Giovanni Tiravanti

A bench study on chromium recovery from tannery sludge

Abstract A process study on separation and recovery of Cr(III) from tannery sludge is presented. The main steps of the process include Cr(III) extraction with sulphuric acid solution at pH 1, Cr(III) oxidation to Cr(VI) with H 2 O 2 , Cr(VI) separation from other cations and its subsequent reduction to Cr(III). The experimental conditions for each of the above steps are optimized on a bench scale. The overall Cr(III) recovery yield from sludges, controlled by the extraction and oxidative phases, amounts to 80%. Cr(III) recovery from segregated exhausted tannery baths, characterized by a low concentration of major cations, is quantitative. The (Fe + Mg)/ Cr molar ratio strongly influences the H 2 O 2 /Cr ratio required to obtain near quantitative Cr(III) oxidation. At present, the running costs may be estimated to be about double that of disposing of sludges in controlled landfills. The tendency to eliminate Fe(II) in tannery wastewaters treatment would significantly reduce the process cost and make it competitive.

BATTERY INDUSTRY WASTEWATER : PB REMOVAL AND PRODUCED SLUDGE

Abstract The process of neutralization with NaOH, in the presence of Fe(III) salt, of sulphuric acid battery industry wastewater seems to be more suitable than any other process for Pb removal because at the same time, it allows the exploitation of the scavenger action of Fe(III), which is often present in the same wastewater and precipitates as hydroxides. In order to optimize sludge production, a laboratory research study has been carried out to minimize the quantity of Fe(III) to be added. All aspects concerning the chemistry of wastewater have been taken into consideration step by step and discussed. As a result, a process which requires the addition of Fe(III) so that an Fe/Pb ratio, both expressed as ppm, of the order of 0.5 is achieved. The process has been applied to two artificial solutions, characterized by a content of H2SO4 of 1000 and 5000 ppm, respectively, and by 10 ppm of Pb, as well as to five samples of wastewater. The proposed process takes about 1 h to perform: 30 min for neutralization up to pH 8.5–9.5 and Fe(III) addition and 30 min for correction in the 9–9.5 pH range and for flocculation with the help of a polyelectrolyte. The Pb concentration of the treated effluent is below 0.2 ppm. Conditions for Pb removal using only NaOH or Na2CO3 have also been considered.

Low and non waste technologies for metals recovery by reactive polymers

Conservative technologies have been investigated in order to utilize natural and synthetic reactive polymers for selective metal removal, recovery and reuse. Examples refer to the treatment of industrial wastes containing silver, cadmium, mercury, chromium, iron and aluminum by different innovative processes. A precipitation process is described, namely Metals Extraction by Xanthate Insolubilization and Chemical Oxidation (MEXICO), using agricultural raw materials to prepare starch or cellulose xanthates as precipitating agents for metals. The process reliability has been proven at pilot scale level (15 m3/d) by a demonstrative campaign treating continuously real chlor-alkali wastewaters for mercury removal and recovery. Results demonstrated Hg concentrations in the filtered effluent in the range of 5–20ppb, with an average removal efficiency of 99.8%. The metal-xanthate sludge was then detoxified by treating with NaCIO solution at acidic pHs. Concentrated (> 10 g/l) chlorocomplex Hg(II) solutions were obtained, to be directly recycled to the electrolytic cell. Other applications deal with the treatment of segregated chromium in exhausted spent baths. The patented Ion Exchange REcovery of CHROMium (IERECHROM) process utilizes a weak cation (carboxylic) ion exchange resin and allows for recovery and reuse of 90% of high purity (> 99%) chromium from other interfering metals such as Fe and Al present in the spent tanning baths. A process for the treatment of tannery sludges has also been studied, particularly for possible restoration of contaminated landfilling sites. The process is based on the acid leaching of the sludges and on treating the leachate by a weak base anion resin, selective to ferrie species, coupled with a strong cation resin (sulphonic) for the separation and recovery of chromium and aluminum. The economical evaluation of the conservative processes described is favorable, without taking into account the economic revenues from the recycling operation of the metals recovered and from the minimized environmental impact related to these technologies.

Degradation of sulphur containing s-triazines during water chlorination

Abstract The reactions of four sulphur containing s-triazines (prometryne, terbutryne, ametryne and desmetryne) with hypochlorous acid (HClO) and chlorine dioxide (ClO 2 ) have been investigated using an 11 ppm/3 ppm oxidant/herbicide ratio. The main objective of the study was the identification of by-products. Additionally, to study the effect of oxidant concentration on the reaction rate, two more oxidant/herbicide ratios (3 ppm/3 ppb and 11 ppb/3 ppb) have been investigated only for prometryne. Oxidation reactions were monitored by high performance liquid chromatography (HPLC), while, the identification of by-products was initially carried out by low resolution HPLC-mass spectrometry (HPLC-MS) and confirmed by accurate mass measurement. Under the experimental conditions ( T = 20°C, pH = 8, reaction time = 48 h), the results indicate that all the investigated triazines react in the same way with each oxidant. The reactions with HClO occur much faster than those with ClO 2 and give rise to three identified oxidation by-products: the sulfoxide, the sulfone and the sulfones hydrolysis product. The reactions with ClO 2 , instead, give rise to a sole oxidation by-product: the sulfoxide. With both oxidants, as expected, the lower the oxidant concentration the slower the oxidation rate. Based on the obtained results, a general pathway for the oxidation of sulphur containing s-triazines is proposed.

Prometryne oxidation by sodium hypochlorite in aqueous solution: kinetics and mechanism.

The reaction of the herbicide prometryne (C 9 H 16 N 5 -S-CH 3 ) with sodium hypochlorite has been investigated from the kinetic and mechanistic stand point. Under the fixed experimental conditions : pH = 7, T = 25 °C, [NaClO]/[substrate] = (10 -3 M)/(10 -5 M), prometryne oxidation takes place according to the following pathway : R-S-CH 3 (P) → R-SO-CH 3 (a) → R-SO 2 -CH 3 (b) → R-O.SO 2 -CH 3 (x) → R-OH (c), where R stands for C 9 H 16 N 5 , i.e., the substituted triazine ring of prometryne and (x) is an unexpected intermediate never previously detected nor identified. After having synthesized the pure intermediates (a), (b), and (x), the values of the pseudo-first-order kinetic constants of the first three steps were experimentally obtained : [k 1 = (0.64 ± 0.03) s -1 , k 2 = (1.81 ± 0.05) x 10 -3 s -1 , k 3 = (1.50 ± 0.03) x 10 -4 s -1 ]. As for k 4 , its value [(2.5 ± 0.2) x 10 -5 s -1 ] has been calculated indirectly on the basis of the kinetic theory concerning consecutive reactions. All the steps were pseudo-first-order reactions with respect to their specific substrate. The effect of pH on the hydrolysis of (b) [(b)→(c)], in the absence of NaClO, has been also assessed to better elucidate the mechanism of the overall pathway.

Exhaustion-regeneration behaviour of carboxylic resins. The Cr(III)/Al(III)/Fe(III) system

Abstract It is not an easy task to regenerate chromium(III) ions from weak electrolyte carboxylate resins using conventional chemicals (e.g., mineral acids, saline solutions, alone or in combination). In areas of environmental control, referring to some specific cases of industrial effluents (e.g., tanneries), where metal complexing agents are present, additional difficulties in separation and recovery of the metals may arise by use of carboxylate form resins. Referring to the Cr(III)/Al(III)/Fe(III) ionic system in a solution simulating a typical spent chrome tanning bath, the exhaustion-regeneration behaviour of a commercial acrylic-based macroporous carboxylic resin (i.e., Purolite C106) is reported. The exhaustion behaviour of the resin is straightforward, whereas regeneration by using chemicals, such as sulphuric acid, sodium hydroxide, sodium sulphate, is difficult. The adoption of hydrogen peroxide in alkaline media, during the regeneration step, seems very promising. The general behaviour is interpreted in terms of ionic speciation and complex formation. A mechanistic explanation is provided in support of the experimental findings.

By-products Formation during the Ozonation of the Reactive Dye Uniblu-A

Abstract The degradation by ozone of the hydrolyzed Uniblu-A (Uniblu-OH), i.e. the compound found in the spent bath resulting from dyeing process with the reactive dye Uniblu-A, has been studied with the aim of identifying intermediate as well as end by-products formed. The experimental results, obtained by ozonating (16 mg O3/L in the feed gas at 100 mL/min) aqueous solutions (500 ml) of Uniblu-OH (40 mg/L), in the presence or not of t-butanol, show that the oxy-dye is completely removed within 30 minutes and that its decay results only slightly affected by the presence of t-butanol. Several organic by-products have been identified by liquid chromatography-mass spectrometry (turboionspray interface) as a result of the oxy-dye degradation. Such by-products result from oxidation of the amino group, hydroxylation of the aromatic rings, cleavage of the alkyl chain, cleavage of the sulfoalkylbenzene group leaving substituted anthraquinone structures and cleavage of the latter structures leading to phthalic acid. Such by-products are further degraded leading to the formation of low molecular weight aldehydes and acids, nitrate and sulphate. As for the formation of inorganic end by-products, after 120 minutes of ozonation, the percentages of nitrate and sulphate conversion resulted 6 and 7 %, 53 and 89% in the absence and in the presence of t-butanol respectively.