This work evaluates phosphate and nitrate ion adsorption from aqueous solutions on calcined adsorbent substrates of variable charge, prepared from three granulometric fractions of an oxidic lithological material. The adsorbent material was chemically characterized, and N₂ gas adsorption (BET), X-ray diffraction, and DTA techniques were applied. The experimental conditions included the protonation of the beds with HCl and H₂SO₄ and the study of adsorption isotherms and kinetics. The lithological material was moderately acidic (pH 5) with very little solubility (electrical conductivity 0.013 dS m⁻¹) and a low cation exchange capacity (53.67 cmol (+) kg⁻¹). The protonation reaction was more efficient with HCl averaging 0.745 mmol versus 0.306 mmol with H₂SO₄. Likewise, the HCl-treated bed showed a better adsorption of PO₄⁻³ ions (3.296 mg/100 g bed) compared to the H₂SO₄-treated bed (2.579 mg/100 g bed). The isotherms showed great affinity of the PO₄⁻³ ions with the oxide surface, and the data fit satisfactorily to the Freundlich model, suggesting a specific type of adsorption, confirmed by the pseudo-second-order kinetic model. In contrast, the nitrate ions showed no affinity for the substrate (89.7 µg/100 g for the HCl-treated bed and 29.3 µg/100 g bed for the H₂SO₄-treated bed). Amphoteric iron and aluminum oxides of variable charges present in the lithological material studied allow for their use as adsorbent beds as an alternative technique to eliminate phosphates and other ions dissolved in natural water.

El objetivo de este estudio fue determinar la reducción de dureza en aguas naturales utilizando lechos de carga variable preparados a partir de dos materiales litológicos oxídicos (MLO), denominados “Material C” y “Material L”, que fueron recolectados de depósitos naturales en Venezuela. Las pruebas de adsorción se realizaron con lechos naturales activados con NaOH. Los resultados revelaron la presencia de óxidos anfóteros de hierro, silicio y aluminio en su estructura. Experimentos de columna muestran una capacidad de retención de 12.51 y 11.02 mgCaCO₃/kg para los lechos L y C, respectivamente. La activación incrementa 2.5 veces el rendimiento del proceso, 29.28 mgCa²⁺/kg Material L y 26.94 mgCa²⁺/kg para el Material C, debido al aumento de la densidad de cargas negativas superficiales. La adsorción es de tipo no específico y los datos se ajustan al modelo de Freundlich. Se concluye que los materiales evaluados representan una excelente alternativa en el tratamiento de la dureza del agua.

The present paper describes the influence of compost/solvent relations on pH and electric conductivity (CE) measurements, using water and CaCl2 0.01 M as extractants, in the extract suspension as well as in centrifuged supernatant. According to the results, pHCaCl(2) seems to be more independent of extractant quantity, contrarily to aqueous extract measurements, which increases with dilution, described by a quadratic model. As a consequence, measurement must be performed where the curve slope is minimum or null, and where the measurement becomes independent of extractant quantity. For aqueous extract, it happens at relations compost/extractant greater than 1:8, and for CaCl2 extracts, greater than 1:5. pH(H2O) is higher than pH(CaCl2) showing that CaCl2 is a better acidity extractant. EC measurements also show polynomical behavior, therefore must be performed on relations compost/extractant greater than 1:6, where EC values are more independent of extractant quantity. EC could be performed in CaCl2 extract, suspension or supernatant, with respective blank correction.

El presente trabajo describe la adsorción de iones cromo(VI) sobre lechos adsorbentes calcinados preparados a partir de materiales litológicos oxídicos refractarios provenientes del sector Lagunillas, estado Mérida, Venezuela. El estudio se realizó sobre el material crudo, así como sobre tres tipos de sustratos calcinados, dos tratados químicamente en medio ácido y alcalino para generar cagas superficiales positivas y negativas, respectivamente. El tratamiento térmico favorece la formación de los óxidos anfóteros de carga variable cuyos PCNC se encuentran alrededor de pH 6,5. Las isotermas de adsorción se ajustan al modelo BET con valores de la constante C entre 0 y 1 y la cual es 10 veces mayor para los sustratos tratados químicamente. La reacción de adsorción tiene rendimiento de 2,5 veces mayor sobre el sustrato tratado en medio ácido. La variación de pH durante la reacción de adsorción sobre los sustratos sugiere un intercambio de protones en la primera monocapa que son sustituidos por los iones dicromato, especialmente en el sustrato tratado en medio ácido. El estudio de desorción indica que se desorbe un 50% menos dicromato de la superficie cargada positivamente, lo que sugiere que hay mayor afinidad entre los iones dicromato y esta última.

Background The paper describes lead ion adsorption on variable charge oxidic calcined substrates with chemically modified surfaces. Amphoteric oxides of iron, aluminum, titanium, and manganese, change their surface electric charge after acid or alkaline treatment, letting cationic or anionic adsorption reactions from aqueous solutions. This property allows using them as adsorbing substrate for heavy metals retention in water treatment systems. Methods Substrate was prepared by extruding cylindrical strips from a saturate paste of the oxidic lithological material-OLM; dries it up and thermally treated by calcination. The study was performed by triplicated trial, on batch mode, using 2 grams samples of treated with NaOH 0.1N and non-treated substrate. Lead analysis was performed by AAS-GF. Freundlich and Langmuir models were used to fit results. Comparing differential behavior between treated and non-treated substrates showed the variable charge nature of the OLM. Results Results show L-type isotherms for the adsorption of Pb(II) ions on the activated substrate, suggesting good affinity between Pb(II) ions and OLM’s surface. Average value of adsorption capacity (K) for activated substrate (1791.73±13.06), is around four times greater than the non-activated substrate (491.54±31.97), during the adsorption reaction, 0.35 and 0.26 mmolH+ of proton are produced on the activated and non-activated substrate respectively using a 1 mM Pb(II) solution and 72.2 and 15.6 mmolH+ using a 10 mM Pb(II) solution. This acidification agrees with the theoretic model of transitional metals chemisorption on amphoteric oxides, present in lithological material used for the preparation of adsorbent substrates, confirming the information given by the L-type isotherms. Conclusions Results suggest that these variable charge oxidic adsorbent substrate show great potential as an alternative technique for water treatment at small and medium scale using granular filtration system. The easiness and low price make them suitable to apply in rural media where no treating water systems is available.

En la presente investigación se evaluó un sistema de filtración lenta en medio granular utilizando sustratos adsorbentes preparados a partir de materiales litológicos oxídicos de dos localidades andinas identificadas como G y V, que fueron caracterizados químicamente. Se prepararon filtros monocapa y bicapa con lechos tubulares de 5 y 8 mm de longitud como sistema de tratamiento de agua residual con turbidez de 76 UNT, DBO 230 mg/L y DQO 445 mg/L. Se analizó la turbidez, el pH y la conductividad eléctrica en alícuotas de 100 mL del percolado hasta la saturación del lecho filtrante. Los resultados muestran que los materiales estudiados depuran el agua residual a través de un proceso de filtración y adsorción, al emplear un filtro bicapa estructurado con el material G se obtuvo una remoción de la turbidez residual en 61 %. Adicionalmente, se logró la reducción del 97.4 de la DBO y 94 % de la DQO, en contraste con el 86 % obtenido con el filtro de arena convencional de tres capas. Los resultados demuestran la potencialidad del uso de estos medios granulares para la depuración de aguas residuales y efluentes como sistemas filtrantes alternativos, ecológicos y de bajo costo.