Science of the Total Environment 521–522 (2015) 37–45

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Effect of biochar on leaching of organic carbon, nitrogen, and phosphorus from compost in bioretention systems Hamid Iqbal a,b, Manuel Garcia-Perez c, Markus Flury a,⁎ a b c

Department of Crop and Soil Sciences, Washington State University, Puyallup, WA 98371, USA Institute of Environmental Sciences and Engineering, School of Civil and Environmental Engineering, National University of Sciences and Technology, Sector H-12, Islamabad, Pakistan Department of Biological Systems Engineering, Washington State University, Pullman, WA 99164, USA

H I G H L I G H T S • Biochar did not reduce leaching of DOC, nitrogen, and phosphorus from compost • Co-composting of biochar did not make a difference in terms of leaching of nutrients • Compost–sand mix was more effective in reducing leaching than compost–biochar mix

a r t i c l e

i n f o

Article history: Received 27 January 2015 Received in revised form 14 March 2015 Accepted 14 March 2015 Available online xxxx Editor: D. Barcelo Keywords: Bioretention Biochar Compost Dissolved organic matter Nutrients

a b s t r a c t Compost is used in bioretention systems to improve soil quality, water infiltration, and retention of contaminants. However, compost contains dissolved organic matter, nitrate, and phosphorus, all of which can leach out and potentially contaminate ground and surface waters. To reduce the leaching of nutrients and dissolved organic matter from compost, biochar may be mixed into the bioretention systems. Our objective was to test whether biochar and co-composted biochar mixed into mature compost can reduce the leaching of organic carbon, nitrogen, and phosphorus. There was no significant difference between the effects of biochar and cocomposted biochar amendments on nutrient leaching. Further, biochar amendments did not significantly reduce the leaching of dissolved organic carbon, nitrate, and phosphorus as compared to the compost only treatment. The compost–sand mix was the most effective in reducing nitrate and phosphorus leaching among the media. © 2015 Elsevier B.V. All rights reserved.

1. Introduction In bioretention systems, compost is often mixed with sand to improve plant growth. Bioretention systems are designed to treat stormwater before contaminants in the water can reach surface or ground waters. Nutrients and pollutants are sorbed and filtered in the bioretention system, thereby preventing them from contaminating receiving waters. Bioretention systems provide an effective and sustainable means for urban stormwater management. However, compost itself can release particulate and dissolved organic carbon (DOC), nutrients, as well as contaminants initially present in the compost. Such release can be of environmental concern if the leachate flows directly into surface or ground waters. Considerable amounts of organic carbon, mainly in the form of DOC, can leach

⁎ Corresponding author. E-mail address: fl[email protected] (M. Flury).

http://dx.doi.org/10.1016/j.scitotenv.2015.03.060 0048-9697/© 2015 Elsevier B.V. All rights reserved.

from mature compost (Christensen and Nielsen, 1983; McLaughlan and Al-Mashaqbeh, 2009; Beesley, 2012), but also inorganic constituents can leach out (Christensen, 1984; Christensen and Tjell, 1984; Li et al., 1997; Hsu and Lo, 2001). If the compost were mixed with biochar, this could reduce the undesired leaching of constituents. Biochar is carbon-rich pyrolyzed biomass produced from a variety of organic feedstocks including municipal, agricultural, and forestry wastes (Sohi et al., 2010). The process of biochar production involves thermal degradation of organic material in the absence of oxygen (Laird et al., 2009). Biochar characteristics are governed by the type of source material and pyrolysis temperature (Chen et al., 2008; Gray et al., 2014). For instance, biochar has a higher specific surface area (mostly on the internal surfaces of micropores) but lower functional groups when processed at temperature higher than 500 °C (Chen et al., 2008; Peng et al., 2011). The high surface area of biochar makes it a useful product for various applications. Biochar can serve as a soil amendment to enhance soil aggregation, water holding capacity, and organic carbon content (Smith

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H. Iqbal et al. / Science of the Total Environment 521–522 (2015) 37–45

et al., 2010; Lehmann et al., 2011). Biochar can also be used for the remediation of contaminants from point sources of polluted water (Beesley et al., 2010). Contaminants adsorbed by biochar include heavy metals, pesticides, and organics (Chen et al., 2008; Cao et al., 2009; Uchimiya et al., 2010; Paz-Ferreiro et al., 2014). Biochar therefore is potentially an effective amendment for bioretention systems to treat stormwater, although it has been pointed out that biochar properties (surface area and functionality), and therefore effectiveness for pollutant sorption, depend on feedstock and pyrolysis conditions (Zhang et al., 2013; Paz-Ferreiro et al., 2014; Cernansky, 2015). Biochar can also be added to compost piles, where the biochar promotes microbial growth (Jindo et al., 2012), increases aeration (Zhang et al., 2014), and reduces NH3 and N2O emissions during the composting process (Steiner et al., 2010; Wang et al., 2013). Biochar can sorb nutrients and DOC during the composting process (Dias et al., 2010; Prost et al., 2013). Cation exchange capacity and the proportion of functional groups on the biochar itself may increase due to co-composting (Prost et al., 2013). The co-composted biochar can be applied together with the compost as soil amendment, and potential benefits include enhanced nutrient use efficiency, activation of biochar, and long-term carbon sequestration (Fischer and Glaser, 2012; Schulz et al., 2013). Biochar and co-composted biochar may be useful amendments to bioretention systems to reduce leaching of nutrients and contaminants either from inflowing stormwater or from the compost itself. It has been reported that bioretention systems themselves can be a source of nutrients and metals (Ding et al., 2010), thereby contributing to pollution of receiving waters when leached out. The use of compost in low-impact development systems therefore needs to be considered carefully. Biochar potentially can prevent leaching of pollutants from compost. The addition of biochar can reduce leaching of nitrate from soil (Knowles et al., 2011; Clough et al., 2013) and reduce pore water concentrations of metals (Brennan et al., 2014; Meng et al., 2014). Dissolved organic matter, however, can negate the increased sorption of metals by biochar (Beesley et al., 2014). No experiments have been reported where cocomposted biochar has been applied to bioretention systems. It is not known whether biochar or co-composted biochar mixed with compost would improve retention capabilities of bioretention systems for nutrients and metals. The objective of this paper was therefore to test whether the amendment of a commercially available biochar to mature compost will reduce the leaching of DOC and nutrients from mature compost. We hypothesized that if biochar is mixed with compost, there is a reduction of leaching of DOC and nutrients. We further hypothesized that co-composted biochar is less effective in reducing the leaching of DOC and nutrients because the biochar may become saturated with DOC and nutrients during composting. We carried out column leaching experiments in simulated bioretention mixes made with compost, biochar, and sand to test these hypotheses. 2. Materials and methods 2.1. Media characteristics A six-month old mature compost made from yard (80 wt %) and food (20 wt %) waste was used for the experiments. The compost was obtained from a local commercial composting facility. Biochar was obtained from a local biochar producer and was made of 100% forest slash (Douglas fir) feedstock at around 650 °C using a gasification procedure. A portion of this biochar was co-composted with a feedstock made of yard (80 wt %) and food (20 wt %) waste obtained from the same composting facility that provided the mature compost. Biochar (350 g) was placed into 10 by 20 cm Nylon meshbags that had a meshsize of 20 μm. The meshbags were then buried in the centers of the compost piles. Bags were removed and replaced when the compost piles were turned over. Composting time was five weeks, and the bags were then removed and the biochar was air-dried. Sand was taken

from the Environmental Restoration Disposal Facility (ERDF) at the semi-arid Hanford site (WA). The sand consisted of 92% sand fraction (2 mm to 53 μm), 5% silt (53 to 2 μm), and 1% clay (b2 μm) (Liu et al., 2013) and contained illite, smectite, kaolinite, vermicullite, mica, quartz, feldspars, and pyroxene (Mashal et al., 2004). The media were characterized for pH, electrical conductivity, nutrients, metals, C/N ratio, and surface area. The pH was measured with 1:1 solid–water (wt:wt) slurry, electrical conductivity was measured from a saturation paste extract, N and C were measured with an Elemental CNS Analyzer with dry combustion, and P, K, Ca, Mg, Zn, Mn, and Cu were analyzed with microwave digestion and ICP-OES (EPA, 2001). The specific surface area was measured by N2 adsorption and analyzed with a Brunauer, Emmett, and Teller (BET) isotherm (ASAP 2010, Micromeritics, Norcross, GA). Cation exchange capacity was measured with a modified barium method (Lee et al., 2010). The ζ-potential was measured with dynamic light scattering (NanoZS, Malvern Instruments, Westborough, MA) on ground samples suspended in a 1 mM NaNO3 solution. The media were further characterized by Scanning Electron Microscopy (SEM) (FEI Quanta 200F, FEI Co., Hillsboro, OR). For SEM imaging, the samples were sputter-coated with gold. 2.2. Treatments and column setup The media mixes were packed into PVC columns, with each treatment replicated three times. The media consisted of compost, biochar, co-composted biochar (CC-biochar), and sand, and mixtures thereof (Table 1). Four columns were filled with 100% compost, 100% biochar, 100% co-composted biochar, and 100% sand, and served as controls. Three columns contained compost thoroughly mixed with the other three media (biochar, co-composted biochar, and sand), and one column was filled with a sand overlaid by compost (Table 1). Two types of columns were used to carry out the leaching experiments, small PVC columns for the two biochar only treatments (100%) and large PVC columns for the rest of the treatments. Two differentsized columns were used because we did not have enough cocomposted biochar material to fill replicated large columns. The small columns were proportionally scaled-down versions of the large columns. Before filling the columns with media, the internal surface of each column was roughened with sand paper and acid washed with 10% HCl. The columns were irrigated with sprinkler heads of 5.7 cm diameter containing 12 syringe needles (22-gauge). Funnels at the bottom were used to direct the outflow into sampling containers. The large columns had a diameter of 10.2 cm and were filled with compost to a height of 25 cm. The sprinkler heads were connected to Mariotte bottles (20-L Nalgene) for constant-head irrigation with deionized water. The columns had perforated PVC plates, overlaid by 1mm mesh sized gauze at the bottom. The smaller columns had a diameter of 5.1 cm and were filled with compost to a height of 12.5 cm. The sprinkler heads were connected to a peristaltic pump, which delivered the same water flux as the Mariotte bottles for the large columns. At the bottom of the columns, a 1 mm mesh sized gauze was attached to hold the compost in place. The 1-mm meshes were coarse enough to let the colloidal and particulate fraction of the leachate pass. 2.3. Column leaching experiment Deionized water (pH = 6 and electrical conductivity = 9.4 × 10−4 dS/m) was used to irrigate the columns. Deionized water was used to provide worst case conditions for colloid mobilization. The columns were irrigated with a flow rate representative for a 6-months 24-hour storm in the Pudget Sound area, i.e., the Seattle–Tacoma location. Such a storm has an intensity of 33.5 mm/day (Washington State Department of Ecology, 2012). The flow rate was calculated such

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Table 1 Bioretention treatments, masses used in the columns, and source materials. Nr.

1 2 3 4 5 6 7 8 a b

Treatments (% by volume)

Compost (100%) a Biochar (100%) b Co-composted biochar (100%) b Sand (100%) a Compost (75%) and biochar (25%); mixed a Compost (75%) and co-composted biochar (25%); mixed a Compost (30%) and sand (70%); mixed a Compost (30%) and sand (70%); layered a

Mass (g)

Source

Compost

Biochar

Sand

905 0 0 0 671 585 453 398

0 50 63 0 172 139 0 0

0 0 0 2697 0 0 1913 1907

Food and yard waste Douglas fir slash Using Nr. 1 & 2 Fluvial deposit Using 1 & 2 Using 1 & 3 Using 1 & 4 Using 1 & 4

Large columns: diameter 10.2 cm and height 25 cm. Small columns: diameter 5.1 cm and height 12.5 cm.

that the ratio of catchment area to bioretention area is 50 to 1, resulting on an overall flux of 77 mm/h. The total duration of the experiment was 36 h, resulting in a cumulative water flux of 2800 mm. The leachate produced by each column was sampled in 3 hour intervals. The samples were collected in acidwashed 250 mL Nalgene bottles, which were then stored in the dark at 3.9 °C until analysis. 2.4. Leachate characterization The leachate was analyzed for pH, electrical conductivity, particulates, DOC, nitrogen, and phosphorus. The pH and electrical conductivity were measured immediately after sampling. The samples were filtered sequentially with a vacuum filtration assembly. Three fractions of particulates were filtered, i.e., N11, 1 to 11, and 0.45 to 1 μm (Whatman 1, Millipore Glass Fibre without binders, and Millipore Mixed Cellulose Ester Membrane Filters, respectively). The filters were dried at 105 °C for 24 h before and after the filtration. The difference in weight of the filters before and after filtration was measured to calculate the amount of each particle size fraction. The DOC was measured by UV/vis spectrometry (Ocean Optics, UV– Vis USB-4000 Fiber Optic Spectrometer, Dunedin, FL). The absorbance was measured at a wavelength of 465 nm and calibrated against DOC (Stevenson, 1994; Zmora-Nahum et al., 2005) measured with a TOC Analyzer (Shimadzu TOC-VCSH, Tokyo, Japan) by acidification/sparging and combustion. A composite sample of the entire leachate produced by each column was sent to the Analytical Science Laboratory, University of Idaho, for analysis of nitrate/nitrite, total nitrogen, and phosphorus (total and ortho). Nitrate/nitrite was analyzed by a Flow Injection Analyzer with a detection limit of 0.1 mg/L, and total nitrogen was analyzed with a high temperature combustion method (ISO 11905-2) having a detection limit of 0.5 mg/L. Total and ortho-phosphorus were analyzed spectrophotometrically (detection limit 10 μg/L for total phosphorus and 6 μg/L for ortho-phosphorus). For measuring total phosphorus, samples were digested (EPA 365.4). Samples for total concentrations were not filtered, samples for nitrate/nitrite, and ortho-phosphorus were 0.45 μm filtered. 2.5. Statistical analysis Differences among treatments were analyzed by Analysis of Variance (ANOVA) and Tukey's test (p = 0.05). The statistical analysis was done with R Version 3.0.2 (R Core Team, 2013). 3. Results and discussion 3.1. Media characteristics Table 2 shows the chemical and physical characteristics of the pure media. As expected, the compost contained the highest amounts of nutrients among the media. The two biochars had high pH, but the pH of

the co-composted biochar was lower than that of the original biochar because during composting neutralization of the biochar occurred. The co-composted biochar picked up nitrogen and other nutrients, as has been observed also by others (Prost et al., 2013). Biochar and cocomposted biochar had by far the highest surface areas of the media. The surface area of the co-composted biochar was about 20% less than that of the original biochar, likely because of clogging of intraparticle pores (Prost et al., 2013). The sand was calcareous and had a fairly high surface area for sand, which is due to the presence of silt and clay particles (Liu et al., 2013). The surface structures of the four media are shown in Fig. 1. The compost shows an assembly of aggregates consisting of particles varying in size. The biochar particles were larger than the compost particles, and the particles had an open porous structure with pore diameters in the order of 20 to 50 μm. The co-composted biochar, on the other hand, was covered with smaller particles similar to those seen in the compost, and therefore likely originating from the compost. The smaller particles can be seen clogging up the porous biochar surface to some extent (Fig. 2). This can explain the smaller surface area measured for the co-composted biochar compared with the pure biochar. The sand particles were covered with clays (Fig. 1), which explains the fairly high surface area measured. 3.2. Comparison of leaching from pure media and media mixes The total particulates and DOC leached from the columns as a function of cumulative flux are shown in Fig. 3. The figure shows particulates and DOC from pure media (a,c) and mixed media (b,d). The leachate concentrations from all treatments were higher initially, and reduced with increasing cumulative flux. Fig. 3a indicates that compost leached the largest amount of particulates. The pure biochars and sand leached less particulates compared to

Table 2 Chemical characteristics of compost, biochar, co-composted (CC) biochar, and sand. Chemical analysis

Units

Compost

Biochar

CC-biochar

Sand

pH Electrical conductivity Total nitrogen (N) Total phosphorus (P) Total potassium (K) Total calcium (Ca) Total magnesium (Mg) Total zinc (Zn) Total manganese (Mn) Total copper (Cu) Total carbon (C) C/N ratio Surface area Cation exchange capacity ζ-Potentiala

– dS/m g/kg g/kg g/kg g/kg g/kg mg/kg mg/kg mg/kg g/kg – m2/g cmolc/kg mV

6.7 2.4 13 8 7 20 6 183 464 45 205 14.3 1.6 14 −22.4

10.1 6.3 1.5 0.7 5.8 4.9 2.1 10 300 20 850 568 610 50 −31.1

8.8 4 5 0.9 6.5 5.5 2.0 10 300 20 820 154 470 57 −32.1

8.2 0.2 0.3 1 0.4 3.8 1 10 100 10 1.4 5.34 11.8 14 −18.7

a

Measured at pH 8 in 1 mM NaNO3.

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(a)

(b)

1 mm

1 mm

(d)

(c)

1 mm

1 mm

Fig. 1. Scanning electron images of different media used: (a) compost, (b) biochar, (c) co-composted biochar, and (d) sand.

(a)

(b)

200 µm

100 µm

(d)

(c)

200 µm

100 µm

Fig. 2. Scanning electron images of (a,b) biochar and (b,c) co-composted biochar. Images are magnifications of the same samples shown in Fig. 1b,c.

H. Iqbal et al. / Science of the Total Environment 521–522 (2015) 37–45

the compost; although, the first outflow sample from the biochar columns contained substantial amounts of particulates. The inset in Fig. 3a and the quantitative analysis of the filters indicate that the particulates leached from the biochar were mostly larger than 11 μm with a smaller fraction between 1 and 11 μm, consistent with the particle size observed by SEM (Fig. 1). When compost was mixed with biochar and sand, we did not observe a significant reduction of particulates from the media mixes as compared to the compost control (Fig. 3b). The major fraction of particulates leached was larger than 1 μm. Neither biochar nor sand were effective in filtering particulates leached from the compost, while the biochar itself contributed to particulate leaching. The compost itself leached high concentrations of DOC (220 mg/L) compared to the other pure media (Fig. 3c). Pure biochars and sand leached some DOC (up to 25 mg/L) in the first outflow sample, but did not leach substantial amounts afterwards. The inset in Fig. 3c corroborates the measured concentrations of DOC by showing the color variation of the samples as a function of cumulative flux. Among the media mixes (Fig. 3d), the compost–biochar mixes leached the highest concentrations of DOC. The compost–biochar mixes leached DOC even in excess of what pure compost leached. The inset in Fig. 3d optically confirms the substantial leaching of DOC from media mixes. Coloration of leachate due to dissolved organic matter can be of concern if leachate flows into surface waters. The excessive leaching of particulates and DOC from the media mixes was surprising because the media mixes contained less compost than the pure compost treatment. We hypothesized that the biochar and sand mixes would reduce the leaching of particulates and DOC due to filtration and sorption. Sorption of organic matter onto biochar during co-composting has been reported (Prost et al., 2013). However, our experimental results did not support that

substantial sorption of dissolved organic matter occurred in our media mixes. 3.3. Dynamics of leaching from media mixes The dynamics of selected leaching parameters from the media mixes is shown in Fig. 4. The pH of the leachate from all the media mixes remained within the range of 7 to 8. The electrical conductivity of the compost–biochar mixes was about two times higher than that of the sand mixes. This is because the biochars had considerably higher salt contents than the sand (Table 2). The peaks of electrical conductivity in the initial outflow samples dropped rapidly and reached background levels of 0.1 dS/m after about 1.5 m of cumulative flux. There was no difference in electrical conductivity between the leachates from mixed and layered columns of compost and sand. Particulates between 1 and 11 μm were, mass-based, the dominating particle size that leached out of the media mixes. The compost–biochar mixes leached the highest amount of particulates of all sizes compared to the compost–sand mixes. The highest amount of DOC was observed from the compost- and co-composted-biochar mixes. Initially, compost–biochar and co-composted-biochar leached high amounts of DOC, and the concentrations dropped only gradually, in contrast to the electrical conductivity, which dropped rapidly. This suggest a continuous release of DOC from the compost; DOC release from the biochars themselves was rapid (Fig. 3c), and the biochars therefore did not contribute to the sustained DOC leaching observed from the media mixes. Mixed and layered compost–sand mixes did not show a pronounced difference in DOC leaching dynamics. We had expected that the layered compost–sand mixture would be more effective in removing DOC from the leachate, but there was no evidence that this was the case in our experiments.

Pure Media

Media Mixes

100

50

(a)

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

C omp + Biochar C omp + C C−Biochar C omp + S and (M) C omp + S and (L)

100

50

0.45 µm 1 µm 11 µm

C ompost Biochar C C−Biochar S and

Particulates (g/L)

150

0.45 µm 1 µm 11 µm

Particulates (g/L)

150

0

(b)

3.5

0.0

350

350

300

300

DOC (mg/L)

DOC (mg/L)

41

250 200 150 100 50

0.5

1.0

1.5

2.0

2.5

3.0

3.5

250 200 150 100 50

(c)

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Cumulative flux (m)

3.5

(d)

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Cumulative flux (m)

Fig. 3. Total particulates (a,b) and dissolved organic carbon (c,d) leached from columns as a function of cumulative flux. Data points represent averages of three replicates. The insets in (a,b) show photos of individual filters for the first outflow samples, and in (c,d) show the color variation (in 2 cm diameter glass vials) of the outflow samples as a function of cumulative flux.

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H. Iqbal et al. / Science of the Total Environment 521–522 (2015) 37–45

12

5

(a) pH EC (dS/m)

pH

10 8 6

1.0

1.5

2.0

2.5

3.0

3.5

0.0 100

(c) > 11 um

Particulates (mg/L)

Particulates (mg/L)

0.5

80 60 40 20 0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

(d) 1 to 11 um

80 60 40 20 0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.0 400

(e) 0.45 to 1 um

80

DOC (mg/L)

Particulates (mg/L)

2

0 0.0

100

Comp + Biochar Comp + CC Biochar Comp + Sand (M) Comp + Sand (L)

3

1

4

100

(b) EC

4

60 40 20 0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

1.5

2.0

2.5

3.0

3.5

(f) DOC

300 200 100 0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Cumulative flux (m)

0.0

0.5

1.0

Cumulative flux (m)

Fig. 4. Dynamics of (a) pH, (b) electrical conductivity, (c) particulates N 11 μm, (d) particulates 1 to 11 μm, (e) particulates 0.45 to 1 μm, and (f) dissolved organic carbon for the media mixes. Data points represent averages of three replicates. Closed symbols are compost–biochar mixes, and open symbols are compost–sand mixes.

3.4. Nitrogen, phosphorus, and carbon leaching 3.4.1. Leachate concentrations Leachate concentrations from composite samples of the entire leaching event are shown in Fig. 5. The left column of the figure shows leachate concentrations from the pure media and the right column shows the leachate concentrations from the media mixes. The main source of nitrogen, phosphorus, and DOC was the compost (Table 2), and the leachate concentrations show that these constituents were mobilized from the pure compost (Fig. 5, left column). Most of the nitrogen in the leachate came from the compost; however, the co-composted-biochar had picked up some nitrogen during composting (Table 2), some of which leached out (Fig. 5c). Adding biochar and sand to compost reduced, but did not eliminate, the leaching of total nitrogen and nitrate/nitrite (Fig. 5d). Phosphorus leached mainly as ortho-phosphorus from the media columns (Fig. 5e,f). The mixing of biochars with compost did not decrease the phosphorus leaching. Similar to the case of phosphorus, amending the compost with biochar did not reduce outflow concentrations of dissolved organic carbon (Fig. 5g,h).

3.4.2. Leachate loads As the compost was the major source of nitrogen, phosphorus, and DOC, concentrations and total amounts of these constituents

leached are affected by the amount of compost used in the columns. To compare the different treatments in our experiments, we therefore normalized the leaching loads by the mass of compost used in each treatment. Fig. 6 shows the normalized loads of nitrogen, phosphorus, and DOC out of the pure compost column and the compost media mixes. For nitrogen, the biochar amendments did not reduce total N and nitrate/nitrite leaching compared to the pure compost (Fig. 6a). When biochar was added to the feedstock during composting, it was reported that ammonia volatilization was reduced, as the negatively-charged biochar sorbed ammonium (Hua et al., 2009; Steiner et al., 2010; Mukherjee et al., 2011). Nitrate and nitrite, on the other hand, are likely not sorbed by our negatively-charged biochar, although it has been reported that high-temperature biochar (N600 °C) can sorb nitrate (Clough et al., 2013). The sand amendments, however, significantly reduced both total nitrogen and nitrate/nitrite leaching. The amounts of total N leached from the compost and compost–biochar treatments were 7 to 8% compared to 4 to 5% for the compost–sand mixes (Table 3). Most of that nitrogen leached was in the form of nitrate/nitrite (Table 3). Based on the negative surface charge of our biochar, we would not expect nitrate and nitrite to be sorbed, and our leaching results confirm this. Knowles et al. (2011) observed reduced nitrate leaching from soil when a pine biochar was amended; however, as their biochar was negatively-charged, nitrate does not sorb to biochar by ion-exchange,

H. Iqbal et al. / Science of the Total Environment 521–522 (2015) 37–45

P ure Media 60

Media Mixes 60

(a)

C oncentration (mg/L )

C oncentration (mg/L )

40

tot−N Nitrate/Nitrite 30

20

40

30

20

10

10

0

0 14

(c)

12 10

tot−P Ortho−P

8 6 4

(d)

12 10 8 6 4

2

2

0

0

200

200

(e)

(f) DOC

C oncentration (mg/L )

C oncentration (mg/L )

(b)

50

C oncentration (mg/L )

C oncentration (mg/L )

50

14

43

150

100

50

0

150

100

50

0

Co

Co

mp

mp

mp

Co

mp

an d( L)

M)

Bio

d(

C-

io

r

an

+S

+S

+C

+B

ha

t

ioc

os

ar

-B

nd

Co

Sa

CC

ch

mp

Bio

Co

Fig. 5. Nitrogen, phosphorus, and dissolved organic carbon (DOC) leached from pure media and media mixes treatment. Data represent averages of three replicates from composite samples, and error bars are ± one standard deviation.

and other mechanisms, possibly inhibition of N mineralization, must be responsible for this finding. For phosphorus, there was no significant difference in the leaching from compost–biochar and compost–sand (layered) compared with the pure compost (Fig. 6b). However, more phosphorus leached from the compost–co-composted-biochar mix and less phosphorus leached from the compost–sand (mixed). In soils, phosphorus leaching is generally limited by the presence of Al and Fe oxides (Sposito, 2008). Iron concentrations were almost seven times higher in sand (2.2 g/kg) than the biochars (0.3 g/kg); therefore, the sand had a higher capacity to suppress phosphorus leaching. Biochar itself leached a substantial fraction of its initial phosphorus (Table 3); however, the amount of phosphorus in the biochar was an order of magnitude smaller than in the compost, so that leaching of phosphorus from biochar was

negligible. The leached phosphorus was almost exclusively in the form of ortho-phosphorus. During the composting process, organic phosphorus is mineralized and becomes soluble when it comes in contact with the water (Sharpley and Moyer, 2000). It appears that biochar amendment promoted release of phosphorus in form of ortho-phosphorus. Phosphorus itself does not sorb to the surface of the majority biochars (Yao et al., 2012; Hale et al., 2013); however, specifically engineered biochars can be used for phosphorus adsorption (Chen et al., 2011). Although the amount of DOC leached from each treatment compared to the amount of C present in the columns was negligible (Table 3), substantial amounts of DOC leached from the columns (Fig. 6c). There were no significant differences between the leaching of DOC among the different treatments (Fig. 6c), indicating that neither

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H. Iqbal et al. / Science of the Total Environment 521–522 (2015) 37–45

L oad (mg N/g)

1.5

(a)

the amount of dissolved organic carbon sorbed was negligible compared to the total amount released from the compost. We had expected that the biochar would sorb a major fraction of the DOC released form the compost. Based literature data, we estimated the amount of DOC that could be sorbed on our biochars. Chen et al. (2008) reported the maximum amounts of selected organic molecules (nitrobenzene, napthalene, and m-dinitrobenzene) sorbed by a pine needle biochar. For high-temperature biochar with a surface area similar to ours, the maximum amounts of the organics sorbed ranged from about 140 to 200 mg/g (Chen et al., 2008). If we consider these numbers also representative for DOC sorption, then the biochars in our compost– biochar mixes could sorb a total of 19 to 21 g DOC. For soils and sediments, it was reported the maximum DOC coverage is 0.86 mg OC/m2 of soil or sediment surface based on a monolayer coverage (Mayer, 1994a, 1994b). If we consider this number as approximate surface coverage of biochar by DOC, then we calculate that the biochars in our columns could sorb a total of 37 to 68 g DOC. Both of these estimates are an order of magnitude greater than the total amount of DOC leached from our compost-only column, which leached a total of 2.8 to 3.2 g DOC. This suggests that our biochar either did not sorb DOC as we had expected, or, which we think is more likely, that the DOC in the leachate did not make thorough contact with the biochar surfaces during the leaching process, thereby making the biochar less effective as sorbent. Flow in unsaturated porous media provides less contact between fluid and sorbent than in a batch reactor, so that sorption determined from a batch sorption test may overestimate sorption under unsaturated flow conditions.

tot−N Nitrate/Nitrite

1.0

*

*

*

*

0.5

0.0

0.5

(b)

L oad (mg P /g)

tot−P Ortho−P

* *

0.4

0.3

* *

0.2

0.1

0.0

8

(c)

Load (mg C /g)

DOC 6

4

4. Conclusions

2

0

mp

mp

Co

Co

L)

M)

d(

d(

Bio

an

an

+S

+S

C-

io

+C

t

+B

os

mp

mp

mp

Co

Co

Co

Fig. 6. Nitrogen, phosphorus, and dissolved organic carbon loads in the leachate normalized by the mass of compost used in each treatment. Data represent averages of three replicates from composite samples, and error bars are ± one standard deviation. Asterisks indicate significant differences (p = 0.05) compared with the pure compost.

biochar nor sand did sorb significant amounts of DOC. Similarly, Beesley et al. (2014) did not observe a significant reduction of dissolved organic carbon concentrations in pore water when compost and biochar were mixed, while others have found that biochar can sorb organic matter during co-composting (Dias et al., 2010; Prost et al., 2013). While some sorption of dissolved organic carbon on biochar may have occur,

Table 3 Percentage of N (total and nitrate/nitrite), P (total and ortho), and dissolved organic carbon (DOC) leached from different treatments relative to the total amounts of N, P, and C in the columns. Treatments

Tot-N

Nitrate/nitrite

Tot-P

Ortho-P

DOC

5.2 0.0 2.4 0.5 4.9 4.8 2.8 3.0

3.0 16.7 8.2 0.0 3.8 4.6 1.7 3.2

2.9 16.7 8.1 0.0 3.7 4.4 1.5 3.1

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

% leached Compost Biochar CC-biochar Sand Compost + biochar Compost + cc-biochar Compost + sand (M) Compost + sand (L)

7.8 0.0 3.3 0.0 6.8 7.7 4.0 4.6

Adding biochar (25% by volume) to mature compost (75% by volume) did not affect the leaching of nitrate/nitrite, ortho-phosphorus, and dissolved organic carbon under an unsaturated flow leaching test. The biochar used in this study was produced at high temperatures around 650 °C and had a high surface area (610 m2/g), and thus our results are only be representative for such types of biochars. Biochars produced at low temperatures have lower surface areas but contain higher quantities of oxygenated functional groups (carboxyl, lactone, phenolic) (Zhao et al., 2013). Such biochars may therefore be better suited for amendments in bioretention systems. Co-composting the biochar did not make a significant difference in terms of its effect on leachates, and co-composting does not appear to be an advantage when incorporating biochar into bioretention systems. While not effective in preventing leaching of nitrate/nitrite and orthophosphorus, biochar applied to bioretention systems may retain metal contaminants. However, as many metals will readily form soluble complexes with dissolved organic carbon, the presence of excess dissolved organic carbon, for instance when compost is applied to the bioretention systems, may circumvent the sorption capacity of biochars for metals.

Acknowledgments We thank Michael Turner for co-composting the biochar used in our experiments. We thank the WSU Franceschi Microscopy Center for access to their facility and Surachet Aramrak for taking the SEM images. The Higher Education Commission of Pakistan (Eg4-074) and the National University of Sciences and Technology, Pakistan, supported the visit of Hamid Iqbal to Washington State University. The financial support from the Washington State Department of Ecology (Interagency agreement: C1400134) is highly appreciated. Funding was also provided by the Washington State University Agricultural Research Center through Projects 0152, 0267, and 0701.

H. Iqbal et al. / Science of the Total Environment 521–522 (2015) 37–45

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Effect of biochar on leaching of organic carbon, nitrogen, and phosphorus from compost in bioretention systems.

Compost is used in bioretention systems to improve soil quality, water infiltration, and retention of contaminants. However, compost contains dissolve...
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