Water resources have been harmed by a variety of toxins, including heavy metals, dyes, surfactants, phenols, and other personal care chemicals [1,2,3]. Since heavy metal waste almost does not dissolve into harmless materials, it accumulates and is toxic to humans. It is also currently amongst the most important environmental concerns. Cadmium is one of the most harmful heavy metals, having been identified as a human carcinogen and teratogen with effects on the lungs, liver, and kidney [4,5,6]. Cadmium is naturally found in the environment as a result of the gradual erosion and abrasion of rocks and soils, as well as one-time events such as forest fires and volcanic eruptions [7,8,9]. As a result, it can be found naturally in the air, water, and soils. Cadmium is also unnaturally extracted from paint pigment, paints, garment manufacturing, battery manufacturing, gasoline manufacturing, and fertilizer manufacturing industries [10,11]. The permissible concentration of cadmium in drinking water has been set very low by the United States Environmental Protection Agency (the US EPA) at 0.005 mg/L, and even lower, at 0.003 mg/L, by the World Health Organization. Therefore, cadmium must be removed from wastewater before it can be discharged into environmental sources and contaminate the water resources [12,13,14]. Membrane-based separation, electrochemical deposition, chemical precipitation, coagulation, solvent extraction, ion exchange, and adsorption have all been explored to reduce the concentration of cadmium to an acceptable level and meet the environmental requirement [15]. Although all the aforementioned technologies are highly effective in removing heavy metals, they have significant drawbacks such as byproduct formation, large sludge production, and high energy requirements [16]. Adsorption is considered an attractive method due to its simplicity, cost-effectiveness, and the possibility of using it on a large scale [17,18,19]. Many adsorbents’ materials have been applied in industries, such as activated carbon, activated alumina, silica gel, molecular sieve carbon, molecular sieve zeolites, and polymeric adsorbents [3,20,21]. An ideal adsorbent material should have a large surface area, maximum adsorption power, mechanical stability, and the ability to be easily separated and regenerated [22]. Biochar has been considered as the promising adsorbent material due to its excellent adsorption capacities for heavy metals and organic pollutants in an aqueous solution [23,24]. Biochar is a solid product of biomass pyrolysis where pyrolysis occurs in the absence of oxygen at the high-temperature heating process for biofuel production [25,26]. Essentially, biochar production aims to produce energy or reduce the amount of biomass feedstock used. However, there has been a lot of emphasis on improving the pyrolysis conditions to increase yield and biochar properties [27,28,29]. The spent biochar suspended in an aqueous medium usually requires centrifugation and filtration measures for recovery. Such recovery process limits the use of biochar in wastewater treatment on a large scale. Furthermore, during these measures, pollutants adsorbed on the biochar may desorb, resulting in secondary pollution [30,31,32]. As a result, it is critical to address biochar’s flaws to increase its effectiveness in water contamination mitigation. Studies from the literature have proven that biochar can remove toxic compounds, but pristine biochar has a limited ability to adsorb heavy metals from wastewater [33]. Several types of feedstocks have been used and treated as biochar-based adsorbents for cadmium removal, such as alamo switchgrass [34], pine wood residues [35], pig manure [36], rice husk [29,37], dairy manure, oak wood [38], pine bark [38], and corn stalk [39]. Surface modification is a proper method to improve biochar properties. Biochar is typically rich in functional groups such as hydroxyl, carboxyl, carbonyl, and methylene on the surfaces of the pore system [40]. Biochar has a large surface area and strong adsorption efficiency that makes it attractive for the removal of heavy metals from wastewater [41]. Surface modification is categorized into activation and formation of composite [42]. Activation is usually carried out by physical or chemical activation, both of which are conventional and have been known about for a long time [43,44]. The biochar-based composite as part of biochar surface modification is usually carried out by modifying the biochar with other materials such as clay, carbonaceous materials, microorganism, organic compound, and metal oxide [45]. Those composites adjust the properties of biochar and improve their functional groups. Biochar surface modification technology is better than primary chemical and physical activation, where the surface modification creates new functional groups that are not present in either biochar or raw materials [42,46]. Biochar-based composite is still in its infancy. It is mainly used in industries, especially those targeting to adsorb specific pollutants where the biochar is specially synthesized to have a specific affinity to the limited pollutants [47]. Biochar has a negative surface charge with high surface area and large pore volume [48]. Those features allow biochar to be a sufficient and promising adsorbent due to distinct adsorption on oxygenated functional groups, electrostatic attraction to aromatic groups, and precipitation on the mineral of biochar [49,50,51]. The biochar-based metal oxide can extract negatively charged oxyanion from an aqueous solution by using the high surface area of biochar as a medium to embed metal oxide with contacting chemical properties [52,53,54]. Recently, biochar surface modification using metal oxide has been employed. Agrafioti et al. [55] used this technology and found out that soaking raw materials (rice husk) and biochar derived from municipal waste with iron powder FeCl3 before pyrolysis created a positive charge which helped to adsorb arsenic from aqueous solution. Vladimir et al. [56] used corncob biochar composite with FeCl3 and found a similar result to Agrafioti, in which the product efficiently adsorbed arsenic. Attempts have been made to use iron’s magnetic properties to produce magnetic biochar that can remove contaminants in pinewood biomass by compositing biochar with hematite, as reported by Wang et al. [57], which resulted in the double performance of sorption capacity in comparison with the pristine biochar. Akila et al. [58] and Khan et al. [59] used magnetic biochar to adsorb lead and cadmium from an aqueous solution and discovered that it was effective and easily separated. In this study, magnetic biochar has been synthesized and developed from kenaf biochar and Fe3O4. The pristine and magnetic biochar were characterized by a vibrating-sample magnetometer (VSM), scanning electron microscope (SEM) X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). Adsorption isotherm, kinetics, and thermodynamic of Cd2+ with this magnetic biochar-based adsorbent have been investigated. Consecutively the effect of pH, initial concentration, and time on the adsorption process was also studied.

Memanfaatkan Biochar untuk Masa Depan yang Lebih Hijau

Biochar, sebuah produk sampingan dari biomassa, telah menjadi bahan yang semakin penting dalam upaya menjaga lingkungan dan memperbaiki kualitas tanah serta air. Dalam berbagai teknik pembuatannya, pirolisis langsung telah menjadi pilihan utama karena kenyamanan, efisiensi, dan dampak ekologis yang positif. Biochar, yang terbentuk melalui proses pirolisis, memiliki kemampuan untuk mengatur pH tanah, meningkatkan kapasitas pertukaran kation, dan menyediakan nutrisi yang diperlukan bagi pertumbuhan tanaman. Selain itu, melalui perlakuan termal ini, biochar dapat mengubah karbon organik menjadi struktur karbon aromatik yang stabil, mengurangi emisi gas rumah kaca, dan membantu mengatasi perubahan iklim.

Namun, biochar tidak hanya bermanfaat untuk pertanian. Ia juga memiliki peran yang semakin penting dalam pengolahan air dan remediasi tanah. Struktur berpori halus dan luas permukaan khususnya meningkatkan kemampuan biochar untuk mengadsorpsi polutan, seperti ion logam berat dan kontaminan organik, baik di dalam tanah maupun dalam air. Ini menjadikan biochar sebagai bahan yang sangat berharga dalam membersihkan lingkungan dan menjaga kualitas air dan tanah.

Salah satu hal yang menarik dari biochar adalah kemampuannya beradaptasi dengan sifat-sifat yang berbeda berdasarkan jenis bahan baku dan kondisi pirolisis. Kita dapat melihat variasi yang signifikan dalam pH, EC, dan karakteristik lainnya tergantung pada jenis biomassa yang digunakan. Selain itu, suhu pirolisis dan waktu tinggal juga memiliki peran kunci dalam menentukan sifat biochar. Ini menciptakan potensi untuk menghasilkan beragam jenis biochar yang sesuai dengan kebutuhan tertentu.

Namun, meskipun banyak penelitian telah berfokus pada perbedaan antara jenis biomassa yang berbeda, perhatian yang lebih sedikit diberikan pada perbedaan yang mungkin terjadi antara biochar yang berasal dari spesies yang sama, tetapi bagian yang berbeda dari tanaman. Studi yang mencoba menjelajahi perbedaan ini membuka jalan untuk pemahaman yang lebih dalam tentang karakteristik biochar.

Selain itu, penting untuk diingat bahwa biomassa yang digunakan dalam produksi biochar juga memainkan peran penting. Seperti yang telah dijelaskan dalam teks di atas, pohon cypress (Taxodium ascendens) adalah salah satu spesies yang populer untuk menghasilkan biochar. Pohon ini tumbuh subur di selatan Cina dan memiliki sifat yang membuatnya sangat cocok untuk gardening dan reforestation di daerah yang memiliki lingkungan dengan tanah yang dalam, lembab, dan bersifat asam.

Dalam penelitian terkait, biochar yang berasal dari bagian berbeda dari pohon cypress (cabang dan daun) telah dipelajari untuk memahami bagaimana sifatnya berubah dengan suhu pirolisis dan waktu. Hasilnya menunjukkan bahwa ada perbedaan yang signifikan antara biochar yang berasal dari bagian berbeda, dan hal ini dapat menjadi landasan untuk pengembangan biochar yang lebih disesuaikan dengan kebutuhan.

Dalam kesimpulan, biochar adalah bahan yang menjanjikan dengan potensi besar dalam aplikasi masa depan. Kemampuannya untuk mengatur pH tanah, mengurangi emisi gas rumah kaca, dan membersihkan air dan tanah menjadikannya alat yang berharga dalam upaya untuk menjaga lingkungan dan mendukung pertanian yang berkelanjutan. Studi lebih lanjut tentang biochar, baik dari segi jenis biomassa yang digunakan maupun kondisi pirolisisnya, akan terus mengungkap potensi yang lebih besar untuk aplikasi masa depan yang lebih hijau. Pohon cypress, dengan sifatnya yang khas, juga memberikan kontribusi yang berharga dalam penghasilan biochar yang efisien dan berkelanjutan.

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