Preparation, Characterization of Bagasse-based Biochar and its Adsorption Performance in Tropical Soils

To study the biochar properties and its influence on Cu (II) adsorption, biochar was prepared from bagasse under the conditions of 350°C, 450°C and 550°C respectively, and biochar was added to three typical tropical soils (paddy soil, laterite and dry red soil). After mixed culture for 30 d, batch equilibrium experiment method was used to determine the effect of biochar on soil adsorption of Cu (II) before the characterization of biochar properties. Results showed that ash content, CEC and pH value of were increased with the increase of pyrolysis temperature. PH values ranged from 5.56 to 8.92. Moreover, the specific surface area and pore structure were affected by pyrolysis temperature. In addition, when biochar was added into soils, the adsorption capacity of Cu (II) in paddy soil, laterite and dry red soil increased by 85.98%, 89.07% and 94.73% respectively. Freundlich and Langmuir equation could be fit for adsorption isotherm of C u (II). Results demonstrated that the Freundlich model (R>0.976) was the better isotherm than Langmuir model (R>0.917). Introduction Large areas of acid soils are distributed throughout the tropical and subtropical regions of southern China. The typical features such as strong acid, aluminum toxicity and low fertility levels have caused poor growth and low yield [1]. Due to low cation exchange capacity (CEC) and pH value, these soils have high mobility and bioavailability of heavy metals [2-4]. Therefore, they are easily polluted by heavy metals, which have been paid more attention [5]. Soil heavy metal pollution mainly comes from the industrial water, pest icides, sludge and atmospheric dust, etc. Excessive heavy metals can cause plant physiological function disorder and malnutrition. As heavy metals cannot be degradated by soil microbes, heavy metals can be persistent and difficult to be removed or degraded once introduced into soils. Moreover, heavy metals can be enriched by plants and endanger human body health through the food chain [6]. It had been heavily reported that many techniques had been developed to remediate soils polluted by heavy metals, including physical methods [7], electro kinetic remediation [8], incorporation of amendments [9], combined remediation technologies [10] and biological remediation [11,12]. Accordingly, it showed that inorganic minerals and organic materials could affect the mobility and bioavailability of heavy metals. Researchers also reported that zeolite [9], lime, red-mud [13] and chicken manure compost [14] could adsorb heavy metals, but rarely applied into practice due to their unsatisfactory effects or high cost. Biochar is the product of biomass through incomplete combustion in the absence of oxygen [15]. Due to microprous structure, active functional, alkaline and high CEC, biochar has extensible potentialities to act as a heavy metal modifier. It had been proposed that biochar had a strong adsorption affinity for heavy metals [16]. Biochar produced from S. alterniflora was employed to remove Cu (II) from aqueous solutions [17]. Biochar could also effectively reduce concentrations of Cu (II), Pb (II) and Cd (II) originated from contaminated soil systems [5]. In addition, a two-year field experiment revealed that biochar amendment could greatly reduce Cd uptake by rice in a contaminated paddy soil [18]. In order to further understand the effects of biochar on adsorption-desorption behavior of heavy metals, with OECD balance experiment method, this paper aimed at revealing the effects of the bagasse-based biochar on environmental behavior of heavy metals in tropical soils, to provide certain reference basis for the remediation of heavy metal contaminated soil in tropical areas. Materials and Methods Soils. The soils (0-20 cm) used for sorption experiments were collected from three cities (Danzhou, Qiongzhong, Ledong) in Hainan province, China. Posthole diggers washed with deionized water between samples were used. All of the soils were typical agricultural soils. The collected soils were air-dried slowly under sheets of paper at room temperature until water content reached about two-thirds of water-holding capacity. Then, the soil samples were sifted through a 60 mesh sieve for analyzing their chemical and physical properties, and a 1-mm sieve for sorption experiments. Characteristics of the soils which were determined according to the routine methods [19] were listed in Table 1. Table 1 Phy-chemical properties of soils