Immobilization of Cd in soil by biochar and new emerging chemically produced carbon

1 Introduction

Rice is cultivated worldwide in all continents of the world except Antarctica. It is a staple food of more than 50 million people and is considered the cultural identity of various countries like China, Oman, Korea, Pongal, Onam, Indonesia, India etc. (Prasad et al., 2017). Due to its high consumption rate and ease of cultivation on various soil types, its cultivation gains much attention. It is considered a foreign exchange source in different countries like India, Thailand, USA, China, Vietnam, Pakistan, Italy, Brazil, Uruguay and Australia (Workman, 2019). According to an estimation of the United States Department of Agriculture, worldwide production of rice is 501.96 metric tons. In Pakistan, its cultivation area increased timely. According to Statistics of Pakistan, in 2007–2008 production rate was 5563 thousand tons which increased up to 7449.80 thousand tons in 2017–18. The area under rice cultivation is 2724 thousand hectares providing an average yield of 7500 thousand tons. After wheat, rice is the second staple food for Pakistani people. Rice contributed about 2.1% in agriculture and 0.6% in GDP of Pakistan's economy (GOP, 2016), among which Punjab and Sindh contributed the most. It is cultivated on 1766.8 and 361.2 thousand hectares, respectively, in Punjab and Sindh. Widely cultivated rice varieties in Punjab and Sindh are Basmati, IRRI-9, IRRI-6, D-98 and Begghi (Chandio and Yuansheng, 2018).

Rice plants are annual grass that is about 4ft tall with a hollow stem and fibrous root systems. Grains are produced on spikelet’s. However, its physical parameters like height, biological yield, stem width, number of spikes and grains vary varieties and growing medium. Various varieties of rice include Basmati 370, JP-5, Basmati Pak, IR6, Basmati 198, Kashmir Nafes, DR 82, KS 282, DR 83, Swat 1 and 2, Basmati 385, Shadab, Sada Hayat, DR 92, Shut 92, Pakhal, Rachna basmati, Super Basmati, Kushboo 95, Saheen basmati, Basmat 2000, NIAB-IR, Sarshar, Jhona-349, Muskan-7, Basmati-370, Muskan-41, Sathra-278, Mahlar-346, Palman-246, IRRI-8 and 6, NIAB-IR9, Shaheen Basmati and super basmati rice. In the case of the nutritional value of rice, it is enriched with various minerals like N, P, K, Ca, Fe, Zn, Na and many more. In 100 g milled white rice, it contains energy (349–373 kcal) proteins (6.3–7.1 g), lipids (0.3–0.5 g), carbohydrates (77–78 g), fibers (0.2–0.5 g), riboflavin (0.02–0.06 mg), niacin (1.3–2.4 mg), thiamine (0.02–0.11 mg) and vitamin E (0.075–0.30 mg) (FAO, 1993).

2 Cadmium contamination and crops

Cadmium contamination in soils leads to an alarming level, as reported by various studies (Liu et al., 2013). In the terrestrial ecosystem, soil act as a major source of transferring heavy metals in the food chain through plants. Naturally, Cd concentration is 0.1–16.5 ppm in soil. Its concentration starts rising due to various anthropogenic (e.g., mining, smelting, sewage sludge and paints, plastic stabilization and metal processing) and natural activities (e.g. weathering, sea sprays and wind dust) (Bi et al., 2007). Natural activities contributed 10% to Cd pollution and anthropogenic activity has 90% contribution. Despite regulatory measures, pollution in the environment started rising. A significant concentration of Cd was also observed in agricultural lands found near the industries where it started accumulation in the surface layer of soil and concentration reduced with depth. The surface layer can easily be taken up by plant roots and translocated in various parts of plants where it accumulates. Among anthropogenic activities, 12% of contamination occurs due to agricultural activities like fertilizer application, irrigation with wastewater and pesticides, or insecticides. Impurities in phosphatic fertilizers seem to contaminate the soil with Cd (Nziguheba and Smolders, 2008).

Cadmium uptake by plants depends upon its bio-availability and concentration. Its entry in the roots of plants followed the same pathway/ membrane carriers used in calcium (Ca), magnesium (Mg), zinc (Zn), iron (Fe) and copper (Cu) uptake (Clemens, 2006; Roth et al., 2006). From roots, it easily gets entered into the plant system, where it negatively influences plant physio-chemical processes and reduced yield (Uraguchi et al., 2009). Plants have a certain level of Cd tolerance (0.01% of plant dry weight), but the rise in Cd concentration from that level can cause phytotoxicity in plants. Studies reported that plants grow on Cd polluted soils usually had low productivity and yield. It negatively affects plant metabolism, vegetative and reproductive growth and thus declines the productivity rate. In severe toxicity cases, plant growth becomes restricted and cells are deformed. Bolan et al. (2013) reported that Asian people uptake 30–35% Cd derived from rice (Fig. 1).

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Fig. 1

Major negative effects of Cd in crops.

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The fate of cadmium in plants was studied by Zhang et al. (2009) by using rice cultivars of sensitive and Cd-resistant cultivar. Application of 0.1–0.5 µM Cd on rice grown in hydroponic culture significantly increased the Cd accumulation in plants up to 108% Cd in susceptible rice cultivar and 55% in Cd-tolerant cultivars. High Cd supply also increased the Cd concentration in the roots of rice. Cd starts localization in the plant cells as Cd concentration increases in the growing medium (vacuoles and cell walls). In Cd-resistant rice, excess Cd also detoxifies in the root cell wall, polysaccharides, organic acids and proteins by forming bonds with them. Thus, translocation of Cd from shoot to root decreased.

Seeds that are sown in the Cd polluted environment usually accumulate Cd, which negatively affects seed physiological processes, thus hindering seed germination (Ahsan et al., 2007). Cadmium inhibitory effect on seed germination rate was studied in rice crop. A wide range of Cd concentration from 0.2 to 1.0 mM was used with the rice test crops. Cd high supply increased the accumulation of Cd in rice seeds and caused an inhibitory effect on seed germination. It also reduced the plant growth, water contents and biomass of rice plants (Kanu et al., 2019). When plants faced the toxicity of Cd, their defence system activates. It binds Cd with protein’s sulfhydryl group, which caused distortion in the protein structure and increases the production of reactive oxygen and free radicals. Hydrogen peroxide, malondialdehyde and leaf electrolyte contents increase with high Cd uptake (Kanu et al., 2019). Cadmium accumulation in rice at a higher level, decrease antioxidant enzymes (superoxide dismutase, peroxidase and catalase) activity and yield components, e.g., panicles, spikelet/panicle, grains number, 1000-grain weight, and total yield of grain (Ahsan et al., 2007; Kanu et al., 2019) (Table 1).

3 Biochar and crops growth

The most critical parameter for determining the fertility and productivity of any soil is organic carbon contents. Pakistan soils usually have less organic matter content (2% only). In such a scenario, the application of biochar significantly increases the organic carbon contents in soil (Abid et al., 2017). Biochar is a high carbon-containing solid product of biomass/ residues pyrolysis. Biochar is heterogeneous material that consists of both kinds, e.g., stable and labile materials (Lehmann et al., 2011). Its major component is carbon, followed by volatile matter, ash, and moisture. Its properties vary with the conditions and procedure adopted during the pyrolysis process and substrates used (Lehmann et al., 2011) (Fig. 2).

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Fig. 2

Some major benefits associated with biochar application in soil.

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Although it is enriched with minerals due to heating at high temperature, volatile components like nitrate and ammonium are lost (Deluca et al., 2009). In a study of wood biochar preparation, as the pyrolysis process starts, N becomes volatile as the temperature reaches 200 °C, at 375 °C S volatile and at 700–800 °C K and P volatilization started. As the temperature reaches above 1000 °C, Ca, Mg and Mn become volatile. Therefore, nitrogen is sensitive to heat and is only 0.18–5.60% is present in biochar. Carbon is the major structural component of biochar which is 17–96% in biochar concentration. Phosphorous is present in 0.27–48% and K is of 0.1–5.8%. A minor quantity of S, H, O and other basic cations are also present (Preston and Schmidt, 2006). Biochar is the nutrient-enriched and environmentally friendly product of pyrolysis. It is prepared from straw residues, crop residues, manures, grains and grasses (Demirbas, 2004).

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Fig. 3

Process involved in Immobilization of heavy metals by biochar in soil.

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Different methods are used for the preparation of biochar e.g. gasification (Sánchez et al., 2009), carbonization and pyrolysis. Muffle furnace is one of the most used method in testing laboratories (Sánchez et al., 2009). Pyrolysis was performed in inert conditions. Nitrogen or helium gas was used it reactor. It is also prepared in Kilns. Kiln is a very simple, cost-effective and easy method (Zhang et al., 2010). This type of production is usually suitable for large-scale biochar production as it yields only 35% and cost-effective (Van Zwieten et al., 2010).

Maroušek et al. (2019) reported that biochar incorporation in the soil significantly enhances the carbon sequestration in the soil, improving soil fertility and aggregate stability. Poor aggregate stability of soil is one reason behind the low quality of soil, which is caused by lower organic carbon contents in soil. Biochar can increase soil aggregation, leading to improved soil properties due to increased aeration, bulk density, water-holding capacity, and ion exchange capacity. Properties of biochar vary with feedstock, pyrolysis temperature and conditions (Freddo et al., 2012).

In three years of field research, biochar effect on the bioavailability of Cd and Pd was analyzed by Bian et al. (2014) using rice as a test crop. Biochar was prepared by wheat straw through pyrolysis at 550 °C. Extractable concentration (through CaCl₂) of Cd and Pd was significantly decreased in all years by 70.9, 64.8, and 60.9% during three years and 33.3, 59.1 and 79.6% Pd availability respectively. Excess available Pd and Cd was immobilized by sorption on clay minerals, precipitate with carbonates due to reduction in soil pH. Except for this, ion exchange and functional groups of biochar form complexes. Furthermore, mineral-organic layers were formed on surfaces of biochar, which was thickened over time. That result proved that immobilization of metals with biochar application increased with the time passage.

Biochar was first produced by Amonette and Joseph (2009), through the oxygen-free burning of crop residues at high temperature (700 °C). It is comprised of inorganic carbon, Mg, Ca and sodium and various other minerals. It can protect the nutrient leaching, reduce greenhouse emission and their detrimental effect on the environment, increased carbon sequestration and agriculture productivity (Oni et al., 2019). Biochar can increase the CEC and anion exchange capacity (AEC) of soil, mediate soil pH, increase N and P availability, boost up plant above and below ground growth, and increase moisture contents, leading to a reduced risk of soil erosion. Biochar has pores in its structure, among which micropores are responsible for its high-water holding capacity and macropores are responsible for aggregation, aeration and ease in root penetration (Freddo et al., 2012). Micropores also allowed the sorption of OM, which is beneficial for high microbial activity and their inhabitation. Microbes are involved in soil remediation from various kinds of soil and environmental pollutants (Hameed et al., 2017). The ability of biochar to degrade pollutants and heavy metals is utilized in removing pollutants from soil, especially for heavy metal remediation.

Improved physical soil properties were reported by Herath et al. (2013) due to biochar application. Corn residues are pyrolyzed at different temperature (350⁰C and 550⁰C) and its effect on soil properties, e.g., bulk density, hydraulic conductivity, aggregate stability and hydraulic conductivity was analyzed. Results showed that biochar amendment improves the aggregate stability, which varies with soil type and other parameters were improved significantly. Reduced agricultural productivity is directed towards soil degradation. Soil degradation leads to reduced agricultural productivity. To lower the soil's degradation rate, biochar application is an effective measure reported by Vinh et al. (2014). In an experiment, biochar, compost and inorganic fertilizer application, biochar addition provides a significantly positive result. It increases micronutrient accumulation in rice plants and increases the yield of rice up to 22.3% (Table 2).

Only soil reduced the nitrogen losses and protected underground water from contamination (Dong et al., 2014). Nitrogen has a high mobility rate and N-fertilizers can caused contamination of groundwater through leaching. In such a scenario, two years of consecutive field experiments showed that the application of biochar could significantly absorb nitrogen on its surface. Increased retention of nitrogen in surface soil significantly increased the productivity of rice plants grown in paddy fields. Biochar can replace inorganic fertilizers, as reported by Lakitan et al. (2018). In a paddy field experiment, 0, 0.4, 0.8, and 1.2 Mg ha⁻¹ biochar was applied. Its application positively affects agronomical rice parameters. Root elongation and shoot length were not much affected, but during the vegetative growth period of rice, biochar addition significantly increased the tillering capacity of rice along with its grain’s quality and quantity.

Silicon (Si) is considered beneficial for plants, especially under stress conditions. It is thought to alleviate nutrient stress. Biochar incorporation in soil provides Si and other macro and micronutrients and increased the mobility of these nutrients in agricultural lands (Li et al., 2019). Different feedstock biochar was prepared and analyzed in field conditions. Analysis of biochar showed that rice husk biochar was enriched with Si and on average, its concentration varies from 3 to 20% in biochar. Other than Si, biochar also had 1.23% N, 0.6% S, 0.3% P, 2.7% K, 1.2% Ca and 0.54% magnesium. The addition of crop residues biochar significantly contributes to nutrient cycling in croplands.

4 Immobilization of heavy metals

Once a site is contaminated with metalloid or metal, it can’t be reversed or removed easily from the soil. Metal is only transformed from more available or more toxic to less bioavailable or less toxic forms (Wu et al., 2015). Otherwise, the remediation process can only be performed by plant uptake of these metals. The metal transformation from toxic to less toxic form is practised more. Biochar is a highly stable product that contains a negative charge on its surface. Due to this negative charge, biochar can retain the metal on its surface. Sorption of metal on biochar surface reduced the risk of metal availability for a longer extent due to the high stability of biochar (El-Naggar et al., 2018). Biochar has ketones, carboxylic and diols groups which are chemically active and bind the Cd and other metals. Instead of this, the high aeration capacity of biochar caused an increased flow rate of oxygen which enhanced microbial activity and aids in the immobilization of these metals (Freddo et al., 2012).

Biochar application reduced the vertical leaching of Cd significantly (Sun et al., 2020). A pot experiment was conducted on Cd-polluted saline-alkaline nature soil. Biochar addition not only lowers the Cd accumulation in plant tissues but also reduced residual Cd concentration. Moreover, biochar reduced the mobility and availability of residual Cd for a longer time. It also protects Cd from vertical leaching, which saves underground water from Cd contamination. But the rate of immobilization was directly related to soil properties and adopted practices. Reduced Cd availability due to biochar addition was also reported by Abbas et al. (2017). Soil was incubated with rice residues biochar, applied at different rate (0, 1%, 1.5%, 3% and 5% w/w). Results reported that the biochar amendment increases the solution pH and silicon (Si) contents and reduces Cd accumulation in plant tissues. In comparison which control plants, biochar amended plants showed more plant height, tillers, biomass and yield of grains. That improved growth was due to reduced oxidative stress, increased antioxidant enzyme activity and photosynthesis rate in biochar amended plants. Biochar application at 1.5%, 3% and 5% reduced 26%, 42% and 57% accumulation of Cd in plant tissues respectively. Except for this, it also reduced the Cd translocation towards grains which was the desired result.

Cadmium translocation and its deleterious effect on rice growth were analyzed by Rizwan et al. (2018). Applied biochar at 1.5, 3.0, and 5.0% w/w significantly reduced the Cd toxicity effect on rice. The harmful effect of Cd was mitigated by increased chlorophyll contents, photosynthesis activity, SOP, POD and CAT enzyme activity. Increased antioxidant enzyme activity helped reduce Cd accumulation in rice tissues. Results demonstrated that 24.4, 36.6, and 57.5% Cd concentration was decreased by 1.5, 3.0, and 5.0% biochar application. However, the duration of reduced mobility of Cd in soil solution was still unknown (Table 3).

Biochar application in Cd polluted soil significantly reduced the bioavailability of Cd in soil solution (Qiao et al., 2018). Reduced bioavailability leads to reduced uptake of Cd by rice roots. Lower uptake of Cd leads to 93% reduced accumulation of Cd in rice grains. Analysis indicated that iron (Fe) concentration was more in biochar amended treatments. High Fe availability increased the immobility of Cd in soil. The study proved that biochar application is a promising strategy for the utilization of Cd polluted sites. The same results were also observed by Bashir et al. (2018) in their pot experiment. Among rice straw biochar and rock phosphate addition, biochar addition increased in situ immobilization of Cd in soil up to 22.91%. Biochar raised the soil pH significantly, which played a key role in Cd immobilization and its transformation/ precipitation into unavailable forms. Reduced uptake and translocation of Cd were reflected from lowering the antioxidant enzyme activity during Cd stress and even no deleterious effect was reported on plant growth and productivity (Fig. 3).

5 Alkaline soils and acidified carbon (AC)

Pakistan soil is alkaline, having high soil pH and high calcium carbonate contents in various regions' soil. The application of biochar significantly raises the soil pH. Alkaline nature of biochar sometimes imparts adverse effect on soil health, especially when soil is already alkaline. In such a scenario, acidified biochar application proves an effective measure for declining soil pH (Sultan et al., 2020). Doydora et al. (2011) explained the production of acidified biochar and its effect on soil properties. Pine chips and peanut hull was pyrolyzed by N₂ gas at 400 °C. After one hour’s residence time, this biochar was grinded and sieved and then treated with 0.5 N HCl for half an hour and shake continuously. It was then allowed to stand for 24 h and then filtered through 0.45 µm sieve. Sieved biochar then dried in an oven for about 48 h at 65 °C. Experimental results showed that it reduced the N leaching losses but did not affect P and K leaching.

Acidified compost and AC results were compared by Ramzani et al. (2017). Three soils were used in the experiment and quinoa was the test plant. AC provided better results than acidified compost under various kind of stress conditions. Production of reactive oxygen species was reduced in AC amended treatments than control or acidified compost treatment. Moreover, AC addition reduced the pH of the soil and improved soil chemical properties. AC application in saline-sodic soil improves the soil quality. AC prepared by rice straw and dicer wood biochar was washed with HCl thrice (Sadegh-Zadeh et al., 2018). AC incorporated in the soil column at 50 g/kg rate and column leachate was analyzed. The analysis showed that AC reduced the EC and sodium adsorption ratio (SAR). Due to the ability to lower the SAR, it was used in the reclamation of saline-sodic soils. Rice straw AC was enriched with Ca and Mg. After incorporating AC in soil, it releases Ca2 + and H + in soil solution that replaced with Na⁺. Except this, AC started dissolution of CaCO₃ which release Ca²⁺ and H⁺ and played role in the remediation of saline sodic-sodic soil.

In calcareous soil, AC improves nutrients bioavailability and positively affects maize plant growth (Sahin et al., 2017). Acid-modified biochar (AB) was prepared by treating poultry manure biochar with phosphoric acid and nitric acid and their combination. Modified biochar was applied at 0.5% w/w rate and their effect on mineral nutrient availability, plant nutrition and plant yield were analyzed. Results demonstrated that AB produced from both acid combinations was enriched with both P and nitrate radicals. That modified biochar also improved the mineral nutrient availability like P, Ca, K, Mg, Fe, Mn, Zn and Cu, and accumulation of these minerals in plant tissues was also enhanced. It also increased the dry weight and yield of maize significantly.

Comparison of different methods of AC production was analyzed by Peiris et al. (2019). Tea-waste was pyrolyzed at different temperatures and produced biochar modified with three acids, e.g. HNO₃, HCl and H₂SO₄. Results denoted that increased pyrolysis temperature decreased the acidic functionalities of biochar and effects on pH and CEC of the biochar. Modified biochar had lower ash contents as compared with simple biochar. Sulfuric acid-modified biochar had higher CEC and then nitric acid modified. The cation exchange capacity of applied biochar is of significant importance because nutrient retention is directly related to CEC. Due to AC incorporation in soil, cationic species become part of soil solution, especially in high pH soils.

6 Conclusion and future perspective

Heavy metals concentration in the soil exceeds their permissible limits (3 mg kg⁻¹) throughout the world due to the increasing rate of green revolution, urbanization and industrialization. Their toxicity and high uptake by plants adversely affect human health, soil and plant productivity and cause serious environmental concerns. Biochar is a stable C enrich pyrolytic product used as an echo-friendly remedy to remediation of heavy metals (Cd) contaminated soils. Owing to its high stability and negative charge, it adsorbs metals, thus significantly reducing metals mobility, bioavailability, and uptake by the plant. It also has active functional groups like ketones, carboxylic, and diols that bind the Cd and other metals. Biochar can also mitigate the harmful effect of Cd by increasing plant chlorophyll contents, photosynthesis activity, SOP, POD and CAT enzyme activity. Biochar can increase the CEC and anion exchange capacity (AEC) of soil, mediate soil pH, and increase N and P availability, boosting plant growth. Its highly porous structure improves soil water and nutrient holding capacity, aggregation, aeration, and ease in root penetration. Researchers claim that biochar should be used as amendments that aid in the remediation of heavy metals contaminated soils and improve C sequestration, soil health, and plant productivity.