Phosphorus recovery from highly concentrated phosphate-ion groundwater by calcium-carrying chaff charcoal

Ogata-Mura was created 40 years ago under a land reclamation project. It is now a leading agricultural region in Akita, Japan. Cyclic use of the water of Hachiro Lake, which is situated in Ogata-Mura, is carried out for agricultural use, and it is causing chronic water pollution.

Another cause of eutrophication is phosphorus highly accumulated into groundwater. It is discharged from the whole region of Ogata-Mura. And for the neighbouring residents of Ogata-Mura, the bad smell of the developmental time of blue-green algae has been a big problem. Hachiro Lake has been receiving 30–40 ton of phosphorus per year, while 25% of it has originated from highly accumulated phosphate ion in groundwater (Katano et al. 1998; Katano 1999).

Akita Prefecture is famous for rice production in Japan. The rice quantity of output per 100,000 people was No.1 of Japan in 2008. Some of the rice straw and chaff produced during the crop season is reused as mulching and cattle feed. Much of it is incinerated, although incineration is prohibited in accordance with Akita prefectural regulations between 1 October and 10 November because of the potential release of formaldehyde (Kayaba et al. 2004).

Proper disposal of chaff and/or reuse is important and has been officially approved. In this study, utilization of calcium-carrying chaff charcoal was examined in order to recover phosphorus from highly concentrated phosphate ion in groundwater in Ogata-Mura.

The phosphorus recovery material manufacturing process is developed by Akita Research Centre for Public Health and Environment (Narita et al. 2011). Calcium was compounded in order to give the phosphorus recovery ability to chaff.

First, nitric acid and suspended mater that contained ion-exchanged water into the calcium hydroxide were added until the solution became colourless. pH was adjusted to 6–7 by adding sodium hydroxide. The calcium solution was compounded, while putting pressure on chaff, and it was made to carbonize for 60 minutes at 600–800 °C. The phosphorus recovery material was manufactured after cooling to room temperature.

Table 1 shows the elemental mass ratio in calcium-carrying chaff charcoal which has been measured using energy dispersive X-ray (EDX) analysis apparatus. The mass ratio of calcium is 29%. Specific surface area for the calcium-carrying chaff charcoal is 211 m²/g and it is measured by using a Micromeritics Automatic Surface Area Analyser (Tristar-II 3020, SHIMADZU).

Groundwater containing high concentration of phosphorus was pumped out from a depth of 7.5 m in Hachiro Lake. As shown in Table 2, the concentration of PO₄-P was very high, 33 mg/L. Hachiro Lake was originally a brackish water lake. At the ocean side, some of the brackish water area was left to control the pressure level from the seawater as to penetrate the seawater from infiltrating into the reclamation land. But, since the reclamation land is below seawater level, a little of seawater from the bottom part of the ocean infiltrated into the reclamation land, thus slightly influenced the concentration of chloride ion and pH value. Furthermore, the electrical conductivity of the groundwater was about 5% of seawater.

Experimental apparatus and their dimensions are shown in Figure 1 and Table 3. The phosphoric groundwater was added from the bottom of the column by varying the hydraulic retention time (HRT) from 1 to 3 days when the flow rate was 0.7 mL/min and from 0.25 to 0.50 day when the flow rate was 2.8 mL/min. The temperature range was 20–25 °C.

By screening calcium-carrying chaff charcoal, the particle size was adjusted >100 μm and filled into the column. A filter paper with a pore diameter of 20 μm and a non-woven fabric was placed between each column to suppress the erosion of chaff.

The sampling of treated water was taken via upper part valve of each column. After the water was filtered using a 0.45 μm membrane filter, SHIMADZU ion chromatograph (Prominence HIC-SP) was used to measure the concentration of phosphate ion. The amount of phosphorus recovery was determined by the differences between the phosphate ion concentration in sample water and treated water during the phosphorus recovery breakthrough time. The phosphorus recovery breakthrough time was defined as the time needed for the concentration of phosphate ion in the treated water exceeding 1.0 mg-P/L.

To evaluate the performance of the calcium-carrying chaff charcoal fertilizer as a reusable product, the phosphorus fraction in each of total phosphorus recovery, citric acid extractable phosphorus and water extractable phosphorus were measured using spectrophotometer according to fertilizer analysis method. The rainwater extractable phosphorus was also measured. Sixty days after the experiment, the sample that had been used was made into an air-dried form of calcium-carrying chaff charcoal.

For the extraction of total phosphorus recovery, 30 mL hydrochloric acid solution and 10 mL nitric acid solution were added into 2 g of chaff. Then, after boiling the mixed solution for about 30 minutes, the total phosphorus in the mixed solution was analysed.

In the determination of citric acid extractable phosphorus, 500 mL of citric acid solution was added into 2 g of chaff. Then, after shaking the mixed solution about 1 hour at 40 rpm, 30°C, the total phosphorus was analysed.

2 g of chaff was added into 500 mL of distilled water to extract the water-soluble phosphorus. Later, the mixed solution was shaken at 30–40 rpm for 30 minutes. This procedure was repeated in the phosphorus extraction test using the rainwater.

After these tests, the solution of each test that was filtered by GF/B glass fibre filter was decomposed by the potassium peroxodisulfate and was analysed using molybdenum blue method for the determination of phosphate.

In this experiment, X-ray diffraction (XRD) device was used to analyze the crystalline phase of calcium presented on the surface of calcium-carrying chaff charcoal before and after phosphorus recovery. While in order to measure the distribution of element and to observe the surface characterization, scanning electron microscopy (SEM) with EDX equipment was used.

As the graph shown in Figure 2, the concentration of phosphorus in treated water increased dramatically in a short period in cases A and B compared to the concentration of phosphorus in case C–D which increased gradually. As well as the breakthrough time for cases A and B, comparatively shorter than the breakthrough time than in case C–D (Table 4). From this result, it could be concluded that longer HRT resulted longer breakthrough time.

The amount of phosphorus recovery obtained in each HRT (case A–case E) was about 1.5–2.0 mg per 1 g of chaff. Although HRT was different in each of the cases, there was not a large difference in the amount of phosphorus recovery. As a conclusion from the results above, since HRT was inversely proportional to the flow rate, variety of HRT did not affect the amount of phosphorus recovery.

An increase of phosphate ions could be seen when the pH value was 9.3 or lower. However, in the case when the pH value was above 9.3, the concentration of PO₄-P was 1 mg/L (Figure 3). Thus, the contribution of pH value in monitoring the indicators of phosphate ion in treated water was confirmed by the result obtained. This might be so because when the calcium-carrying chaff charcoal is in contact with highly concentrated phosphate ion groundwater, the calcium oxide in calcium-carrying chaff charcoal is isolated in the groundwater. The isolation supposedly caused an increase in the pH value (Watanabe et al. 2012). At the same time, the formation of phosphorus compound has consumed the calcium component, and thereby approached the pH value of groundwater.

When the extraction test was carried out using hydrochloric acid and nitric acid, 1.5 mg of phosphorus per 1 g chaff was recovered. On the other hand, there was no phosphorus elution detected when using citric acid solution and distilled water. However, 0.06 mg/g of phosphorus was eluted slightly into rainwater. The result was concluded as shown in Table 5. Based on this result, it could be understood that most of the recovered phosphorus might exist as hardly soluble phosphorus. Since the amount of phosphorus recovered in the elution test was smaller than the amount recovered in the water flow test, it was necessary to review the decomposition rate for the elution test, the phosphorus recovery time in the water flow test, etc.

Although the phosphorus that has been recovered is mostly hardly soluble phosphorus, it still can be used as fertilizer due to the role of mycorrhizal fungi to demineralise and uptake phosphate ions in soils (Wasaki 2011). Moreover, the chemical properties of rhizosphere also affect the nutrient dynamics; for example, mobilization of sparingly-soluble phosphorus by organic acids.

The differences between images (a) and (b) in Figure 4 represent the characteristic of calcium-carrying chaff charcoal surface before and after phosphorus adsorption, especially at the central part of the surface. Images (c) and (d) below show the results of EDX mapping. The entire surface of calcium-carrying chaff charcoal indicates the presence of phosphorus in image (c) while image (d) indicates the overlapping of calcium and phosphorus distribution. It can be assumed that the dispersion between phosphate ion contained in groundwater and calcium as a phosphorus compound, has occurred in calcium-carrying chaff charcoal.

Figure 5 shows the XRD pattern obtained before and after phosphorus adsorption experiment. As the result, calcium carbonate pointed the highest peak compared to calcium oxide and calcium carbide. That means, calcium contained in calcium-carrying chaff charcoal mostly consisted of calcium carbonate. Even after the phosphorus recovery, the XRD pattern is the same as before. But, strength reduction of the calcium carbonate peak and minor peak of calcium phosphate compound has been detected. Meanwhile, due to qualitative analysis, the presence of Ca₈H₂(PO₄)₆·H₂O-NaHCO₃·H₂O (Dibasic calcium phosphate hydrate-sodium hydrogen carbonate hydrate) has been identified at the disappearance peak of calcium carbonate and calcium carbide. This result shows clearly the qualitative and quantitative changes occurred in calcium-carrying chaff charcoal.

Phosphorus is a highly reactive element that is essential for life, and forms a variety of compounds in terrestrial and aquatic ecosystems. In water, phosphorus presents as the orthophosphate ion (PO₄³⁻) and is also presented in all life forms as an essential component of cellular material (Domagalski & Johnson 2012). Although phosphorus is one of the most important essential elements especially in agriculture, excess application of phosphorus will cause eutrophication (Wasaki 2011) as it occurred in Hachiro Lake, Ogata-Mura. Besides that, we are facing the depletion of phosphorus as well. It is necessary to find an alternative management of phosphorus in order to make sure of the sustainability of agricultural activities. Therefore, the focus of our research is to find ways to keep phosphorus more efficient, as well as to improve the usage of waste products such as chaff.