Adsorption of Phosphorus Infiltrating Andosol in Aso Caldera, Kumamoto

doi: 10.2965/jwet.19-068 Journal of Water and Environment Technology, Vol.18, No.1: 37–44, 2020

Original Article Adsorption of Phosphorus Infiltrating Andosol in Aso Caldera, Kumamoto

Daisuke Yumioka a, Takehide Hama b, Koichiro Kitamura a, Satoshi Hatate a, Hiroaki Ito b, Yasunori Kawagoshi b

a Department of Civil and Environmental Engineering and Architecture, Kumamoto University, Kumamoto, Japan b Center for Water Cycle, Marine Environment and Disaster Management, Kumamoto University, Kumamoto, Japan

ABSTRACT High concentration (more than 1.0 mgP/L) of phosphorus was detected in groundwater in Aso caldera, Kumamoto. The fact indicates phosphorus can be transported by groundwater flows. The objective of this study is to clarify the characteristics of paddy soil (Andosol) in phosphorus adsorption process. Adsorption capacity of the Andosol was evaluated by continuously-flowing system using the soil column. In addition, soil phosphorus was categorized into five fractions; water-extractable phosphorus (Water-P), phosphorus extracted by sodium bicarbonate and sodium dithionate (DB-P), phosphorus extracted by sodium hydroxide (NaOH-P), phosphorus extracted by hydrochloric acid (HCl-P) and residual phosphorus (Res-P). As a result, it is confirmed that the flow rate of water is an important factor to control the phosphorus adsorption by the Andosol. The lower the flow rate is, the more phosphorus is adsorbed on the upstream soil. On the other hand, when the flow rate was high, phosphorus was evenly distributed in column soil. In addition, the fraction of NaOH-P accounted for 52% of the adsorbed phosphorus, suggesting that NaOH-P is the most important fraction which controls phosphorus adsorption to Andosol.

Keywords: phosphorus fraction, paddy soil, infiltration rate

INTRODUCTION amounts of phosphorus have been applied to soil for a long time [6]. Recent researches suggest that not only surface flow Eutrophication of water bodies such as rivers, lakes and such as rivers but also groundwater flow have large impact marine areas will cause degradation of their water qual- on wetlands and coastal ecosystems [7]. ity [1]. They may cause abnormal proliferation of plankton Andosol, which is widely distributed in volcanic area and cyanobacterial blooms, anoxia and fish perish, can and cover more than 50% of upland areas in Japan [8], has produce food web alterations, and freshwater red tides a high phosphorus adsorption capacity [9]. For this reason, [2–5]. The main cause of eutrophication is excessive input phosphorus fertilizer is applied to Andosol fields with larger of nutrients such as phosphorus and nitrogen, which are amount compared to other soils. In paddy fields, the amount mainly discharged from agricultural lands. Phosphorus of fertilizer applied to Andosol is about 1.5 to 3 times as is one of limiting factors of eutrophication. It is important much as other soils [10]. This may be due to the high P/N for improvement of eutrophication to identify the source of ratio in Japanese fertilizer consumption [11]. phosphorus and control its transport. In general, phosphorus However, high concentration of phosphorus (more than has been thought to hardly reach aquifer because phosphorus 1.0 mgP/L) was detected in groundwater in Aso caldera, is adsorbed to soil in the process of infiltration. However, Kumamoto. This may be because the adsorption capacity in the phosphorus adsorption capacity of soil decreases if large the groundwater pathway is saturated or decreased due to

Corresponding author: Takehide Hama, e-mail: [email protected] Received: June 28, 2019, Accepted: October 4, 2019, Published online: February 10, 2020 Open Access This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial (CC BY-NC) 4.0 License. http://creativecommons.org/licenses/by-nc/4.0/

37 38 Journal of Water and Environment Technology, Vol. 18, No. 1, 2020 continuous phosphorus input associated with high fertilizer Continuously-flowing test using the soil column application to the agricultural fields. In particular, paddy Continuously-flowing test using the soil column was con- fields have large contribution to the phosphorus contamina- ducted to evaluate adsorption capacity of the paddy. The soil tion because large amount of water percolates to ground- sample was mixed with glass beads at mass ratio of 1: 3 to water through the land surface during the irrigation period. enhance the permeability before the test. The glass-mixed Therefore, it is important to clarify the characteristics of soil sample was packed with height of 30 cm (soil mass = phosphorus adsorption of Andosol to control excessive phos- 62.5 g) in a glass column with inner diameter of 2.6 cm, phorus fertilization. outer diameter of 3.0 cm and length of 40 cm (Fig. 1). The Although many researches, in which batch tests were com- porosity of column soil is about 78%. To prevent clogging, monly used, estimate the phosphorus adsorption by Andosol the top and bottom 5 cm of the sample were filled with glass in a steady state, knowledge on phosphorus adsorption in beads and 40-μm filters of glass fibers were set at both ends infiltration process is limited. Therefore, the objectives of of the glass column. Deionized water was supplied to the soil this study are 1) to evaluate the effects to the phosphorus column for 10 days to stabilize the soil environment. Then, adsorption of infiltrating rate in Andosol soil column, 2) to KH2PO4 solution of 5.0 mgP/L was supplied for 30 days at clarify the phosphorus fraction that has large contribution constant rate (11.2 mL/hour, 24.8 mL/hour, 62.6 mL/hour, to the phosphorus adsorption in the infiltrating process. The 85.1 mL/hour). The experiment was conducted in an incuba- adsorption capacity was evaluated by a continuously-flowing tor with constant temperature (25°C). The effluent water was system using the soil column. collected almost everyday. The PO4-P concentration was determined by the molybdenum blue method [13]. MATERIALS AND METHODS Adsorption capacity of Andosol

Study site and soil sampling The amount of PO4-P adsorbed to the soil Q(mgP/g) was The study site is a paddy field located in Aso Caldera, calculated using equation (1), based on the initial PO4-P con- Kumamoto. In summer season, maximum temperature in centration Ci (5.0 mgP/L), the PO4-P concentration of the ef- daytime is over 30°C and minimum temperature is below fluent water Ce (mgP/L), the flow rate V(mL/h), elapsed time 20°C. In winter season, maximum temperature is about 2°C T(hour), and the dry weight of the soil in the glass column and minimum temperature is about −5°C. The annual rain- W(62.5 g) fall exceeds 3,000 mm and rainfall in rainy season (June and

July) accounts for 40% of the annual rainfall. Cie VT−∫ V C dT Q = (1) The paddy soil is Andosol, in which amorphous and qua- W sicrystalline minerals such as allophane and imogolite are the main clay minerals. Chemical composition of Andosol in The amount of net adsorbed phosphorus is determined by the balance of adsorption and desorption of phosphorus. Aso region as follows; The percentage of SiO2, Al2O3, Fe2O3, Therefore, temporal change in the amount of phosphorus CaO, Na2O, SO3, TiO2, K2O, Mn2O3 were 55.5, 14.55, 13.78, 8.03, 3.11, 2.01, 1.56, 1.39, 0.207% respectively. In addition, adsorbed onto the soil can be described as equation (2): Andosol has a lot of humus and has excellent water retention, dQ water permeability and breathability [12]. In general, the =KK(1 −−γγ) (2) dt ad phosphate absorption capacity of Andosol is high because large amounts of activated aluminum in Andosol adsorb where Ka is the adsorption rate (mg/g ∙ h), Kd is the desorption and insolubilize phosphoric acid. However, high concentra- rate (mg/g ∙ h), γ is the ratio of the uncovered area by phos- tion (more than 1.0 mgP/L) of phosphorus was observed in phorus in the adsorption site, and t is elapsed time (hour). groundwater in the caldera. On the other hand, since γ is expressed by equation (3) using Soil sample was collected from the paddy field in April the maximum adsorption amount Qmax, equation (4) can be 2017 before transplanting of rice. The soil sample was taken derived from the initial condition (Q= 0 at t = 0): to the laboratory in sealed plastic bags and air-dried at room temperature. Then, the soil sample was sieved through a 250 QQ− γ = max μm mesh to remove small plant roots and gravels. Q (3) max Journal of Water and Environment Technology, Vol. 18, No. 1, 2020 39

Fig. 1 Continuously-flowing test system.

Fig. 2 Continuously-flowing test using the soil column. [a] PO4-P concentration of the effluent water, and [b] the amount of PO4-P adsorbed to the soil.

by using equation (4) (Fig. 2 [b]). k −α Qe=a (1 − t ) (4) α Phosphorus fraction scheme Phosphorus fractions in the soil are important indicators However, the coefficient α is expressed by equation (5): for assessing phosphorus availability and mobility [14]. kk+ Extraction methods of phosphorus have been developed by α = ad Q (5) many researches [15,16]. max The soil column after the continuously-flowing test was In this study, adsorption capacity of Andosol was analyzed divided into three layers of 10 cm, and then phosphorus 40 Journal of Water and Environment Technology, Vol. 18, No. 1, 2020

Table 1 Comparison of the extraction schemes. Pshnner and Willson [17] Ruttenberg and Zhang [18] Present study

Step1 Deionized water 1 M MgCl2 Deionized water Step2 0.1 M Na2S2O4 / NaHCO3 0.1 M Na2S2O4 / NaHCO3 0.1 M Na2S2O4 / NaHCO3 Step3 0.1 M NaOH Acetate buffer 0.1 M NaOH Step4 0.5 M HCl 1 M HCl 1 M HCl Step5 1 M NaOH Ashed / 1 M HCl Ashed / 1 M HCl fractions in the soil of each layer were determined using adsorption. The first stage is rapid adsorption that finishes the sequential extraction method, which is developed by within 0.5 hours after phosphorus is added to the Andosol Pshhnner and Willson [17] and Ruttenberg and Zhang [18]. [19]. Previous researches [20,21], which evaluate phospho- In this study, the extraction method was partially modified rus adsorption capacity by using batch tests, report that the as Table 1. In the first step, deionized water was added amount of PO4-P adsorption in the rapid adsorption process to the soil sample with solid liquid ratio of 1:50 to extract accounts for about 70% to 80% of the maximum adsorption easily-soluble phosphorus (Water-P). Water-P was extracted capacity. In this experiment, when the flow rate was 11.2 ml by shaking the solution for 2 hours at 25°C. In the second / h, amount of adsorbed phosphorus did not reach 70% of step, sodium bicarbonate (0.1 M) and sodium dithionate the maximum adsorption amount even after 720 hours. This (0.1 M) was added to the residual sample after the Water-P result suggests that the actual amount of phosphorus adsorp- extraction, to extract phosphorus (DB-P), which was sensi- tion in the infiltration process is lower than the adsorption tive to oxidation reduction condition. DB-P was extracted by capacity of the soil, which is estimated from the adsorption shaking the solution with solid liquid ratio of 1:50 for 8 hours isotherm in a batch test. at 40°C. Then, sodium hydroxide (0.1 M) was added to the The PO4-P concentration in the effluent water increased residual sample after the DB-P extraction, extract phospho- with the elapsed time. For example, the PO4-P concentration rus (NaOH-P), which was mainly combined with aluminum was 3.26 mg/L (the elapsed time = 360 h) and 3.88 mg/L oxide of the soil surface. NaOH-P was extracted by shaking (the elapsed time = 720 h) on the condition that the flow rate the solution with solid liquid ratio of 1:50 for 16 hours at was 11.2 mL/h. In addition, the PO4-P concentration in the 25°C. In the fourth step, hydrochloric acid (1 M) was added effluent water depended on the water flow rate (Fig. 2[a]). In to the residual sample after the NaOH-P extraction, extract other words, the amount of PO4-P adsorption was high when phosphorus (HCl-P), which was associated with calcium. phosphorus was supplied slowly to the soil column. HCl-P was extracted by shaking the solution with solid liq- The maximum adsorption capacity was 0.366 mg/g, uid ratio of 1:50 for 16 hours at 25°C. Finally, after the soil 0.397 mg/g, 0.410 mg/g, and 0.568 mg/g in the order of the was ashed at 550°C, hydrochloric acid (1 M) was added to flow rate (Fig. 2[b]). The higher the water flow rate was, the the to the residual sample after the HCl-P extraction, extract higher the maximum PO4-P adsorption capacity of Andosol the remaining phosphorus (Res-P). Res-P was extracted by was observed. In general, the amount of adsorbed phosphorus shaking the solution with solid liquid ratio of 1:50 for 24 was proportional to phosphorus concentration in the ambient hours at 25°C. Phosphorus concentration in each extracted water [22]. This is because the frequency of the interaction solution was determined by the molybdenum blue method. between the adsorption site on the soil surface and phosphorus in the solution increases with rising concentration of RESULT AND DISCUSSION phosphorus. Similarly, our results indicate that increase in the flow rate contributes to increase in phosphorus supply to Phosphorus adsorption by the soil column the adsorption site.

At the initial stage of the experiment, the PO4-P concentra- However, when the water flow rate was low, Qmax and the tion in the effluent water of each soil column was 0.127 mg/L adsorption amount determined by phosphorus fraction did (flow rate = 11.2 mL/h), 0.158 mg/L (flow rate = 24.8 mL/h), not match. When flow rate is low, phosphorus is adsorbed 0.689 mg/L (flow rate = 62.6 mL/h), and 0.467 mg/L (flow preferentially by the upstream layer, and then the phosphorus rate = 85.1 mL/h), respectively. In general, PO4-P adsorption adsorption in the downstream layer remains to be insuf- on Andosol can be divided into two stages, rapid and slow ficient. Therefore, there is a possibility that the adsorption Journal of Water and Environment Technology, Vol. 18, No. 1, 2020 41

Fig. 3 Phosphorus fraction of Andosol. phenomenon does not occur uniformly. When the flow rate is Fig. 4 shows increment of each phosphorus fraction for low, it may require much longer time to saturate the adsorp- three layers. The amount of phosphorus adsorption tended to tion site with phosphorus than expected. be larger in the upstream layer (0–10 cm from the surface of the soil) compared the other layers. In particular, DB-P and Phosphorus fraction Water-P were clearly adsorbed to the upstream layer. On the The main phosphorus fraction of the soil was NaOH-P other hand, the difference of NaOH-P and HCl-P among the (Fig. 3), except Res-P. This fraction represents phosphorus layers became smaller with increase of the water flow rate. that is adsorbed to the soil by chemical combination between It is confirmed that Andosol has various adsorption sites of phosphorus ion and hydroxyl functional group of aluminum phosphorus. It is also indicated that the adsorption rate varies hydroxide on the soil surface [23] or ligand exchange between with the adsorption site (fraction). Phosphorus is readily ad- phosphorus ion and polymerized hydroxyl aluminum [24]. sorbed onto Water-P or DB-P fractions while the adsorption This result is consistent with the report that phosphorus is of phosphorus to NaOH-P or HCl-P fractions is relatively dominantly adsorbed by aluminum in Andosol [25,26].DB-P slow. The ununiform distribution of phosphorus in the soil is related to phosphorus associated with iron and manganese column may be explained by the intensity of phosphorus and and that may release under an anoxic condition [27] and the reactivity of the adsorption site. When flow of the phos- the increase of adsorbed phosphorus is the second largest phorus solution is slow, phosphorus is adsorbed to NaOH-P (about 40% for all phosphorus increment). The increment of fraction in the upstream soil. Then, the amount of adsorbed HCl-P and Water-P in the experiment (about 3% and 5% for phosphorus in the downstream soil become lower because all phosphorus increment) are negligible compared to other phosphorus concentration is decreased by the upstream soil. phosphorus. HCl-P is related to phosphorus associated with When flow of the phosphorus solution is fast, phosphorus calcium [28,29] and difficult to release23,30 [ ]. However, is not significantly reduced by the adsorption to NaOH-P this fraction is susceptible to low pH. In other words, it may fraction. Then, phosphorus is adsorbed to DB-P and Water-P release under weakly acidic condition [31,32]. In contrast, fractions uniformly in the total soil column. However, the Water-P is available because it is labile easily release to the amount of HCl-P adsorption at 85.1 mL/h tended to larger in pore water [14,33]. In this sequential extraction method, the upstream layer. The cause is not clear. It is future issue to phosphorus fraction, that is the most difficult to extract, is quantify the adsorption rate of each fraction. Res-P. The amount of Res-P did not significantly increase in the experiment. 42 Journal of Water and Environment Technology, Vol. 18, No. 1, 2020

Fig. 4 Increment of each phosphorus fraction for three layers. [a] NaOH-P, [b] DB-P, [c] HCl-P, and [d] Water-P.

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