Cadmium (II) Removal from Aqueous Solutions Using Mineral Wool Waste

Introduction:
Water pollution with heavy metals is a serious and spreading problem. Elevation in heavy metal concentration in environment has been caused health problems with human and other living organisms. Heavy metals are accumulated in the organisms’ tissues following ingestion or feed and water absorption(Omid Sayar et al. 2015). Rapid growth of modern industry and human population are the main reasons of heavy accumulation in the environment that is a great global concern. Heavy metals’ concentration is rapidly elevating in industrial wastewaters. Heavy metals in industrial wastes are main environmental pollutants classified as class 1 toxicants. The class 1 toxicants are materials with environmental risks; heavy metals are non-degradable and harmful for human health(Zhanga et al. 2015). Naturally, 2500 t cadmium enter environment. Forest fire, volcanic activity, industrial wastewaters from textile and smelting industry phosphorous fertilizers, pesticides, cadmium batteries, fossil fuels and cement industry are the sources of cadmium distribution in environment(Zouboulis et al. 2004). In aquatic environments, cadmium is accumulated in riverine clams, shrimps, crab and fish. Hypertension, hepatic disorders, cerebrospinal damages and itai itai disease are of cadmium effects in human(Aksu et al. 2006). Maximum allowable cadmium concentration in drinking water for a 70-kg person is 0.03 mg/l based on daily water consumption of 5.2 l (WHO, 2004). Therefore, heavy metals’ removal or concentration suppression in wastewaters prior to iterance to ground water or agriculture water is very important. Common methods for metal removal from wastewaters are chemical participation, chemical oxidation-reduction, ionic mobilization, electrochemical treatment, reverse osmosis and filtration(Ahluwalia et al. 2007). These methods might be less efficient and expensive, thus, development of less-expensive and environment-friendly methods for heavy metals’ removal is important. Surface adsorption is a physiochemical process occurring at the intersection of two phases. It is an economic method for heavy metal removal with several advantages such as ease of use, high adsorption capacity, flexibility in designation and application(Talut et al. 2011).”Artificial mineral fibers” is a general term used to describe materials including mineral wool, slag wool, glass wool and refractory ceramic fiber(Luoto et al. 1998; Alvez et al. 2018). Researches in this field show that producers attempt to produce products with high quality, low biological stability and less harmful effects in body(Kamstrup et al. 1998; Seraji et al. 2018). Mineral wool belongs to a family of mineral fiber thermal insulation(Luoto et al. 1998; Alvez et al. 2018). Metal oxides(MgO, TiO2, Al2O3, B2O3, SiO2) are presented in chemical structure of mineral wool. Mineral wool is extensively used in building construction and about 60% of total insulation in market is mineral wool(Papadopoulos 2005). Thus, the aim of the present study was to investigate the feasibility of using mineral wool wastes for heavy metal removal from aqueous medium by surface adsorption.
Materials and methods
In the present study, a madden wastewater was used for metal removal test. Four factors(pH, initial metal concentration, time and adsorbent concentration) were test for their effects on removal rate.
Experimental adsorbent
Mineral wool wastes were prepared from Asia Mineral Wool Co. (industrial town of Najafabad, Isfahan).
Mineral wool wastes traits determination
At first, some characteristics of mineral wool waste including pH, EC, pHZPC, chemical composition, structure and morphology were determined using XRD(Philips PW1800; for mineral woo; structure determination), XRF (Philips PW1480; for chemical composition determination) and scanning electron microscopy(Philips XL30; for morphological study and particle size determination) (Kumar and Rani 2013).
Effects of different factors on adsorption process
Different pH(3, 5, 7 and 9), initial cadmium concentrations(1, 5, 10, 20, 50 and 100 mg/L), contact time(5, 15, 30, 90 and 120 min) and adsorbent concentrations(1, 2, 5, 10 and 20 g/L) were tested. The fixed level for pH was 3, for adsorbent concentration was 1 g/L, for initial cadmium concentration was 1 mg/L and for contact time was 90 min. At all stages, the samples were mixed with shaker at 180 rpm for 90 min. then the solutions were filtered and cadmium concentration was determined in the supernatant using atomic absorption spectrophotometer(Perkin Elmer 3030) with three replications(Osasona et al. 2011). Finally, cadmium removal percentage and adsorption isotherms were calculated for all stages of adsorption using formula(1) and(2).
(1)
%R=(C0-Ce)/C0×100

C0 is initial cadmium concentration in aqueous solution and Ct is cadmium concentration after the trial(Huang 2018)
(2)
Qt=((Ci-Ct))/M×V
Qt= amount of adsorbed metal per adsorbent mass unit, Ci= initial metal concentration, Ct= final metal concentration at the time t, M= adsorbent mass, V= solution volume(Huang 2018)
Surface adsorption isotherms
To determine surface adsorption isotherms, different concentrations of cadmium, 1, 5, 10, 20, 50, 100 mg/L were prepared and 100 mL of suspensions containing 1 g of the adsorbent with pH 3 were used for 90 min in triplicate. After 90 min, the solution were passed through filter paper and cadmium concentration was measured in the filtered solution. Then, surface adsorption isotherms at different concentrations were graphed and Langmuir and Freundlich models were used for data fitting (Spark 2003).
Langmuir equation is as follow(Langmuir 1916):
(3)
x/m = Kcb/ (1+ kc)
x/m: adsorbed mass per adsorbent mass(qe), K: constant of adsorption energy, b: the maximum metal being adsorbed(one complete molecular layer), C: equilibrated concentration of adsorbed
Linear form of Langmuir equation:
(4)
c/x/m= 1/kb+ c/b
Appropriate nature of adsorption and specific traits of Langmuir isotherm can be described using a non-united constant named separation factor or equilibrium parameter suggested by Hall et al(1966):
(5)
RL=1/(1+bC0)
b=KL= Langmuir constant
C0= initial concentration of metal (mg/L), When RL is zero, it means irreversible adsorption; 0< RL1 means inappropriate adsorption (Roop et al. 2005; Rinkumani et al. 2018).
Freundlich equation is an experimental model with general form of(Freundlich 1906; Rinkumani et al. 2018):
(6)
Log q‌e= Log kf+1/n Log Ce


Results and Discussion
XRD analysis showed that the mineral wool was in amorph and non-crystalline form. XRF analysis showed that SiO2, CaO, Al2O3, MgO, Fe2O3 and K2O encompass about 95% of the chemical composition. Silicate was dominant compound, thus the mineral wool waste was chemically similar to other adsorbents such as zeolite and other silicate minerals. Erdem et al(2004) investigated a natural zeolite potential to removed heavy metals and reported that the zeolite composed of SiO2(69.31%), Al2O3(13.11%), CaO(2.07%), NaO(0.52%) MgO(1.13), Fe2O3(1.31%) and K2O(2.83%).
The lowest adsorption percentage was observed after 5 min(19.03%), which was significantly different compared to the other times(P<0.05). the highest adsorption percentage was obtained after 30 min(95.11%); further increase in contact time had no significant effects on adsorption percentages. In this case, studies show that adsorption percentage increased along with time, because active sites at the adsorbent surface are more occupied by the adsorbed material.
Among the different pH, the highest adsorption percentage was observed at pH 9(83.5%; P<0.05), while the lowest was obtained at pH 3(60.5%; P<0.05). pH affects metal speciation so that the metal can be as ionic or hydroxyl form; this affects adsorption efficiency. Cadmium is found as Cd2+, Cd(OH)+, Cd(OH)2, Cd(OH)3-, Cd(OH)42- at different pH in aqueous media(Geological Survey of Japan 2005; Chowdhury et al. 2013). Moreover, pH affects functional groups and active sites on adsorbents’ surface, thus markedly affects adsorption efficiency. Optimum pH for maximization of adsorption depends on the nature of the adsorbent. Navish et al(2018) studied the synthesis of iron oxide nanoparticle from wood meal ash to remove cadmium and found that increase in the medium pH from 2 to 8 resulted in increase in adsorption of cadmium from 5 to 56%. Osasona et al(2018) studied the use of active carbon derived from oat for cadmium adsorption and found that elevation in the medium pH from 1.5 to 6 led to increase in cadmium removal from 75 to 99.75%. Nazar et al(2015)