain were noncalcareous. On the other hand, Gopalpur (calcaric fluvisol), Ishurdi (calcaric fluvisol) and Ghior (calcaric fluvisol) series of low Ganges river floodplain were calcareous. The three calcareous soil series comprised a catena, where Gopalpur and Ghior soil series located at the highest and the lowest elevation, respectively. Soil samples at a depth of 0-15 cm were collected from 20 spots from an area of ~1 km² under a soil series. Equal proportions of theses samples were mixed to form a composite sample. The soils were then air dried, ground and sieved through a 2 mm sieve. Phosphorus sorption studies: Triplicate 1 g soil samples were equilibrated in a 50 mL centrifuge tube with 20 mL 0.01 M CaCl₂ solution containing 0, 1, 2, 4, 8, 16, 25, 50, 100 and 150 μg phosphorus mL^-1 (equivalent to 0, 20, 40, 80, 160, 320, 500, 1000, 2000 and 3000 μg phosphorus g^-1 soil) as KH₂PO₄. The soil samples were then incubated at room temperature for 3 days prior to the sorption study. This incubation time was chosen in accordance with a previous experiment. The amount of adsorbed P was increased with increasing incubation period from 1-15 days. The P availability in soil was gradually decreased with increasing the incubation time for both organic and inorganic sources of P. After centrifugation at 3000 rpm for 15 min, the supernatant was collected. The amount of P sorbed by the soil was calculated from the difference in P concentration between the initial P and equilibrium P concentrations in the solution. The P in the filtered solutions was measured colorimetrically by a Shimadzu model UV-120-02 spectrophotometer. Soil samples were analyzed for textural classes, total P and available N, P, K, organic carbon, free carbonate, fractions of iron, etc. Particle size analysis was done by hydrometer method. Soil organic matter was determined by wet oxidation method. Cation Exchange Capacity (CEC) was estimated by NH₄OAc saturation. Available P was extracted by 0.5 M NaHCO₃ at pH 8.5 and P in the extract was determined by ascorbic acid blue color method. Free carbonate content was measured by rapid titration method. Among the three iron fractions, free iron oxides were estimated. “Active” or “Amorphous” iron oxides were determined by using a modified procedure. Sodium pyrophosphate extractant (pH 10) was used for the estimation of organically bound Fe. Subsequent iron determinations were carried out by the process. Microsoft Office Excel and SPSS-12 computer programs were used to estimate relationships between phosphate sorption and different soil properties. Correlation coefficients were calculated between phosphate sorption parameters like P-sorption capacities (k_f), Freundlich constant (n), P-sorption maximum (S_max) and Equilibrium Phosphate Concentration (EPC₀) values with soil properties like percent clay, organic matter content and CDB-extractable iron content. Correlation coefficients among various P-sorption parameters were also calculated. Suitability of different adsorption equations were studied by calculating the R² values of the respective equations. Regression curves were drawn by the Microsoft Office Excel program.

Source of Information: International Journal of Soil Science, 7 (3): 91-99,2012; (ISSN 1816-4978)

Article Link (Online Journal): http://scialert.net/fulltext/?doi=ijss.2012.91.99&org=10

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

The soils differed considerably in their capacity to sorb the added P and in the estimated labile pool of native soil P. Soils with the lower clay and iron oxide content showed the lowest retentive capacity. Except b, other phosphate sorption parameters like k_f, S_maxand EPC₀ values were positively correlated with both the soil parameters. In general, calcareous soils sorbed more phosphorus in their labile pool than non-calcareous soils when provided with same concentrations of P in solution.

Knowledge Management: Journal