Biochar as adsorbent in purification of clear-cut forest runoff water: adsorption rate and adsorption capacity

Abstract

Forest management practices in boreal peatlands increase nutrient export and suspended solids to watercourses calling for development of new water protection methods. One potential solution could be adsorption-based purification of runoff water using biochar. The aim of this study was to determine the adsorption rate and capacity for Norway spruce and silver birch biochars to design a biochar-filled reactor for a ditch drain. In a 10-day laboratory experiment, biochar was stirred with runoff water from a clear-cut peatland forest, and changes in water pH, total nitrogen, nitrate nitrogen, ammonium nitrogen, phosphorus, and total organic carbon concentrations were measured. Based on the concentration changes, adsorption was quantified and adsorption model containing the adsorption rate and capacity was fitted to the data. Our results indicate that biochar effectively adsorbs both inorganic and organic nitrogen from runoff water. Birch biochar had higher adsorption capacity of nitrogen than spruce biochar. This study demonstrates that the adsorption of nitrogen compounds onto biochar surfaces increases with increasing initial concentrations. Thus, aquatic ecosystems exposed to high nutrient loads from fertile peatlands would particularly benefit from biochar-based water purification.

Nitrogen Recovery from Clear-Cut Forest Runoff Using Biochar: Adsorption–Desorption Dynamics Affected by Water Nitrogen Concentration

“…The current study showed that birch and spruce biochar effectively adsorbed TN and NO 3 − -N in forest runoff waters. Our results support previous studies by Saarela et al (2020) andKakaei Lafdani et al (2020).…”

“…2 and 3), which is probably due to the increased N availability in the solution and increased TN concentration gradient along the flow path from the solution to external surfaces and finally to internal surfaces of biochar (Lagritte & Pasquier, 2016;Ahmadvand et al, 2018). The results are consistent with the results obtained by Ahmadvand et al (2018) and Saarela et al (2020), showing that with an increase in the initial N concentration of the solution, the adsorption amount of biochar tends to increase. The low initial concentrations of NH 4 + -N (< 0.02 mg L −1 ) was the reason why we could not determine the adsorption parameters for this compound, but based on the literature, it is known that biochar is an effective adsorbent for NH 4 + -N (Wang et al, 2015;Takaya et al, 2016;Yin et al, 2017).…”

“…Therefore, the adsorbent must recover nutrients from a rather low concentration, high water volume, and in the presence of high dissolved organic matter concentration and should persist the adsorption under fluctuating nutrient concentrations. It has been reported that the adsorption amount increases with increasing element concentration in water (Ahmadvand et al, 2018;Saarela et al, 2020). This implies that the adsorption amount is improved when the runoff nutrient concentrations increase in the seasonal cycle.…”

“…Knowing the desorption of the recovered N is crucially important when biochar is applied as a water protection tool. Only a few studies before have examined the utility of biochar in peatland forest areas (Kakaei Lafdani et al, 2020;Saarela et al, 2020), and according to our knowledge, this is the first study to address the N adsorption-desorption processes in peatland runoff water. The current experiment was conducted to examine the effect of N concentration on the adsorption-desorption behavior of spruce and birch biochars in the water collected from a ditch draining a clear-cut peatland forest area.…”

“…Biochar has been shown to be an effective adsorbent (Laird et al, 2010;Jin et al, 2016;Wang et al, 2015;Liu et al, 2020;Saarela et al, 2020). Biochar is formed when organic material is heated under limited oxygen concentration in the pyrolysis process.…”