Jerapan Fosfat pada Kleinano dari Tuf Volkan menurut Model Langmuir, Freundlich, Brunauer-Emmett-Teller, dan Dubinin-Radushkevich

Abstract

Sebagai terapan Ilmu Eko-Kimia Tanah dan Mineralogi Tanah, reaksi jerapan sering dimanfaatkan dalam proses dan pengembangan teknik pengelolaan lingkungan dan adsorbennya, antara lain dalam remediasi perairan eutrofik akibat pengayaan fosfat. Dari bahan tuf volkan yang melimpah di Indonesia dapat diekstraksi fraksi kleinano dengan ukuran partikel <200 nm, yang antara lain terdiri atas mineral aluminosilikat XRD-amorfus alofan dan imogolit. Kleinano yang terekstraksi memiliki permukaan reaktif silanol dan aluminol dengan sifat muatan bergantung-pH, sehingga dapat dimanfaatkan sebagai adsorben alami fosfat jika proses ekstraksinya dikondisikan asam. Penelitian ini bertujuan menganalisis dan membandingkan karakteristik jerapan fosfat kleinano bermuatan positif hasil ekstraksi tuf volkan G. Salak berdasarkan model isotermal Langmuir, Freundlich, Brunauer-Emmett-Teller (BET), dan Dubinin-Radushkevich (D-R). Bahan tuf volkan diambil pada lapisan ke-3 (tv3) dari profil Andisol di lereng G. Salak, Bogor. Kleinano (kn3) diekstraksi menurut prosedur Patent USA nomor US 2010/0187474 A1 (Lo 2010) yang dimodifikasi dengan penambahan perlakuan HCl hingga larutan ekstraksi ber-pH 4.0. Hasil penelitian menunjukkan dengan waktu ekuilibrasi 48 jam, aplikasi model Langmuir dan D-R masing-masing menghasilkan jerapan maksimum 460.78 mg P/g kn3 (nilai qe Langmuir; dua-tapak) dan 439.66 mg P/g kn3 (nilai qe D-R; tapak heterogen). Model D-R juga menghasilkan parameter E (0.63 kJ/mol, energi transfer adsorbat fosfat dari larutan ke permukaan adsorben kleinano). Nilai E <8 kJ/mol mengindikasikan terjadinya jerapan secara fisik, non-spesifik, atau elektrostatik. Aplikasi model BET menghasilkan jerapan fosfat 111.11 mg P/g kn3 pada tapak pertama (nilai qmono BET; multi-tapak). Nilai qmono dapat diinterpretasikan sebagai kadar fosfat yang terjerap secara kimia, spesifik, atau pertukaran ligan. Model Freundlich menghasilkan konstanta empirik 0.28 (1/n Freundlich) yang mengindikasikan kn3 merupakan adsorben bermutu tinggi (1/n 0.3 ‒ 0.5). Keunggulan model Langmuir dan D-R keduanya menghasilkan parameter jerapan maksimum. Keunggulan model Freundlich adalah penggunaan persamaan sederhana dan menghasilkan parameter 1/n pengindikasi mutu adsorben, sedangkan model BET adalah penggunaan asumsi jerapan multi-tapak, pelibatan data kadar fosfat awal dan saat kesetimbangan dari larutan uji jerapan ke dalam model, dan menghasilkan kapasitas jerapan pada tapak pertama yang lebih tepat, sebagai qmono. Keempat model disarankan untuk diaplikasikan dalam pengujian suatu adsorben, karena masing-masing memiliki keunggulan spesifik. Namun model yang paling sesuai dengan data hasil penelitian ini adalah model linier Langmuir karena menghasilkan R2 tertinggi, yaitu 0.90 (tapak pertama) dan 0.99 (tapak kedua).

As an application of Soil Eco-Chemistry and Soil Mineralogy, adsorption reactions are often utilized in the process and development of environmental management techniques, such as remediation of eutrophic waters due to enrichment of nutrients, particularly phosphates. From the volcanic tuff materials that are abundant in Indonesia, it can be extracted nanoclay fraction with a particle size of <200 nm, which among others consists of XRD-amorphous aluminosilicate minerals, allophane and imogolite. The extracted nanoclay exhibits reactive surfaces, silanol and aluminol, with pH-dependent charge characteristics, therefore it can be utilized as a natural phosphate adsorbent if the extraction process is conditioned acidic. This research was aimed at to analyzing and comparing phosphate adsorption characteristics of positively charged nano-clays (nc3) extracted from volcanic tuff (vt3) of Mt. Salak based on Langmuir, Freundlich, BET, and D-R isothermal models. The nanoclays were extracted by applying US Patent procedure No. US2010/0187474 A1 (Lo 2010) that was modified with the addition of HCl step to set the extraction solution up at pH 4.0. The results showed that after 48 hours equilibration, application of the Langmuir and D-R models resulted maximum adsorption of respectively 460.78 mg P/g nc3 (Lang-muir qe value; two-layers) and 439.66 mg P/g nc3 (D-R qe value; heterogeneous sites). Model D-R also resulted E parameter (0.63 kJ/mol, transfer energy of phosphate adsorbate from solution onto surfaces of the nanoclay adsorbent). E <8 kJ/mol indicated that physical, non-specific, or electrostatic adsorptions were taken place. Application of the BET model resulted phosphate adsorption of 111.11 mg P/g nc3 at the first layer (BET qmono; multilayers). The qmono could be interpreted as phosphate concentration that adsorbed with chemical, specific, or ligand exchange reactions. The Freundlich model resulted in an empirical constant of 0.28 (Freundlich 1/n), which indicated that nc3 was an high quality adsorbent (1/n 0.3 ‒ 0.5). The advantages of the Langmuir and D-R models were that both resulted in maximum adsorption parameter, while those of the Freundlich model were the use of simple equation and resulted in 1/n parameter as a quality indicator of an adsorbent, and those of the BET model because it was based on assumption of multilayers adsorption, incorporating phosphate concentrations at the initial and equilibrium state of the adsorption test solution into the model, and resulted in a more precise adsorption capacity at the first layer, as qmono. The four models are recommended to apply in performing tests on an adsorbent because each results in specific advantages. However, the most suitable model for the data of this research was the Langmuir linier model because it produced the highest R2, i.e. 0.90 (first layer) and 0.99 (second layer).